Delineating the effect of substituent and π-bridge flip on the photophysical properties of pyrene derivatives: answers from DFT/TD-DFT calculations

Pyrene-based Schiff base derivatives have attracted enormous potential as fluorescent probe for multifarious research applications due to their fashionable photophysical properties. In the present work, pyrene-based Schiff base derivatives of 4-[(pyren-1-ylmethylene)amino]phenol (PAP1-12) were computationally designed with hydroxyl (-OH) substitution at different positions on phenyl rings and have been studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. All the designed molecules were found to possess a coplanar geometry. PAP1-6 molecules with the phenol moiety substituted on the nitrogen atom of the C=N pi-bridge were found to possess relatively small dipole moment values than PAP7-12 with phenol moiety substituted on the carbon atom of the C=N bridge that is flipped. Frontier molecular orbital (FMO) analysis divulged that HOMO to LUMO transition in PAP1-6 molecules is of pi -> pi* character signifying that these molecules are potential aggregation-induced emission (AIE) molecules while PAP7-12 showed ICT character in correspondence with the order witnessed in dipole moment values. With regard to the HOMO-LUMO energy gap, PAP4 was estimated to show relatively small value than other molecules and as a result it exhibited the largest absorption maxima of 425 nm and large first hyperpolarizability value of 40 x 10(-30) esu. On the other hand, PAP6 molecule possessed relatively larger emission lambda(max) value of around 500 nm. In essence, the present work demonstrates that change in the substituents on the PAP derivatives and the flipping of C=N pi-bridge rendered PAP1-6 suitable for AIE application and PAP7-12 for ICT application. This substituent dependency and effect of pi-bridge flipping could be capitalized upon for future development of similar organic dyes for diverse applications. [GRAPHICS] .

Automated summary

In this study, a series of pyrene-based Schiff base derivatives were computationally designed and their optical properties were investigated. The results showed that different substituents and flipping of carbon-nitrogen bridge had significant effects on the optical properties of these molecules, with some being suitable for aggregation-induced emission applications and others for charge transfer applications. The substituent dependency and the effect of bridge flipping have potential implications for the future development of diverse applications of similar organic dyes.