Abstract
Three novel fluorescent NLOphoric push-pull fluorophores (2a-c) constituted by different cyano containing acceptors linked to morpholine-thiourea (donor) and acenaphthene (rotor) hybrid were synthesized and characterized. They exhibit positive absorption and emission solvatochromism. The solvent polarity function based Lippert-Mataga plot provides the validation of charge transfer (CT) characteristics whereas, Rettig plot 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 dyes 2a-c. Viscosity induced emission studies show 4.18, 17.61, and 5.30 fold increase in the emission intensity for dyes 2a, 2b, and 2c respectively which recommends these dyes as fluorescent molecular rotors (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 difference in dipole moment (ca. 4.86–8.23 D) results in a strong non-linear optical (NLO) properties which increases in the order 2a<2b < 2c. Z-scan studies reveal that the dyes possesses reverse-saturable i.e. positive type of absorption non-linearity resulting in a positive values of β (2c > 2b > 2a). The lower optical limiting threshold values for dyes 2a-c, made them suitable for the use in the application of NLO materials.
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