Supramolecular Approach in Energy Conversion Devices

Abstract

This review summarizes investigations carried out at the Laboratory of Photochemistry and Energy Conversion (LFCE) in the University of São Paulo dealing with design and characterization of ruthenium(II), rhenium(I) and iridium(III) polypyridine complexes with desired photochemical and photophysical properties in light of the development of optoelectronics and photoinduced energy conversion systems. First, the breakthroughs on molecular engineering of emissive Re^I, Ru^II and Ir^III complexes for the development of highly efficient light-emitting devices, such as organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs), are presented. Then, the photochemical and photophysical properties of fac-[Re(CO)₃(NN)(trans-L)]⁺ complexes (NN = bidentate polypyridyl ligands and trans-L = stilbene-like ligand), which find use in molecular machines and photosensors, are discussed. Finally, dye-sensitized energy conversion devices based on Ru^II complexes and natural dyes, such as dye-sensitized solar cells (DSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs), are reviewed, highlighting some strategies for photoanode engineering aiming at improved device efficiencies.

Keywords:
supramolecular chemistry; energy conversion; molecular devices; solar fuels; dye-sensitized solar cells; OLEDs/LECs

1. Introduction

A supramolecular approach gives rise to the design of organized systems and can be conveniently exploited through metal complexes for the control of photoproperties and development of molecular devices through structurally organized and functionally integrated chemical systems.^1-5 One of the strategies is the use of coordination compounds with proper ligands to pursue a high chemical stability and intense absorption in the visible^6,7 with suitable redox properties for energy and/or electron transfer processes.^8-12

The Laboratory of Photochemistry and Energy Conversion (LFCE) at Chemistry Institute of the University of São Paulo has been carrying out investigations in photochemistry and photophysics of coordination compounds and supramolecular systems toward molecular design and devices fabrication.

The initial studies were focused on kinetics, electrochemistry and spectroscopy of cyanoferrate(II) complexes,^13-17 with emphasis on the reactivity of the ground state. The extensive investigation on kinetics and mechanisms of formation and substitution of pentacyanoferrate(II) complexes, [Fe(CN)₅L]^3- (L = monodentate neutral ligand, such as NO or CO),^13,18,19 was then extended to the reactivity of coordinated NO to the cyanoferrate moiety.^14,19 These studies and the solid knowledge in the ground state spectroscopy, electrochemistry and reactivity of coordination compounds complexes were proven to be fundamental for exploitation of more complex chemical systems and application in several new areas. Subsequent investigation on the photoreactivity of these compounds came as a natural step. The photosubstitution of the L ligand in [Fe(CN)₅L]^3- was first investigated in the early 80s.^18,20-24 The data for substitutionally inert [Fe(CN)₅L]³⁻ with L = CO, AsPh₃ (triphenylarsine), SbPh₃ (triphenylantimony(III)), P(OCH₃)₃ (trimethyl phosphite), PPh₃ (triphenylphosphine) and en (ethylenediamine) are shown in Table 1.

Table 1
Quantum yields for the photosubstitution of L in [Fe(CN)₅L]^3-

Complex	λirradiation / nm	Φ	Reference
[Fe(CN)5CO]3-	366	0.9	20
	366	0.37	22,23
[Fe(CN)5(AsPh3)]3-	366	0.15	22
[Fe(CN)5(SbPh3)]3-	366	0.12	22
[Fe(CN)5(P(OMe)3)]3-	366	0.14	22
[Fe(CN)5(PPh3)]3-	313	0.14	22
	336	0.15
	404	0.12
[Fe(CN)5(en)]3-	366	ca. 0.09	20

• λ_irradiation: irradiation wavelength ; Φ: photochemical quantum yield; AsPh₃: triphenylarsine; SbPh₃: triphenylantimony(III); P(OMe)₃: trimethyl phosphite; PPh₃: triphenylphosphine; en: ethylenediamine .

The photochemistry of aqueous solutions of [Fe(CN)₅(en)]³⁻ in the presence of a large excess of the diamine ligand under continuous photolysis was the first example reported in the literature,²⁰ having a cyanide photolabilization as the reactive deactivation pathway. The neighboring effect in the excited state was exploited as a strategy by using the ability of the dangling free amino group to capture a lower coordinate intermediate to undergo subsequent ring closure after labilization of the Fe-CN bond, Figure 1.²⁰

Figure 1
Neighboring effect and subsequent ring closure.

The inertness of the photoproduct, [Fe(CN)₄(NN)]^2-, after chelating enabled its further isolation and quantitative analysis.^25,26 The polarity and viscosity of the solvent have a key role on the quantum yields for the CN^- release in [Fe(CN)₅(NN)]^3- complexes.^27,28 While the wavelength dependence accounts for different efficiencies of primary radical formation in the initial step before deactivation occurs, the dependence on medium parameters is related to the dynamics of deactivation for the formation of the final photoproduct. The medium characteristics can shift the balance from one pathway to another, tuning the efficiency of the photochemical process.

Gradually, the focus has been directed on understanding the photophysical and photochemical properties of Re^I, Ru^II and Ir^III complexes as well as the development of photoinduced energy conversion devices, such as dye-sensitized solar cells (DSCs), dye-sensitized photoelectrosynthesis cells (DSPECs), organic light-emitting diodes (OLEDs), light-emitting electrochemical cells (LECs), molecular machines and photosensors, many of them through fruitful collaboration with Prof Carlo A. Bignozzi and Prof Thomas J. Meyer, along with highly motivated students to face such an interdisciplinary area.

In the following sections, we show that these compounds are strategical for the development of such supramolecular devices as they can exhibit a high chemical stability and intense absorption in the visible due to metal-to-ligand charge transfer (¹MLCT) transitions,^6,7 with suitable redox properties for energy and/or electron transfer processes.^8-12 Some of these complexes also exhibit intense luminescence at room temperature, usually ascribed to a phosphorescence from the ³MLCT counterpart arisen from a high spin-orbit coupling (SOC).^29-31

2. Light-Emitting Devices

Sustainable development demands to reduce energy consumption by developing modern optoelectronics and efficient light-emitting devices. In this context, light-emitting diodes (LEDs) are alternatives for replacing old lighting technologies for their less power consumption,^32,33 low voltage and current operation,³³ fast response time,³³ long durability,^33,34 high performance,³⁵ low maintenance³⁴ and eco-friendly fabrication processes.^33,35

A LED is based on inorganic semiconductors such as GaN and GaAs. However, crystals usually need to be highly pure to ensure high performances in most optoelectronic applications.³⁶ On the other hand, OLEDs are characterized by more facile processing, although they lack the long-term chemical and thermal stability usually observed for inorganic ones.³⁷

An OLED is assembled in a supramolecular multi-layer configuration of organic semiconductors with an active emissive film positioned in between p- and n-type charge transport layers (hole transport layer (HTL) and electron transfer layer (ETL)). Detailed mechanisms for the OLED’s operating principle has been reviewed extensively elsewhere^38-40 and Figure 2 shows a simplified view. When an external voltage is applied between the electrodes, holes are injected through the anode and electrons are injected through the cathode.⁴¹ The holes/electrons are transported through HTL/ETL to reach the active emissive layer and fill its highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO), respectively.^40,41 Within the emissive layer, electrons and holes reach a recombination zone, where both particles form a bound state called exciton. Excitons populate the lowest-lying excited state, from which a luminescent decay occurs, resulting in the emission of light from the device.⁴²

Figure 2
The multilayered architecture, the usual components and the operating mechanism of an OLED (simplified; more details in the main text).

