N‐Ethyl Carbazole Derived D‐π‐A‐π‐D Based Fluorophores: Consolidated Spectroscopic, Viscosity and DFT Studies

Abstract

AbstractThis article reports design and synthesis of three new fluorophores (5 a‐c) with the donor‐π‐acceptor‐π‐donor motif, where dicyano vinylene group is the central acceptor, N‐ethyl carbazole group is a fixed donor and varying the N‐substituted secondary donors so as to study their photophysical behavior. Based on the systematic photophysical and theoretical reconnaissances, the fluorophores structure‐property relationships are defined. Their spectral and photophysical behaviors are affected by the solvent polarity, with a significant bathochromic shift in emission and large Stoke shifts being observed in polar solvents. Contrary to dipolar asymmetric analogue this system shows an efficient intramolecular charge transfer as observed from various solvent polarity plots. For 5 a, 5 b, and 5 c, the viscosity induced emission studies show respectively 3.78, 6.53, and 6.82 times enhancement in their fluorescence intensity and hence act as a fluorescent molecular rotor (FMR). The structure and electronic properties of these fluorophores were evaluated using density functional theory computations using B3LYP/6‐311++G(d,p). The frontier molecular orbitals and the optimized geometry reveal that secondary donors in each fluorophore donate the electron and not the N‐ethyl carbazole group which is more twisted. The non‐linear and linear optical properties were determined employing solvatochromic and computational methods in solvents of varying polarities with results falling within the expected limiting values. The linear and non‐linear optical properties of the fluorophores change with the strength of different secondary donors used and solvent polarity. The two photon absorption cross‐section described using two level approximation is highest for 5 b (115‐172 GM).