Influence of electron donors in fluorescent NLOphoric D-π-A derivatives with acenaphthene rotor: Photophysical, viscosity, and TD-DFT studies

Abstract

The novel fluorescent NLOphoric push-pull fluorophores (2a-c) constituted by the different N-substituted donors linked via π-bridge as a spacer to 1,1′-dicyanomethylidene group as an acceptor with acenaphthene rotor were synthesized and characterized. These dyes exhibit positive absorption and emission solvatochromism in solvents of different polarities. Solvent polarity plots viz. Lippert-Mataga, McRae, and Weller models provide the validation of charge transfer (CT) characteristics whereas, the Rettig model furnishes an alternative relaxation channel due to twisting around the σ-bonds between donor and acceptor on photoexcitation leading to the twisted intramolecular charge transfer (TICT) state in 2a-c. Viscosity induced emission studies show 4.40, 8.78, and 5.63 fold increase in the emission intensity for dyes 2a, 2b, and 2c respectively which recommends the titled dyes as a fluorescent molecular rotor (FMR). Density Functional Theory (DFT) calculations [(B3LYP/6-311++G(d,p)] give complete information of structural as well as electronic properties of dyes 2a-c. The large difference in dipole moment (ca. 7.92–20.0 D) results in a strong non-linear optical (NLO) properties. The NLO properties were estimated theoretically as well as spectroscopically in solvents of different polarities. The computed values for these dyes show high first-order hyperpolarizability (β) in the range of 210–577 × 10−3° esu and second-order hyperpolarizability (γ) in the range of 506-2,325 × 10-36 esu.