Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis.

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH2) and electron acceptor (-NO2) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH2 and -NO2 groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Theoretical Investigation of Steric Effect Influence on Reactivity of Substituted Butadienes with Bromocyclobutenone.

The Diels-Alder reaction (DA) between various mono- and disubstituted 1,3-butadiene (Dn-1 to Dn-10) and 2-bromocyclobutenone (DPh) was carried out in gas phase using density functional theory (DFT) at the M06-2X/6-31+g** level. The reaction was found to proceed through a concerted asynchronous transition state. Further, the asynchronous and early nature of the transition state was clearly pinpointed with the frontier molecular orbital (FMO) and bond order index (BOI) analyses. The intermolecular hydrogen bonding interaction along with steric encumbrance in the transition state were found to be the predominant factors in controlling the reactivity of the dienes. Among the investigated dienes, Dn-6 was found to be the most reactive diene which is attributed to its low activation barrier due to the presence of strong intermolecular H-bonding interactions. These factors were further supported by quantum mechanical calculations using global descriptor indexes, natural bond orbital analysis, and quantum theory of atoms in molecules analysis. These theoretical results were found to be in good agreement with the previous experimental findings.