Metal-free Chromophores Research Articles

Toward Metal-free Photocatalysis: Photochemical Regeneration of Organic Hydride Donors Using Phenazine-Based Photosensitizers

Earth-abundant chromophores and catalysts are important molecular building blocks for artificial photosynthesis applications. Our team previously reported that metal-free hydride donors, such as biomimetic benzoimidazole-based motifs, can reduce CO2 selectively to the formate ion and that they can be electrochemically regenerated using the proton-coupled mechanism. To enable direct utilization of solar energy, we report here the photochemical regeneration of a benzoimidazole-based hydride donor using a phenazine-based metal-free chromophore. The photochemical regeneration was investigated under different experimental conditions involving varying sacrificial donors, proton donors, solvents, and component concentrations. The best hydride regeneration yield of 50% was obtained with phenol as a proton source and thiophenolate as a sacrificial electron donor. The mechanism of photochemical regeneration was studied using steady-state and time-resolved UV/Vis spectroscopies. Based on the results of these studies, we hypothesize that the initial photoinduced electron transfer from photoexcited phenazine chromophores involves the benzoimidazole cation and that this process is likely coupled with proton transfer to generate protonated benzoimidazole-based radical cation. The second photoinduced electron transfer is hypothesized to generate the hydride form. Our findings provide the requisite information for the future development of reductive photocatalysts for solar energy and light-harvesting applications utilizing earth-abundant metal-free materials.

Visible-Light-Driven Triplet Sensitization of Polycyclic Aromatic Hydrocarbons Using Thionated Perinones.

Metal-free chromophores that efficiently generate triplet excited states represent promising alternatives with respect to transition metal-containing photosensitizers, such as those featuring metal-to-ligand charge transfer excited states. However, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy metal-free triplet photosensitizers. We demonstrate that thionation of the lone C═O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O2 quantum yields at 1270 nm (ΦΔ = 0.78-1.0). Electronic structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in the proximity of a slightly higher energy S2 (π → π*) excited state. The distinct character in each of the two lowest-lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S1 → T1 intersystem crossing was thereby facilitated in this manner through spin-orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C═O moiety residing within its structure, valuable for the design of metal-free triplet photosensitizers.

Investigation of new carbazole based metal-free dyes as active photo-sensitizers/co-sensitizers for DSSCs

Herein, we report the molecular design, synthesis and characterization of three new D-D-π-A configured metal-free chromophores D1-3 for their application in DSSCs as sensitizers as well as co-sensitizers. The new entities comprise carbazole as donor scaffold, 4-methoxyphenyl group as auxiliary donor and three different units, viz. cyanoacetic acid, 2, 4-thiazolidinedione and barbituric acid as acceptor/anchoring groups. Their photochemical, electrochemical and theoretical studies were carried out in order to assess their feasibility as active sensitizers. Further, D1-3 were exploited as co-sensitizers along with NCSU-10 dye. Their photoelectrochemical performances and charge transport properties in fabricated DSSCs were studied. The results revealed that D1 sensitizer displayed the highest PCE of 2.20% among the three dyes. D3 when co-sensitized with NCSU-10 displayed an improved PCE of 8.32% (JSC = 19.25 mA.cm−2, VOC = 0.680 V, FF = 63.7%) while NCSU-10 alone exhibited PCE of 8.25% (JSC = 20.41 mA.cm−2, VOC = 0.667 V, FF = 60.6%).

Photoemission Studies on N-Substituted Dithienylated Phenothiazines.

Dithienylated phenothiazines (DTPTs) with different functional groups attached to the central nitrogen atom are presented as a class of versatile metal-free chromophores for the design of dye-sensitized solar cells (DSSCs) and organic light-emitting diodes (OLEDs). The electronic characteristics of spin-coated thin films on polycrystalline gold were studied using photoelectron spectroscopy assisted by theoretical calculations, scanning force microscopy, and UV/Vis spectroscopy. Complementary fluorescence spectra show light emission in the blue region (465 nm). The absorption properties and good hole-transporting abilities make DTPTs feasible hole-transporting materials (HTM) and metal-free chromophores in UV-sensitive solar cell designs.

EFFECT OF THE CHROMOPHORES STRUCTURES ON THE PERFORMANCE OF SOLID-STATE DYE SENSITIZED SOLAR CELLS

The effect of metal-free chromophores on dye-sensitized solar cell performance is investigated. Solid state dye-sensitized solar cells (ssDSCs) using different molecular sensitizers based on triphenylamine (TPA) with thiophene linkers and different alkyl chain in the donor unit have been characterized using impedance spectroscopy (IS). We show that different molecular structures play a fundamental role on solar cell performance, by the effect produced on TiO 2 conduction band position and in the recombination rate. Dye structure and its electronic properties are the main factors that control the recombination, the capacitance and the efficiency of the cells. A clear trend between the performance of the cell and the optimization level of the blocking effect of the dye structure has been identified in the solid state solar cells with Spiro-OMeTAD hole conductor.

