Carbazole-based π-conjugated 2,2′-Bipyridines, a new class of organic chromophores: Photophysical, ultrafast nonlinear optical and computational studies

Abstract

The developmen of donor-acceptor (D-A) architecture based organic chromophores with large two-photon absorption (2 PA) cross-sections are essential for myriad of applications, ranging from nonlinear microscopy to biomedical imaging. Here, we present the results from a comprehensive study of 2 PA cross-sections of a new series of carbazole mono substituted-π-conjugated-2,2′-bipyridine derivatives with D-π-A architecture, possessing carbazole as the donor moiety and bipyridine core as an acceptor. Further, we have extended the π-conjugation by introducing the phenyl and butoxy substituted phenyl linkers. The fluorescence properties of these D-A chromophores are highly sensitive to solvent polarity and alteration of electron donor functionalities. The relevant computation studies support our experimental results, e.g., a shift in emission maxima and band gaps of the molecules. The third-order nonlinear optical (NLO) properties of the title donor-acceptor (D-A) chromophores demonstrated that 2 PA cross-section values are in the range of 6–39 GM, obtained using ~50 fs (fs) laser pulses at a wavelength of 800 nm. Further, the nonlinear refractive indices (n2) of these chromophores were found to be ~10−14 cm2/W, rendering them potential optical switching candidates since the coefficients were obtained with kHz, fs pulses extracting pure electronic nonlinearities. Based on our experimental findings and theoretical calculations presented in this study, we believe that carbazole-based π-conjugated 2,2′-bipyridines, demonstrated in this work, would be potential ligands to obtain a series of transition metal coordination complexes of interesting physical properties (for example, superior NLO behaviour).