Doping the organic emissive layer with a phosphorescent guest molecule (such as heavy-metal d⁶ coordination compounds) has been a successful strategy for increasing luminous efficiencies and controlling the emitted color, standing as the second generation of OLEDs.^38,39,42-44 Under operation, excitons are generated in the organic layer, then posteriorly transferred to the guest complex excited state, from which a phosphorescent deactivation occurs.

Conversion of electricity into light can be also reached using LECs in a simpler architecture. These devices share a similar base structure with OLEDs, in which an emissive film is placed in between a metallic and a transparent electrode; however an active layer in LECs contains mobile ions, changing the operating mechanism and properties of the device.^45-50 As summarized in Figure 3, application of bias causes the mobile charge carriers to drift within the active film towards the proper electrode with an opposite charge.^51,52 The ion redistribution leads to a formation of double layers close to the electrodes. The double layers facilitate injection of electrons and holes into the emissive film when the applied bias is enough to overcome the HOMO-LUMO gap (or bandgap in the case of a semiconductor) of most usual emitters.⁴⁴ After injection, electrons and holes produce excitons which then recombine resulting in luminescence.⁵²

Figure 3
The sandwich type architecture, the usual components and the operating mechanism of a LEC (simplified; more details in the main text).

Usually, efficient green- and red-light emission can be reached with these devices; even though obtaining suitable blue-light systems remains a challenge. The LFCE has been investigating numerous phosphorescent coordination compounds, mostly of Ru^II, Re^I and Ir^III for applications in light-emitting devices with focus on molecular engineered control of their color and emission quantum yields along with thorough photophysical elucidation.

2.1. Ruthenium(II) complexes

The emission of [Ru(NN)₃]²⁺ complexes (NN = bidentate polypyridinic ligands, such as 2,2’-bipyridine (bpy), 1,10-phenanthroline (phen) and their derivatives) has been extensively investigated over the past 70 years with detailed photophysical elucidation discussed elsewhere and reviewed in several publications.^53-58 The ³MLCT_Ru⇋NN state of [Ru(NN)₃]²⁺ complexes is a strong oxidizing and a reducing agent⁵⁹ and its phosphorescent deactivation to the singlet ground state occurs at room temperature in microseconds, with a broad non-structured spectrum and emission quantum yields from 10 to 0.1%³⁹ and usually in the orange-red spectral region.⁶⁰

Our initial investigation on light-emitting devices with ruthenium(II) polypyridinic complexes, Figure 4, to build single-layer LECs, Figure 5, by spin coating on indium tin oxide (ITO) substrate, had the ITO/Ru-1:PMMA/Al architecture (PMMA: poly(methyl methacrylate)).

Figure 4
Ruthenium(II) complexes investigated by LFCE for light-emitting devices.

Figure 5
(a) ITO/Ru-1:PMMA/Al LEC under operation, emitting orange light with CIE color coordinate indicated in diagram (b).

Both the electroluminescence and photoluminescence spectra (and CIE coordinates) of Ru-1 and Ru-2 are similar (with maximum ca. 630 nm). The Comission Internationale d’Eclairage (CIE) quantified the color perceived by humans in three matching functions or spectral sensitivity curves (x(λ), y(λ) and z(λ)) based on trichromatic stimuli of the human virtual cortex (for more details, see literature).⁴⁴ Their CIE coordinates (x, y) is (0.64, 0.36). The best device efficiency was obtained for the Ru-1 device with ca. 10 µW optical output power at the band maximum with a wall-plug efficiency higher than 0.03%.^61,62

2.2. Rhenium(I) complexes

Tricarbonyl polypyridinic rhenium(I) complexes can find a great variety of applications in biological sensors,^63-69 photocatalysis of CO₂ to CO,^70,71 photosensitization of ¹O₂,^72,73 polymer sensors⁷⁴ and light-emitting devices.^75-77

Lifetimes as high as ten microseconds are observed in fac-[Re(CO)₃(NN)(L)]^0/+ complexes (NN = bidentate polypyridinic ligands and L = halogenates, phosphines and pyridine derivates),^74,78-80 due to the strong spin-orbit coupling (SOC) exerted by the Re^I metal center (with an estimated SOC constant ξ_Re between 700 and 900 cm^-1)⁸¹ facilitating an efficient population of the emissive ³MLCT_Re⇋NN excited state. The strong π back-bonding between the carbonyl ligands and the metal center usually leads to a restricted-energy gap between Re^I d orbitals and π* ligand orbitals involved in the MLCT transition, therefore emission wavelengths lie predominantly in the green-yellow to orange-red spectral region.⁶⁰

The LFCE investigated photophysical behaviors of fac-[Re(CO)₃(NN)(L)]^0/+ complexes, Figure 6, and application in OLEDs.^77,82,83 The emission of these complexes is ascribed to ³MLCT_Re⇋NN (³MLCT_Re⇋bpy, ³MLCT_Re⇋quin and ³MLCT_Re⇋isoquin for Re-1, Re-2 and Re-3, respectively) with strong rigidochromic effects, highly sensitive to the media rigidity, as exemplified in Figure 7 for Re-1.^83,84

Figure 6
Luminescent rhenium(I) complexes investigated by the LFCE for light-emitting devices.

Figure 7
Emission spectra for Re-1 in fluid acetonitrile at 298 K (^___), in rigid PMMA (poly(methyl methacrylate)) at 298 K (─•─•─) and in rigid EPA (5:5:2 diethylether:isopentane:ethanol) at 77 K (─ ─ ─), with λ_excitation = 300 nm.

