Structural modulation of π-conjugated linkers in D-π-A dyes based on Triphenylamine dicyanovinylene framework to explore the NLO properties

A donor-π-acceptor type series of Triphenylamine-dicyanovinylene based chromophores (DPMN1-DPMN11) was designed theoretically by the structural tailoring of π-linkers to exploit changes in the optical properties and their NLO behaviour. Density functional theory (DFT) computations were employed to understand the electronic structures, absorption spectra, charge transfer phenomena and the influence of these structural modifications on NLO properties. Interestingly, all investigated chromophores exhibited lower band gap (2.22-2.60 eV) with broad absorption spectra in the visible region, reflecting the remarkable NLO response. Furthermore, natural Bond Orbital (NBO) findings revealed a strong push-pull mechanism in DPMN1-DPMN11 as donor and π-conjugates exhibited positive, while all acceptors showed negative values. Examination of electronic transitions from donor to acceptor moieties via π-conjugated linkers revealed greater linear (<α>=526.536-641.756 a.u) and non-linear (β_tot =51313.8-314412.661 a.u) response. It was noted that the chromophores containing imidazole in 2^nd p-linker expressed greater hyperpolarizability when compared with the ones containing pyrrole. This study reveals that by controlling the type of π-spacers, interesting metal-free NLO materials can be designed, which can be valuable for the hi-tech NLO applications.