Theoretical investigation of the donor group related electronic structure properties in push-pull organic sensitizers

The judicious selection of donors is one common tactic to elaborately tailor metal-free chromophores, which can harness visible as well as near-infrared solar photons and modulate the charge transfer kinetics at titania/dye/electrolyte interface in dye-sensitized solar cells. In this paper, to gain insight into the effect of the donor on spectroscopic properties, electronic structures, excited state lifetimes and reorganization energies which are related to light-harvesting capacity and charge transfer reaction, we carried out a computational investigation on a variety of ordinary arylamine electron donors, by use of three cyclopentadithiophene dyes (C246, C218 and K201) with the respective N-hexyl-carbazole (HCZ), dihexyloxy-substituted triphenylamine (HTPA) and bisfluorenylaniline (HFA) electron donors. Amongst these electron donors, N-hexyl-carbazole is found to display the weakest electron-donating capacity, accounting for the corresponding C246 dye with evidently blue-shifted light absorption in comparison with the other two congeners, and endowing the hole mainly localizing on the conjugated segment, which will slow down the regeneration reaction. The HFA electron donor group with large and rigid structure not only presents longer excited state lifetime, which could be beneficial to the photoinduced electron injection, but also features smaller reorganization energy for the regeneration process.

Efficient Dye-Sensitized Solar Cells with an Organic Photosensitizer Featuring Orderly Conjugated Ethylenedioxythiophene and Dithienosilole Blocks

In view of the limited ruthenium resource, metal-free organic dyes may play a prominent role in the coming large-scale application of cost-effective dye-sensitized solar cells, if their efficiency and stability can be considerably improved. In this paper we utilized a binary π-conjugated spacer of ethylenedioxythiophene and dithienosilole to construct a high molar absorption coefficient push−pull dye, characteristic of an intramolecular charge-transfer band peaking at 584 nm measured in chloroform. In comparison with the standard ruthenium sensitizer Z907, this metal-free chromophore C219 endowed a nanocrystalline titania film with an evident light-harvesting enhancement, leading to an unprecedented 10.0−10.3% efficiency at the AM1.5G conditions for dye-sensitized solar cells with nonruthenium dyestuffs, although a highly volatile electrolyte was used. Transient absorption measurements have revealed that even if the kinetics of back-electron transfer and dye regeneration are considerably different for Z907 and C219, the branching ratios of these two charge-transfer channels are over 35 for both dyes, ensuring a high yield of net charge separation at the titania/dye/electrolyte interface. A solvent-free ionic liquid cell with C219 as the sensitizer exhibited an impressive efficiency of 8.9% under a low light intensity of 14.39 mW cm−2, making it very favorable for the indoor application of flexible dye-sensitized solar cells.

Influence of the electrolyte cation in organic dye-sensitized solar cells: lithium versus dimethylimidazolium

We investigate the influence of electrolyte cations such as lithium and dimethylimidazolium on the spectroscopic and electrical characteristics of dye-sensitized solar cells based on a metal-free chromophore C218. An evident bathochromic effect of lithium with respect to dimethylimidazolium is noticed for the C218 dye-coated nanocrystalline titania film via measuring electronic absorption and photocurrent action spectra. In comparison with dimethylimidazolium, the use of lithium as the electrolyte cation evokes a downward shift of the excited-state redox potential of the C218 sensitizer by 120 mV, and that of the conduction band edge of the nanocrystalline TiO2 film by 390 meV. The resultant remarkable variation in the energy alignment at the titania/dye interface brings on dissimilar electron injection yields as revealed by transient emission measurements. Modulating the titania surface states distribution with lithium relative to dimethylimidazolium induces a slightly declining electron diffusion coefficient in the mesoporous titania film. However, the adsorption of lithium cations on titania strongly retards the interfacial charge recombination compared to dimethylimidazolium, contributing to an over one order of magnitude of enhancement of electron diffusion length. The improvement of electron diffusion length has a noticeable effect on the charge collection yield, which can also be observed by measuring monochromatic incident photon-to-electron conversion efficiencies. Further interface engineering is needed to enhance both the photovoltage and photocurrent, making use of the full power of metal-free organic dyes.