The electroluminescence spectrum of OLED using Re-1 as a dopant in a thin film of polyvinylcarbazole (PVK), an organic semiconductor,⁸³ is ascribed solely to the very intense ³MLCT emission of the guest complex at 580 nm, due to an efficient energy transfer from the host to the dopant.^77,83 The electroluminescence spectrum of spin-coated films of PVK without Re-1 complex in the ITO/PEDOT:PSS/PVK/butyl-PBD/Al OLED (PEDOT: poly(3,4-ethylenedioxythiophene), PSS: poly(styrenesulfonate), butyl-PBD: 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, Al: aluminum) exhibits a characteristic blue emission, λ_max = 420 nm, assigned to the PVK excimer (CIE coordinates: 0.19, 0.12), Figure 8a, while ITO/PEDOT:PSS/PVK:Re-1/butylPBD/Al architecture device, resulted in an eye-observed apparent white emitter device (CIE coordinates: 0.42, 0.45), Figure 8b.

Figure 8
(a) ITO/PEDOT:PSS/PVK/butyl-PBD/Al and (b) ITO/PEDOT:PSS/PVK:Re-1/butyl-PBD/Al OLEDs under operation, with their CIE color coordinates indicated in diagram (c).

2.3. Iridium(III) complexes

The mer-[Ir(NC)₂(LX)]⁺ complexes (NC = 2-phenyl pyridine or similar bidentate ligands, organometallated to Ir^III through NC in a 5-membered metallacycle, and LX = diimines, picolinates, acetylacetonates or similar bidentate ligands) usually present excellent thermal and photochemical stabilities with a variety of ligands and find applications in biological phosphorescent labels and sensors,^85,86 photodynamic therapy,^87,88 metallopharmaceuticals with antitumoral activities,^89,90 dye-sensitized solar cells^91,92 and catalysis.^93,94

These complexes exhibit microsecond-lived excited states with impressive emission quantum yields (close to 100%) as a consequence of iridium’s very-strong SOC (with estimated SOC constant ξ_Ir around 4430 cm^-1).⁹⁵ They also undergo color and efficiency tuning through judicious molecular engineering by controlled changes in the ligands, allowing emission in all three primary colors-blue, green and red.^44,96,97

mer-[Ir(NC)₂(LX)]⁺ complexes present overlapped excited states in the visible with strong SOC-induced mixing of electronic characters, leading to hybrid excited states (see more information in the literature).⁴⁴ The weak lowest-energy band in most mer-[Ir(NC)₂(LX)]⁺ complexes is ascribed to a SOC-induced direct absorption to the normally spin-forbidden lowest-lying triplet excited state, T₁.³⁸ After excitation at any wavelength, SOC facilitates rapid population of T₁ via a series of intersystem crossings and internal conversions and deactivation from T₁ usually occurs through intense phosphorescence.⁹⁸

The emission color can be judiciously tuned by addition of electron-donating or -withdrawing groups to one of the ligands, which, respectively, lead to destabilization or stabilization of the energies of the ligand orbitals.⁹⁹ More specifically, modifications in NC ligands modulate the energy of HOMO while those in a LX ligand affect the energy of LUMO.^100,101

These investigations started in 2010 and, since then, many complexes, Figure 9, had their photophysics detailly elucidated.^39,102 The LFCE was one of the first groups to propose that the degree of SOC-induced mixings in T₁’s excited-state character can also be rationally tuned towards enhancements in the radiative rate constant.⁹⁵ By this approach, improved emission quantum yields can be reached, as thoroughly discussed and revised.⁴⁴

Figure 9
Luminescent Ir^III complexes investigated by the LFCE for light-emitting systems.

The emission of complexes Ir-1 to Ir-4 varies systematically from blue-green to orange with variations in electron-donating or -withdrawing substituents on both the NN and the NC ligands, Figure 10.⁹⁵ Time-dependent density functional theory (TD-DFT) calculations and Franck-Condon band shape analyses indicated a SOC-induced mixed MLCT/LC character for the emission of Ir-1, which resulted in an emission quantum yield (ϕ) = 96%, at least four times higher than that observed for the similar complex Ir-2, with ϕ = 23%. Later, complex Ir-5 was synthesized to exhibit blue-sky emission using the strong electron-donating 3-methylpyridine-2-carboxylate (Mepic) ligand, coordinated to Ir^III through NO instead of NN.¹⁰³ Ir-5 was inspired in the archetypal blue emitter FIrPic (see Figure 9),¹⁰⁴ yet with an additional methyl group in the picolinate ligand to enhance the mixing character in T₁ hence increasing ϕ from 80% in FIrPic to 98% in Ir-5. On the other hand, changing the electron-donating methyl group to the electron-withdrawing CF₃ group, as in the Ir-6 to Ir-8 series, increased the non-radiative rate constant in detriment to the radiative one, decreasing their ϕ (< 13%).¹⁰⁵

Figure 10
(a) Emission spectra and (b) CIE color coordinates for Ir-1 to Ir-5 in 298 K fluid acetonitrile.

The photophysical properties of the series Ir-9 to Ir-11, with 2-phenylquinoline as cyclometalated NC ligand, were also investigated.¹⁰⁶ These complexes exhibit high ϕ and high efficiency of singlet oxygen photosensitization, showing that they can find use in many applications, from the active layer of electroluminescent devices^{43,44,48,50,52,91,97,98,107,108} to photosensitizers for photodynamic therapy and theranostics.^{88,90,109-111}

Complexes Ir-12 to Ir-14 have the ability to produce micelles combining the photophysical properties of Ir-2 to Ir-4.¹¹² These complexes were mixed with an appropriate surfactant to result in micelles that served as templates for the synthesis of highly-emissive mesoporous silica host supramolecular materials.

LEC devices were fabricated by employing complexes Ir-1 and Ir-2 as emissive active layers, exhibiting green and yellow light, respectively, Figure 11.^113-115 The FTO/PEDOT:PSS/Ir-2/Al device was sealed using a treatment developed by the LFCE,¹¹⁶ which allowed the optoelectronic characterization to be carried out in air, outside of a glovebox.¹¹⁴ A blue-emissive FTO/PEDOT:PSS/PVK:Ir-5/Al OLED was fabricated using Ir-5-doped PVK as the emissive layer.¹¹³ The PVK:Ir-5 films are subjected to an intense energy transfer from the polymer to the complex, resulting in a complete quenching of the PVK emission. These works are a successful proof of concept for feasible low-cost light-emitting devices with simple architectures and fabrication methods that require non-specific instruments.

Figure 11
(a) ITO/PEDOT:PSS/Ir-1/Al and (b) ITO/PEDOT:PSS/Ir-2/Al LECs and (c) ITO/PEDOT:PSS/PVK:Ir-5/Al OLED under operation, with their CIE color coordinates indicated in diagram (d).

