Four anthracene based sensitizers, 3-(anthracene-9-yl)-2-cyanoacrylic acid (M1), 2-cyano-3-(10-methoxyanthracene-9-yl)acrylic acid (M2), 2-(anthracene-9-ylmethylene) malonic acid (M3), and 2-((10-methoxyanthracene-9-yl)methylene)malonic acid (M4) were designed and synthesized to understand the binding modes of anchoring groups ( and ) on the nanocrystalline TiO 2 (101) surface and on the efficiency of dye-sensitized solar cells (DSSCs). All four sensitizers have been fully characterized using ATR-FTIR, UV-vis, and CV. These sensitizers were tested in DSSCs using 0.05 M I 2 , 0.5 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), and 0.5 M lithium iodide (LiI) in methoxypropionitrile (MPN) redox electrolyte. The sensitizers having a monocarboxylic acid group, i.e., M1 and M2, have shown marginally higher IPCE and efficiency than M3 and M4 having dicarboxylic acid groups. To have a detailed understanding of this behavior, the adsorption and binding energies to the TiO 2 surface of these dyes have been investigated using computational techniques (periodic DFT). The studies show that the cyanoacrylic acid anchoring group has a stronger binding to the TiO 2 surface compared to the malonic acid anchoring group.

Black dye (BD), isomer 1 ([Ru(II)(H3-tctpy)(NCS)3](-1), where H3-tctpy = 4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) is known to be an excellent sensitizer for dye-sensitized solar cells and exhibits a very good near-IR photo response, compared to other ruthenium dyes. Because isothiocyanate is a linear ambidentate ligand, BD has three other linkage isomers, [Ru(H3-tctpy)(NCS)2(SCN)](-1), isomer 2 and 2', and [Ru(H3-tctpy))(SCN)3](-1), isomer 3. In this study, we have calculated the geometry of BD and its isomers by DFT. Further, we have analyzed the bonding in these isomers using NBO methods. TDDFT calculations combined with scalar relativistic zero-order regular approximations (SR-ZORA) have been carried out to simulate the absorption spectra. Calculations have been performed for the isomers both in vacuo and in solvent (ethanol). The inclusion of the solvent is found to be important to obtain spectra in good agreement with the experiment. The first absorption bands are dominated by the metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT).

The dynamics of interfacial charge transfer across (E)-3-(5-((4-(9H-carbazol-9-yl)phenyl)ethynyl)thiophen-2-yl)-2-cyanoacrylic acid (CT-CA) and TiO 2 nanocomposites was studied with femtosecond transient absorption, fluorescence upconversion, and molecular quantum dynamics simulations. The investigated dye, CT-CA is a push−pull chromophore that has an intramolecular charge-transfer (ICT) excited state and binds strongly with the surface of TiO 2 nanoparticles. Ultrafast transient absorption and fluorescence measurements, in both solution and thin film samples, were carried out to probe the dynamics of electron injection and charge recombination. Multiexponential electron injection with time constants of <150 fs, 850 fs, and 8.5 ps were observed from femtosecond fluorescence measurements in solution and on thin films. Femtosecond transient absorption measurements show similar multiexponential electron injection and confirm that the picosecond electron injection component arises from the excited ICT state of the CT-CA/TiO 2 complex. Quantum dynamics calculations also show the presence of a slow component (30%) in the electron injection dynamics although most of the electron injection (70%) takes place in less than 20 fs. The slow component of electron injection, from the local ICT state, is attributed to the energetic position of the excited state, which is close to, or slightly below, the conduction band edge. In addition, the transient bleach of CT-CA on the TiO 2 surface is shifted to longer wavelengths when compared to its absorption spectrum and the transient bleach is further shifted to longer wavelengths with charge recombination. These features are attributed to transient Stark shifts that arise from the local electric fields generated at the dye/TiO 2 interface due to charge-transfer interactions.

Tuning the parameters to enhance the efficiencies of a novel set of metal free sensitizers for dye sensitized solar cells (DSSC) is carried out by varying the πconjugated spacers that link the donor benzocarbazole to the acceptor cyanoacrylic acid. The molecules are synthesized by different combinations of spacers, namely fluorene-thiophene (BFT), fluorene-furan (BFF), fluorene-phenyl (BFB), and thiophene-phenyl (BTB). The molar extinction coefficients of all the dyes are high which is attributed to benzocarbazole, but it is higher in the dyes in which fluorene is one of the spacers. But interestingly, in the photovoltaic device when the nonfluorene dye BTB is the sensitizer, red-shifted and broader incident photon-to-current efficiency (IPCE) curves are obtained leading to larger short circuit current density, J sc , almost double when compared to BFB-based cell. The efficiency of the device with this dye as the sensitizer is also the highest in this series. The reasons behind these observations are investigated using computational techniques.

We have designed and synthesized two new diketopyrrolopyrrole (DPP) based organic sensitizers (DPPCA and DPPCN) with the dithiafulvalene (DTF) unit as donor and cyanoacrylic acid/malononitrile as acceptor moieties. These dyes showed excellent efficiency of photocatalytic hydrogen production over a Pt-TiO2 composite via solar-induced water splitting. The sensitizers showed broad absorptions over the wide visible regime (500-800 nm). In DPPCN, the malononitrile moiety led to strong intra-molecular charge transfer, as evidenced by red shifted (∼24 nm) absorption maxima with highly enhanced molar absorptivity (108 190 M(-1) cm(-1)). The electrochemical characterization of as-prepared sensitizers confirmed the feasible electron injection from the dye to the TiO2 conduction band (CB) which has been further validated by theoretical studies. In this study, the rate of the photocatalytic activity was found to be dependent on the acceptor part of the dye molecule as DPPCN sensitized Pt-TiO2 (DNPT) exhibited remarkable (1208 μmol) hydrogen evolution yield in comparison to DPPCA sensitized Pt-TiO2 (DAPT) (840 μmol). The rigid DPP core made the sensitizers significantly photo-stable as affirmed by their high hydrogen production efficiency over 80 h of prolonged irradiation. As predicted from density functional theory (DFT) calculations, ground state geometry of the dyes was almost planar, facilitating continuous conjugation throughout the molecule. Time-dependent DFT (TD-DFT) calculations were also carried out to make clear the understanding of charge transfer transition of the dye molecules.