Heteroleptic ruthenium (II) complexes featuring donor functionalized phenyl‐terpyridine (ph‐tpy) and a monocarboxylic‐(ph‐tpy)/(tpy) are synthesized and characterized. Reactions of ruthenium (II) precursors at 80 °C favored heteroleptic complexes formation over the homoleptic side products. Visible light excitation of these complexes resulted in the metal‐to‐ligand charge transfer (MLCT) transitions. The inter‐planar torsional angle between the atoms of donor functionalized phenyl ring and the central pyridine (py) of the tpy core strongly influences visible light absorption and photovoltaic properties. The lower inter‐ring py‐ph torsion in the acceptor end of the MLCT structures and its increase in the oxidized doublets could prevent the back electron transfer. The ruthenium atom and the acceptor functionalized tpy host the triplet‐MLCT spin density. Ambient temperature excited‐state decay followed the energy gap law and occurred in the order of a few nanoseconds. Herein, we evaluate the photosensitizing ability of these complexes via a combined experimental and computational approach.

We present the syntheses of ruthenium (II) complexes bearing an N-heterocyclic carbene (NHC)-based C^N donor set and an NCS ligand and evaluate their use as photosensitizers in dye-sensitized solar cells (DSSCs). These complexes deploy a monocarboxylic acid-functionalized terpyridine (tpy)/phenyl-tpy to anchor with the TiO 2 of the photoanodes. Results show that the complexes devoid of the phenyl spacer between the acid anchor and the tpy harvest the visible light more effectively. Absorption of the visible light transfers the electron density from the ruthenium, NHC, and the NCS donors to the tpy acceptor ( 1 M Ru L donor L acceptor CT). Stronger M Ru L donor σ-bonds in the complexes with opposite NHC and tpy ligand configuration render facile ruthenium-centered oxidation. In contrast, the metal-centered oxidation of complexes with trans-oriented NHC and NCS ligands is relatively difficult. However, these complexes display higher photon conversion efficiency (PCE) in DSSCs. One of them shows PCE of 3.44%, which is $70% of the standard N3 dye, under similar conditions. A longer electron lifetime and the lowest charge transfer resistance at the TiO 2 /electrolyte interface derived from the electrochemical impedance spectra accounts for the enhanced PCE. Insights into the oxidized dye regeneration in a DSSC setup, obtained from the computed Hirshfeld charges and spin density, depict the essential role of iodide anion in dye regeneration. This report summarizes our investigation of photophysics, electronic structure calculations, and the electrochemical study of all newly prepared complexes and their use as photosensitizers in DSSCs.

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The quality of human life in the modern era seems to be highly reliable on the technology advancements and energy consumption. [1] Globally, around 85% of the energy depends on nonrenewable sources which may extinct within next few years. [2] In this context, solar cells in specifically dye-sensitized solar cells (DSSCs), which come under thin-filmbased third-generation solar cells, are considered to play a vital role of fulfilling the future energy needs by means of its proficient energy conversion potential, in addition to its lightweight, low-cost, reliable efficiency, multicolor options, and feasible for both indoor and outdoor applications. It is one of the most promising energy conversion devices from its technological discovery of early 1990s by Brain O'Regan and Michael Gratzel [3,4] In general, the working of DSSC depends on five major components, namely, 1) substrate, 2) semiconductor nanolayer, 3) the sensitizer, 3) redox electrolyte, and 5) counter electrode. The schematic representation of a typical DSSC with components is shown in Figure 1.Initially, the photosensitizer (dye) of the DSSC usually in its ground state and the illumination of light over the DSSC excite the ground state dye molecules from highest occupied molecular orbital (HOMO) level to the lowest unoccupied molecular orbital (LUMO) level (1), and then the electron from the LUMO level jumps over the conduction band of the semiconductor in 150 ps (2) and reaches the TCO. The holes produced during the excitation of dye molecules are scavenged by the electrons from the redox electrolyte (4) in 0.5 μs. The electrons reached the TCO then move to the cathode through the external circuit and power up the electrical load. Finally, the oxidized electrolyte gets reduced at the cathode (5) in 10 μs. [5,6] Besides these carrier-transport processes, some adverse reactions like nonradiative carrier-recombination paths will occur that desperately reduces the overall device performance. [7] Such unfavorable reactions are follows: nonradiative decay of immediate excitation of dye molecule to its ground state occurs in ns (6), plausible recombination of the electrons from the conduction band of the semiconductor material to the ground state of dye molecule occurs in ms (7), and to the electrolyte occurs in 100 ns to 1 ms (8), respectively. [8] The substrate for the photoanode and cathode is the most important component. Apart from this metal foil or polymeric film, transparent conductive oxide (TCO)-coated transparent glass substrate, such as fluorine-doped tin dioxide (FTO) or