3. Molecular Machines and Photosensors

3.1. Photochemical and photophysical properties of Re^I complexes

Those fac-[Re(CO)₃(NN)(L)]^0/+ complexes mentioned in sub-section 2.2 are also effective in sensitizing the trans⇋cis photoisomerization of stilbene-like molecules (which can coordinate to Re^I as a monodentate L ligand), Figure 12.^6,79,117-142 This photochemical reaction is appealing for allowing molecular geometry control by means of light absorption,^5,143 which can be conveniently exploited in molecular machines, gears and motors with yes-no or on-off logical responses for applications in sensors and biological systems, such as deoxyribonucleic acid (DNA) transcription¹⁴⁴ and regulation of cations in membranes.^145-147 The main advantage of using coordination complexes is to improve and/or sensitize the photoreaction to the visible region.⁶

Figure 12
trans⇋cis photoisomerization for (a) free stilbene-like molecules or (b) as a ligand in fac-[Re(CO)₃(NN)(L)]⁺ complexes.

Absorption of light is restricted to the UV region for the non-coordinated stilbene molecules while the coordination to the fac-[Re(CO)₃(NN)]⁺ moiety overcomes this limitation, sensitizing the trans→cis photoisomerization process toward visible light.^6,81,139 The photoisomerization mechanism of fac-[Re(CO)₃(NN)(trans-L)]⁺ is not straightforward being subject of many investigations. It depends not only on the L stilbene-like molecule but also on electronic interactions with the NN ligand and the Re^I center (spin-orbit coupling inducer).^{124,127,129,134,138,148} In summary, the absorption of light occurs through singlet excited states, either a ligand-centered ¹LC_L transition (equivalent to the S₁ state in the free organic molecule) in the UV region or a ¹MLCT_Re→NN toward visible. Then, the excited molecule relaxes (by intersystem crossings and/or internal conversions) to the lowest-lying triplet ligand centered state ³LC_L (equivalent to the T₁ state in the free organic molecule), which is now less spin-forbidden due to the influence exerted by the Re^I core.

trans→cis photoisomerization of fac-[Re(CO)₃(NN)(L)]⁺ can be properly monitored by UV-Vis and ¹H nuclear magnetic resonance (NMR) spectral changes as a function of photolysis time, as exemplified in Figure 13. Upon irradiation, absorption spectral changes usually lead to well-defined isosbestic points, which indicate no competitive photoreactions.^{6,84,117,121,127,141,149,150} Also, ¹H NMR signals for the trans-isomer decrease gradually, while new signals ascribed to the cis-isomer build up in intensity. It is noteworthy that the hydrogen coupling constant between H_c and H_d of the trans-isomer is J³ ca. 16 Hz^121,123 while for H_c’ and H_d’ in the cis-isomer is J³ ca. 12 Hz,⁶ which confirm a successful photoisomerization.

Figure 13
(a) Absorption (CH3CN) and (b) ¹H NMR (800 MHz, CD₃CN) spectral changes for Re-15 as a function of photolysis time (λ_irradiation = 436 nm, Δt = 30 s, T = 298 K).

Usually, trans- and cis-isomers absorb in the same region, Figure 13, thus the the photoisomerization quantum yield (Φ) values determined by UV-Vis spectral change are just apparent. On the other hand, ¹H NMR spectroscopy has been successfully proven by the LFCE^121,151 to be an important tool to determine accurate quantum yields since the hydrogen signals for photoproducts and reactants appear in very distinct regions, Figure 13. As a consequence, quantum yields so determined are the true ones while those determined by variations in absorption spectra are apparent.

The LFCE has proven that ¹H NMR spectroscopy is an important tool to determine accurate quantum yields, since the hydrogen signals for photoproducts and reactants appear in very distinct regions, Figure 13.^84,141,150

The cis-isomer complex is usually emissive while the trans-isomer is non-emissive, Figure 14, and emission spectral changes can also monitor the photoisomerization process. The gradual increase in emission (ascribed to the lowest-lying ³MLCT_{cis-L(Re→NN)} state) via trans→cis photoisomerization can be exploited in the development of optoelectronic devices for photosensors, emission on-off photoswitches and polymerization sensors.^{117,122,123,125,128,129}

Figure 14
(a) trans-to-cis photosiomerization reaction for Re-14 and (b) changes in its emission (λ_exc = 420 nm) spectra as a function of photolysis time in acetonitrile (λ_irr = 436 nm, Δt = 30 s, T = 298 K). The inset graph in (b) exhibits the increase in the emission intensity at 620 nm as a function of photolysis time.

Figure 15 and Table 2 show fac-[Re(CO)₃(NN)(trans-L)]⁺ complexes investigated by the LFCE. The Re-4 to Re-7 bpe series (bpe = 1,2-bis(4-pyridyl)ethylene) typically exhibits higher trans-to-cis photoisomerization quantum yields (Φ_trans→cis) than the non-coordinated ligand, independent on irradiation wavelenghts.^{6,121,127,148,151-153} For example, for complex Re-4 the bpe ligand photoisomerizes under blue light irradiation with outstanding Φ_trans→cis (Φ_{313 nm}= 0.81 ± 0.07, Φ_{365 nm}= 0.80 ± 0.07 and F_{404 nm}= 0.77 ± 0.09),^121,148 more efficient than the UV restricted non-coordinated bpe (Φ_{313 nm}= 0.26 ± 0.04).¹²¹ Also, an increasing luminescence centered at 570 nm is observed at room temperature as the cis-isomer is formed.^123,131

Table 2
trans-cis photoisomerization quantum yields for the fac-[Re(CO)₃(NN)(trans-L)]⁺ complexes investigated by LFCE

Compound	trans-cis photoisomerization quantum yield					Reference
	λ irradiation
	313 nm	334 nm	365 nm	404 nm	436 nm
Re-4	0.81 ± 0.07	-	0.80 ± 0.07	0.77 ± 0.09	no absorption	121
Re-5	0.43 ± 0.03	-	0.44 ± 0.02	0.43 ± 0.02	no absorption	121
Re-6	0.55 ± 0.03		0.55 ± 0.05	0.56 ± 0.05	no absorption	148
Re-7	0.32 ± 0.05	0.29 ± 0.04	0.33 ± 0.04	0.29 ± 0.03	no absorption	129
Re-11	0.64 ± 0.02	-	0.69 ± 0.03	0.48 ± 0.04	no absorption	117
Re-12	-	-	-	0.48 ± 0.03	no absorption	152
Re-13	-	-	-	0.31 ± 0.07	no absorption	152
Re-14	0.54 ± 0.04	0.55 ± 0.03	0.53 ± 0.02	0.43 ± 0.03	0.44 ± 0.05	149
Re-15	0.49 ± 0.06	0.51 ± 0.03	0.54 ± 0.05	0.51 ± 0.04	0.50 ± 0.03	150
Re-16	0.59 ± 0.05	-	0.60 ± 0.06	0.43 ± 0.02	no absorption	121
Re-17	0.60 ± 0.05	-	0.64 ± 0.09	0.42 ± 0.03	no absorption	121
Re-18	0.53 ± 0.02		0.57 ± 0.02	0.41 ± 0.05	no absorption	148
Re-19	0.52 ± 0.06	0.57 ± 0.05	0.57 ± 0.06	0.35 ± 0.02	no absorption	129
Re-20	0.44 ± 0.02	0.44 ± 0.01	0.45 ± 0.02	0.39 ± 0.03	no absorption	84
Re-21	0.43 ± 0.02	0.47 ± 0.02	0.48 ± 0.06	0.36 ± 0.03	no absorption	84
Re-22	0.65 ± 0.05	0.64 ± 0.04	0.62 ± 0.05	0.55 ± 0.05	0.56 ± 0.04	149
Re-23	0.58 ± 0.05	-	0.61 ± 0.05	0.42 ± 0.02	no absorption	137
Re-24	0.38 ± 0.08	0.38 ± 0.08	0.38 ± 0.03	0.37 ± 0.02	no absorption	141

• λ irradiation: irradiation wavelength.

