Influence of thiophene spacer and auxiliary acceptor on the optical properties of 4-(Diethylamino)-2-hydroxybenzaldehyde based D-π-A-π-D Colorants with N-alkyl donors: Experimental, DFT and Z-scan study

Abstract

We report here design and synthesis of four new D-π-A-π-D type dyes (4 and 5) with triphenylamine/N-ethyl carbazole as fixed rotor/donor and N, N-diethyl amine as a secondary donor having dicyanovinylene as a common acceptor. The Influence of π-linker (thiophene) along with an auxillary cyano group on the nonlinear and linear optical properties is investigated. Molecules with thiophene spacer (4b and 5b) exhibit progressive variation in their absorption and emission profiles reflecting the effect of π-linker, compared to those devoid of thiophene spacer (4a and 5a). The dyes display solvent dependent emission [e.g. 4b (toluene = 689 nm) and 5b (DMSO = 795 nm)] suggesting a more polarized photo-excited state arising as a result of enhanced intramolecular charge transfer in the excited state. The solvatochromic nature is supportive of a general solvent effect, which is confirmed by solvent polarity plots (Weller’s and Rettig’s plot). The trends in the optical properties were also validated by time dependent density functional theory computations which further reveals charge transfer (from secondary donor to acceptor) for the long wavelength absorption. Dye with the highest charge transfer dipole moment relatively has the maximum two-photon absorption cross-section area (4b = 246–399 GM) which was established using theoretical two-level model. Non-linear optical properties were investigated using solvatochromic two-level model and computationally using global and range separated hybrid functionals. The dyes show high first-order hyperpolarizability (β0) in the range of 191–1162 × 10−30 e.s.u. and second-order hyperpolarizability (γ0) in the range of 491–6,704 × 10−36 e.s.u. Z-scan results show 4a (3.79 × 10−13 e.s.u.) possesses a higher value of third order nonlinear susceptibility in DMSO. Thus, the dyes can be considered as promising candidates for NLO application in material science.