Aromaticity-Photovoltaic Property Relationship (APPR) of Triphenylamine Based D-π-A Dyes: Leads from DFT Calculations.

D-π-A based dyes find wide range of applications in molecular electronics and photovoltaics in general and dye sensitized solar cells (DSSC) in particular. We speculated if there exists a relationship between the degree of aromaticity of the π-spacers used in the D-π-A type dyes and their structural, electronic, energetic, photophysical and intramolecular charge transfer properties. Triphenylamine (TPA) and cyanoacrylic acid (CAA) have been chosen as the donor and acceptor, respectively. In order to carry out the investigation systematically the π-spacers have been logically chosen based on their experimental resonance energies which follows the order, furan < pyrrole < thiophene < pyridine < benzene. All the properties have been discussed based on the degree of aromaticity of the π-spacers. Geometric properties such as dihedral angles and bond lengths have been discussed extensively. Energy levels of the frontier molecular orbitals, electrochemical properties, namely: ground and excited state oxidation potentials (GSOP/ESOP), change in Gibbs free energy for electron injection and regeneration (ΔGinj/ΔGreg) have also been evaluated. Photophysical properties like wavelength of maximum absorption (λmax), oscillator strength (f), light harvesting efficiency (LHE) and intramolecular charge transfer properties, viz., charge transfer distance (D_CT), fraction of charge transferred (q_CT) and change in dipole moment (μ_CT) have been assessed. The adsorption characteristics of dye with (TiO2)9 nanocluster have been studied along with their optical properties. Results reveal that the nature of relationship between the afore-mentioned properties and extent of aromaticity of the π-spacers is inherently multifaceted. It thus turns out that, it is highly difficult to quantify the relationship. These properties of D-π1-π2-A molecules can be regarded to be arising from two groups, namely: π-spacers with lower and higher resonance energies. This results in a natural tradeoff in selection of competing properties. The qualitative aromaticity photovoltaic property relationship thus obtained, may serve as a guide to tailor design various properties of D-π-A type dyes for application in the intramolecular charge transfer devices.