Figure 15
trans→cis L photoisomerizable rhenium(I) complexes investigated by the LFCE for photosensors and molecular machines.

The photoisomerization process was also observed for rhenium(I) binuclear complexes, in which bpe was used as a bridge photoisomerizable ligand attaching both bulk units. Similarly to mononuclear complexes, irradiation of Re-8 led to spectral changes with clear and well defined isosbestic points ascribed to the trans→cis photoisomerization.¹⁵³ On the other hand, no photoisomerization was observed for rhenium(II)-iron(II) (Re-9) and rhenium(II)-osmium(I) (Re-10) bimetallic complexes,^126,154 due to the presence of the lower-lying ³MLCT state of iron (³MLCT_Fe→bpe)¹⁵⁴ or osmium (³MLCT_Os→bpe)¹²⁶ that quenches the photoisomerization channel.

Differently than the Re-4 to Re-7 bpe series, the Re-16 to Re-19 stpy series (stpy = 4-styrylpyridine) exhibit irradiation-wavelength dependent Φ_trans→cis , suggesting different photoisomerization mechanisms, subject of several studies.^{81,133,134,138} The progress of these studies led to the fac-[Re(CO)₃(NN)(trans-stpyCN)]⁺ series (Re-20-24) achieving the first Re^I series to present a truly reversible (trans→cis and cis→trans) photoresponsive molecular motion.^{84,115,137,141,149} This impressive photoreversibility of fac-[Re(CO)₃(NN)(trans-stpyCN)]⁺ opens new perspective for application in molecular machines as molecular motors and geometry regulators for switch on/off devices.

Re-11 and Re-15 complexes were engineered for device applications taking advantage of carboxylic acid as anchoring group. The photochemistry had been investigated both in fluid solution and adsorbed on TiO₂ surface with detection of photoisomerization for both complexes independent on the media. The Re-11 cis-photoproduct, emissive in solution, had its emission quenched in favor of electron injection into TiO₂, with increasing photocurrent as the concentration of cis-isomers increases.¹¹⁷ The breakthrough of Re-15 is the effective sensitization to the visible with photoisomerization occurring even at 436 nm; in particular, Re-15 exhibits characteristics for solar energy conversion devices.¹⁵⁰ The undergoing investigation revealed that it adsorbs more efficiently on the TiO₂ surface. Furthermore, it is successfully photoisomerized under 436 nm irradiation, Figure 16. These systems exemplify their use as a proof of concept for molecular devices.

Figure 16
trans-to-cis photoisomerization of Re-15 adsorbed on the TiO₂ surface (complex and nanoparticle are out of scale).

4. Dye-Sensitized Energy Conversion Devices

The abundant and potentially infinite energy generated by the sun is one of the most promising sources to supply and complement the world energy matrix,^155,156 in special in Brazil, where the solar irradiation index is high practically throughout the whole year.¹⁵⁷ Therefore, there is a great interest in developing devices capable of converting solar energy efficiently, as perspectives for sustainable and renewable sources of clean energy.¹⁵⁸ The LFCE extended investigations on the solar energy field in 1995, with researches on DSCs emerged in collaboration with Prof Carlo A. Bignozzi.

4.1. Dye-sensitized solar cells

DSCs belong to the third generation of photovoltaics and their working mechanism makes use of supramolecular approaches for device concepts.^158-161 Differently from other photovoltaic technologies, DSCs are photoelectrochemical cells in which light-to-electricity conversion occurs via dyes chemically adsorbed on the electrode surface and the separation of charge carriers is kinetically controlled by the chemical reaction involved.^162,163

The typical DSC is composed by a photoanode and a counter electrode in a sandwich-type arrangement, Figure 17. The photoanode consists in a glass substrate with a transparent conductive oxide (TCO, usually fluorine-doped tin oxide, FTO) on which a mesoporous thin film of a wide bandgap semiconductor oxide (commonly TiO₂) is deposited. Sensitizers (dyes, e.g., cis-[Ru(dcbH₂)₂(NCS)₂], N3, or cis-[Ru(dcbH₂)₂(NCS)₂][TBA]₂, N719) are adsorbed on the semiconductor nanoparticles surface to absorb the visible light. The counter electrode consists of a TCO substrate with a thin layer of a catalyzer (usually transparent Pt film). A mediator electrolyte solution is inserted between both electrodes (usually, a concentrated I^-/I₃^- solution) to complete the regenerative circuit.^164,165

Figure 17
Simplified working mechanism of DSCs (more details in the main text).

Under sun shining, the sensitizer dye (S) absorbs solar irradiation to result in excited state (S^*), which injects electrons to the semiconductor conduction band (CB). The oxidized sensitizer (S⁺) is then reduced by the mediator I^-, regenerating S. Meanwhile, photoinjected electron percolate through the semiconductor film and is collected at the counter electrode, where a catalyzer reduces I₃^- to I^-, closing the cycle.^159,164 Therefore, in this regenerative electrochemical device, visible light is efficiently converted into electricity without any permanent chemical change. The highest efficiencies reported so far is around 14.3%^166,167 and have proven to be commercially feasible and several products had been released.^168,169

The LFCE developed several aspects related to the DSC technology, including the pathway from fundamental academic scientific knowledge acquired from research to technological innovation and intellectual protection by patents.^170-175

These activities included the preparation of colloidal TiO₂ using distinct methods and different deposition techniques, such as spin-coating, painting and screen-printing.^176,177 Homogeneous semiconductor films with controlled transparency had been obtained with the proper colloidal TiO₂ for automated process. Conducting plastic materials were also tested as the electrode substrate in association with a polymeric gel as an electrolyte medium resulting in wholly flexible solar cells. The electron transfer from the excited dye to semiconductor and the charge recombination/quenching processes were further investigated by transient UV-Vis absorption spectra on dye-sensitized TiO₂ films deposited onto glass substrates. A fast quenching of the oxidized complex in the presence of iodide emphasized the importance of a proper concentration of donor species in the redox mediator for the effective regeneration of the oxidized sensitizer.^178,179

Molecular engineering of cis-[Ru(dcbH₂)₂LL’] compounds (dcbH₂ = 4,4’-dicarboxylic acid-2,2’- bipyridyl and LL’ = ancillary ligands) that act as panchromatic charge transfer sensitizers of TiO₂ is still in continuous study, Figure 18. Our investigation efforts now progress toward device improvements through new synthetic and natural dyes.

