Carbazole/fluorene-substituted 5-phenyl-2,2′-bipyridine D–π–A fluorophores: photophysical data, hyperpolarizability and CT-indices

Abstract

Push–pull fluorophores exhibiting pronounced intramolecular charge transfer have a low fluorescence quantum yield. Here we report a new series of D–π–A push–pull fluorophores containing a unified 2,2′-bipyridine domain acceptor with terminal electron-donating carbazole/fluorene groups that exhibit good fluorescence performance. All fluorophores demonstrated high fluorescence quantum yields of up to 0.99 and large Stokes shifts of up to 10 394 cm⁻¹ (157 nm). For the first time, the solvatochromic method was successfully applied to 2,2′-bipyridine compounds of this type to evaluate their static and dynamic hyperpolarizability coefficients. Due to their remarkable fluorescence capability, two-photon emission cross sections were measured for three dyes using the two-photon fluorescence method. Both experimental studies and calculated charge transfer indices (CT-indices) confirmed the greatest charge separation in carbazole-substituted isomers. The annelation of the cyclopentene moiety to the 2,2′-bipyridine core inhibited the formation of aggregates in the solid state and significantly increased the energy gap. Finally, a planarized ICT (PLICT) process was confirmed for fluorene-containing fluorophores based on theoretical calculations in the ground and excited states in various solvents. The synthesized carbazole-containing 2,2′-bipyridines are very promising ligands for constructing metallic NLO-phores, while fluorene-based fluorophores with a predominant PLICT state can be considered as promising environment polarity sensitive fluorescent dyes and biological probes to detect structural changes in proteins/biomolecular interactions and optical switches.