Figure 18
Ruthenium(II) complexes investigated by the LFCE as sensitizers in DSCs.

Ruthenium-based dyes are the most-commonly used sensitizers due to their intense (high molar absorptivity) and broad absorption bands in the visible region ascribed to the ¹MLCT_Ru→NN transition. They also present high chemical and thermal stability in both ground and oxidized states and have favorable redox potentials for electron injection into CB of TiO₂.^180-182

The proper selection of ancillary ligands provides the suitable energetic control of the overall properties of these complexes. This approach was successfully applied to improve the incident photon-to-current efficiency (IPCE) and consequent the electron injection into TiO₂-CB sensitized by complexes Ru-3 to Ru-6.

The incident photon-to-current efficiency obtained for Ru-3 (56%, Table 3) is higher than that for Ru-4 (ca. 40%).^178,183,184 The Ru-3 complex, with two different ancillary ligands, presented a broader spectral response at longer wavelengths when compared to the bis-coordinated 4-phenylpyridine (ppy) derivative Ru-4, showing an important role in the characteristic of the donor ligand in tuning photoelectrochemical properties. The Ru-6 complex exhibited higher IPCE (ca. 60%)¹⁸⁴ when compared with its analogous ppy- (Ru-3 and 4) and isoquinoline-derivatives (Ru-5).¹⁸⁵

Table 3
Photoelectrochemical parameters for DSCs sensitized with the ruthenium(II) dyes investigated by the LFCE

Complex	JSC / (mA cm-2)	Voc / V	ff	η / %	IPCEmax / %	Reference
Ru-3					56	178,183,184
Ru-4					ca. 40	178,183,184
Ru-5					ca. 60	185
Ru-6					ca. 60	184
Ru-10	8.0	0.66	0.51			152
Ru-11	3.5	0.58	0.46	0.95	25	186
Ru-12	6.6	0.67	0.65	2.82	50	186

• J_SC: short-circuit photocurrent density; V_OC: open-circuit potential; ff: fill factor; η: photoconversion efficiency; IPCE: incident photon-to-current efficiency.

DSCs sensitized by Ru-10 exhibited a fair short-circuit photocurrent density (J_SC) of 8.00 mA cm^-2, open-circuit potential (V_OC) of 0.66 V and fill factor (ff) of 0.51.¹⁵² Variations in photoelectrochemical parameters obtained for DSCs with Ru-11 and Ru-12 sensitizers were ascribed to the different properties of the ancillary ligands.¹⁸⁶

Synthetic dyes, such as coordination compounds and organic molecules, usually lead to the best photon conversion efficiencies in DSCs.^182,187-189 However, the use of certain natural dyes as sensitizers is an advantageous environmental-friendly alternative, for their low toxicity, easy obtention and preparation of low-cost devices.^190-196 Natural dyes are obtained from different parts of plants, such as fruits, flowers, leaves and roots.¹⁹⁷ Those used in DSCs usually present an intense blue/violet color ascribed to the presence of anthocyanins with anchoring groups to promote an effective adsorption on TiO₂, Figure 19, similarly to the Ru^II-based dyes.¹⁸² The LFCE investigated DSCs sensitized by many anthocyanin-based fruit-extract: blueberry (Vaccinium myrtillusLam.),^198,199 cabbage-palm fruit (Euterpe oleracea Mart.),²⁰⁰ calafate (Berberis buxifoliaLam.),²⁰¹ chaste tree fruit (Solanum americanum Mill.),²⁰⁰ jaboticaba’s skin (Myrtus cauliflora Mart),^198,201,202 java plum (Eugenia jambolana Lam),^202,203 mulberry (Morus albaL.),^198-200,202 pomegranate (Punica granatum)²⁰² and raspberry (Rubus idaeus L.),¹⁹⁹ as summarized in Figure 19.

Figure 19
Schematic adsorption of anthocyanin on TiO₂ and the fruits investigated by the LFCE.

The strategies exploited by the LFCE to enhance the efficiencies of DSCs sensitized by mulberry, jaboticaba’s skin, java plum and pomegranate have focused mainly in variating the extraction medium or pH control. For instance, DSCs sensitized with jaboticaba’s skin showed an improvement when water was substituted by ethanol as extraction medium, enhancing the device efficiency up to 115% (h from 0.47 to 1.15%).²⁰¹

Adding pyridine in mediators for DSCs sensitized by anthocyanins resulted in a considerable J_SC loss. For mulberry, for example, this led to a 79% decrease in photoconversion efficiency (h from 4.31 to 0.92%).²⁰⁰ The current drop was due to a decrease in the medium pH, leading to deprotonation of anthocyanin and consequent desorption from the TiO₂ surface, as previously observed by Calogeroet al.²⁰⁴ Therefore, pyridine-free electrolytes are more appropriate for anthocyanin-sensitized DSCs.

The stability of natural-dye-sensitized DSC was also investigated with mulberry, jaboticaba’s skin and blueberry.^199-201 DSCs sensitized by mulberry exhibited constant photoelectrochemical parameters after 14 weeks of continuous evaluation, remaining stable even after 36 weeks with a fairly good efficiency when sealed under proper condition.¹⁹⁸

4.2. Dye-sensitized photoelectrosynthesis cells

Dye-sensitized photoelectrosynthesis cells (DSPECs) use the concepts of DSCs for energy storage.^205,206 They have been investigated toward the production of solar fuels inspired in natural photosynthesis and referred as artificial photosynthesis.^207,208 DSPECs can have lower-cost production in comparison to other crystalline-semiconductor-based systems for photooxidation of water.²⁰⁹ Although a promising strategy, some parallel reactions in their interfaces decrease considerably their efficiency.²¹⁰ Researches on this field are usually focused on new materials and understanding of these phenomena for decreasing electronic recombination from the mesoporous semiconductor to the oxidized dye and diminishing the time the photoinjected electrons take to reach the TCO substrate.²¹¹

The operation mechanism of a DSPEC is similar to a DSC, in which a sensitizer molecule is excited to a higher-energy state that injects electrons in the conduction band of a semiconductor oxide (usually TiO₂), initiating a series of molecular and interfacial electron transfer processes to lead to the production of the photoproducts (solar fuels), with higher chemical energy content,^206,208,212 Figure 20.

Figure 20
Main components, device architecture and simplified mechanism of a DSPEC for light-driven water oxidation and proton reduction under operation (more details in the main text). PEM is a proton exchange membrane.

The LFCE, in collaboration with Prof Thomas J. Meyer and his research group from the University of North Carolina at Chapel Hill, investigated reactivity toward water oxidation in a class of molecules whose properties can be systematically tuned by synthetic variations based on mechanistic insights.²¹³ These molecules work as catalyst for the water oxidation driven either electrochemically or by Ce^IV. The first two, [Ru(tpy)(bpm)(OH₂)]²⁺ and [Ru(tpy)(bpz)(OH₂)]²⁺ (bpm = 2,2’- bipyrimidine and tpy = 2,2’:6’,2’’-terpyridine), undergo hundreds of turnovers without decomposition using Ce^IV as oxidant. Detailed mechanistic studies and DFT calculations revealed a stepwise mechanism, addressed in our previous work.²¹³ In a brief summary, there is an initial 2e^-/2H⁺ oxidation of the Ru^IV-H₂O center leading to Ru^IV=O²⁺ followed by oxidation to Ru^V=O³⁺. A nucleophilic attack by H₂O gives Ru^III-OOH²⁺ with further oxidation to Ru^IV(O₂)²⁺ leading to oxygen loss followed by coordination of another water molecule, regenerating the initial Ru^IV-H₂O center. An extended family of the catalyst series based on tpy and Mebimpy (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) ligands shares a common mechanism.²¹³ Interfacial dynamics at the derivatized TiO₂ (TiO₂-Ru^II) were also investigated under appropriate condition to water oxidation, by means of nanosecond laser flash photolysis.²¹⁴

The water oxidation catalyst [Ru(bda)(4-O(CH₂)₃P-(O₃H₂)₂-py)₂], (py = pyridine and bda = 2,2’-bipyridine-6,6’-dicarboxylate) in a series of chromophore-catalyst assemblies has been investigated as light-driven water splitter in DSPECs.²¹⁵ Device performance for both coloaded and layer-by-layer assemblies with phosphonate-Zr^IV bridging on SnO₂/TiO₂ core-shell electrodes was evaluated by both photocurrent and direct O₂ measurements in collector-generator cells. The photoelectrodes displayed favorable photocurrents (0.72-1.5 mA cm^-2) and Faradaic efficiencies for O₂ generation (71-97%), providing important mechanistic insights into the microscopic details for DSPEC water splitting.²¹⁵

4.3. Photoanode engineering

DSCs based on nanocrystalline mesoporous semiconductors, in special TiO₂, have been extensively investigated since 1991²¹⁶ with continuous improvement. For high performance DSCs, the desired characteristics of mesoporous films are a high specific surface area with good porosity, long electron diffusion length and a pronounced light scattering effect.²¹⁷

One of approaches of photoanode engineering is integrating small nanocrystallites into one single bifunctional film with advantages of increased light scattering and dye-loading, by using mesoporous microspherical titania particles comprised of small nanocrystallites.^218-221 The development of a screen-printable paste with improved morphological and optical characteristics for the automated deposition of submicrometer TiO₂ resulted in 32% enhanced conversion efficiency.^176,177

Another successful approach is the photoanode engineering to minimize recombination effects by treating the TiO₂ or FTO surface with a thin cover layer of compact semiconductor oxide²²² or even insulating oxide²²³ that efficiently blocks charge recombination decreasing power conversion efficiencies.²²⁴

The recombination at FTO/electrolyte interface occurs through the direct physical contact between the FTO and the mediator that percolates through the mesoporous TiO₂ semiconductor film. This can be decreased with the use of a compact layer obtainable by many deposition techniques, such as spray-pyrolysis,²²⁵ sputtering,²²⁶ dip-coating,^227-229 spin-coating,²³⁰ atomic-layer deposition^231,232 and layer-by-layer (LbL).²³³ In particular, the LbL technique is a low-cost procedure for a thin film deposition that offers rigorous control of morphology with possibility of scaling up and applications in different areas.^234-237 This technique is based on the electrostatic interactions between oppositely charged materials; therefore a chosen substrate, as FTO, is immersed into cationic and anionic solutions (or suspensions) in a cyclic procedure,²³⁸ as shown in Figure 21.

Figure 21
Transparent DSC improved with compact layers assembled using opposite charged nanoparticle by the LbL technique.

Our first approach using LbL assemblages started with TiO₂ nanoparticles as cations pairing with polyelectrolytes, such as sodium sulfonated polystyrene (PSS), sulfonated lignin (SL) and poly(acrylic acid) (PAA), as anions.^239,240 The nature of polyelectrolyte had a key role on the efficiency of N3-sensitized DSCs, with the best performance achieved by the use of the TiO₂/PSS compact layer, that increased the overall efficiency of DSCs in 30%, Table 4.²⁴⁰ The lower thermal stability of PAA resulted in a more porous film and, therefore, unable to block the contact of electrolyte and the FTO substrate.

Table 4
Photoelectrochemical parameters for LbL-compact film improved DSCs investigated by the LFCE

Compact layer	Mesoporous layer	Dye	Jsc / (mA cm-2)	Voc / V	ff	η / %	Efficiency improvement / %	Reference
-	a	N3	10.2 ± 0.5	0.68 ± 0.02	0.64 ± 0.03	5.7 ± 0.3		239
TiO2/PSS			12.6 ± 0.5	0.73 ± 0.01	0.62 ± 0.03	7.3 ± 0.3	28
-	a	N3	13.0 ± 0.8	0.66 ± 0.02	0.66 ± 0.01	5.6 ± 0.5		240
TiO2/PSS			15.8 ± 0.3	0.70 ± 0.01	0.67 ± 0.01	7.3 ± 0.1	30
TiO2/SL			15.0 ± 0.5	0.68 ± 0.01	0.69 ± 0.01	6.9 ± 0.3	23
TiO2/PAA			11.8 ± 0.3	0.69 ± 0.01	0.72 ± 0.04	5.8 ± 0.3	3
-	a	N3	8.3 ± 0.1	0.67 ± 0.01	0.57 ± 0.01	3.3 ± 0.1		241
TiO2/Nb2O5			13.9 ± 0.1	0.70 ± 0.00	0.60 ± 0.01	6.2 ± 0.1	87
-	a	N3	9.6 ± 0.1	0.670.01	0.71 ± 0.01	4.9 ± 0.1		242
TiO2/ZnO			15.6 ± 0.3	0.70 ± 0.01	0.70 ± 0.02	8.2 ± 0.2	67
-	a	N719	15.2 ± 0.7	0.75 ± 0.01	0.62 ± 0.02	7.1 ± 0.3		243
TiO2/TiO2			16.4 ± 0.9	0.74 ± 0.01	0.64 ± 0.04	11.5	62
-	a	N3	10.8 ± 0.3	0.47 ± 0.01	0.52 ± 0.01	2.68 ± 0.01		202
TiO2/TiO2			15.1 ± 0.2	0.51 ± 0.01	0.57 ± 0.01	4.38 ± 0.12	63
-	a	mulberry	4.31 ± 0.21	0.38 ± 0.01	0.56 ± 0.01	0.90 ± 0.02		202
TiO2/TiO2			6.08 ± 0.23	0.40 ± 0.01	0.62 ± 0.02	1.49 ± 0.04	66
-	a	jaboticaba's skin	3.59 ± 0.14	0.34 ± 0.00	0.48 ± 0.03	0.59 ± 0.03		202
TiO2/TiO2			5.17 ± 0.02	0.36 ± 0.00	0.52 ± 0.02	0.98 ± 0.03	6
-	a	java plum	2.57 ± 0.11	0.37 ± 0.01	0.47 ± 0.03	0.53 ± 0.05		202
TiO2/TiO2			4.03 ± 0.13	0.38 ± 0.01	0.58 ± 0.01	0.85 ± 0.01	6
-	a	pomegranate	2.44 ± 0.39	0.38 ± 0.01	0.53 ± 0.01	0.50 ± 0.01		202
TiO2/TiO2			3.36 ± 0.36	0.41 ± 0.01	0.52 ± 0.01	0.71 ± 0.01	42
-	a	N3	8.0 ± 0.3	0.69 ± 0.01	0.73 ± 0.02	4.0 ± 0.2		176
TiO2/TiO2			11.2 ± 0.2	0.72 ± 0.01	0.72 ± 0.02	5.8 ± 0.2	45
-	b	N3	10.7 ± 0.1	0.73 ± 0.02	0.75 ± 0.03	5.9 ± 0.1		176
TiO2/TiO2			15.1 ± 0.3	0.74 ± 0.04	0.72 ± 0.01	8.0 ± 0.3	36
-	c	N3	11.3 ± 0.2	0.75 ± 0.01	0.74 ± 0.02	6.3 ± 0.2		176
TiO2/TiO2			15.9 ± 0.3	0.75 ± 0.01	0.75 ± 0.03	9.0 ± 0.4	43
-	a	Ru-13	2.56 ± 0.57	0.56 ± 0.02	0.71 ± 0.11	1.00 ± 0.10		244
TiO2/TiO2			3.83 ± 0.44	0.57 ± 0.01	0.78 ± 0.02	1.71 ± 0.14	71

• Mesoporous semiconductor:
• ^a
  nanocrystalline TiO₂,
• ^b
  nanocrystalline TiO₂/mesoporous anatase phase microspheres,
• ^c
  nanocrystalline TiO₂/rutile phase microspheres. J_SC: short-circuit photocurrent density; V_OC: open-circuit potential; ff: fill factor; η: photoconversion efficiency.

A nanostructured thin film consisted of TiO₂/ZnO bilayers from acid TiO₂ and basic ZnO had also been successfully applied as a blocking and contact layer. This film revealed a significant improvement in the J_SC and V_OC, leading to a remarkable 67% improvement in the conversion efficiency of N3-sensitized DSCs.²⁴²

The innovative ultrathin LbL all-nano-TiO₂ film as an interfacial layer created excellent adhesion of the TiO₂ layer on the FTO substrate²⁴⁵ leading to an efficient electronic transport from TiO₂ to FTO and consequent impressive enhancement up to 62% in performances of N719-sensitized DSCs.²⁴³ This nano-TiO₂ compact layer when employed in DSCs sensitized by mulberry, jaboticaba’s skin, java plum and pomegranate fruit led to enhanced global efficiencies up to 66%.²⁰²

The LbL deposition of nano-TiO₂ compact layers on the underlying mesoporous oxide had been successfully employed in DSPECs leading to an impressive improvement of 53% for photocurrent with additional enhancements of sustainable currents over time photolysis with characteristics favoring O₂ evolution at the photoanode. This innovative approach in DSPECs with a significant impact on the device performance provided a suitable platform to decrease back electron reactions and/or improve the electron collection at the photoanode due to an improved insulation at the back contact, opening up new possibilities of gathering suitable molecular assemblies using a low-cost deposition method for thin films to enhance the performance of water-splitting devices.²⁴⁶

5. Technological Innovation and Closing Remarks

The development within the framework of supramolecular chemistry gives rise to the possibility of designing organized systems and components of molecular level devices. This organization is particularly interesting for building molecular assemblies capable of performing useful functions, such as energy conversion/storage, information purpose and lightning-devices.

The multidisciplinary but fundamental research outlined herein resulted in innovation and technological developments, as well as proof of concept for several applications, such as luminescence-based sensors and displays, photoresponsive polymers, visible sensitization of solar cells and photoelectrosynthesis devices.

Contacts with industries/companies led to the first trademark (Dye-Cell^®) of the University of São Paulo in 2001²⁴⁷ and some patents for intellectual protection. In this occasion, the project developed at the LFCE had been selected by the Brazilian Ministry of Science and Technology (MCT) as an innovative project/product in the energy sector, and took part during the whole year of 2002 in a series of events entitled “Exhibition of Innovative Technological Products for Energy Production”. In 2002, one of the patents received São Paulo State Governor honorable mention award. From 2006 to 2008, the development of dye-sensitized solar cells was supported by Petrobras/CNPES till the end of the company renewable program. During these 2 years, several prototypes had been assembled and the feasibility of the device proven.

The development of solar cells became the main subject of a special program called CIUPE (Interinstitutional Collaborative Program for Strategic Researches), which was supported by the University of São Paulo and attracted researchers from several areas in the development of photovoltaics and had provided encouragement for cooperative efforts among different laboratories.²⁴⁸

Research on OLEDs and LECs focused on multicolor, highly-emissive complexes and new techniques for device fabrication. For this purpose, many phosphorescent coordination compounds have played a prominent role in several applications. Recent breakthroughs and the state of the art on highly-efficient emissive complexes elucidating the role of molecular and electronic structures to control photophysics in light-emitting systems had been reported in the pursuit of smart white-emitting devices.⁴⁴

A successful molecular engineering of coordination complexes and supramolecular assemblies in many applications of systems cited here is inspiring and illustrates the fascinating strategies to conceive and understand artificial photoresponsive or highly emissive compounds. Future molecular design strategies must head beyond energy and color control.

• Dedicated to Prof Henrique Eisi Toma on the occasion of his 70th birthday.

Acknowledgments

This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - finance code 001 and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/CTEnerg).

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Publication Dates

• Publication in this collection
  30 Oct 2020
• Date of issue
  Nov 2020

History

• Received
  02 Feb 2020
• Accepted
  20 May 2020