Evaluation of the excited state dynamics, photophysical properties, and the influence of donor substitution in a donor-π-acceptor system

There have been numerous attempts for the theoretical design of a better donor-pi-acceptor structural framework with improved absorption and emission properties. However, for effective dye designing, it is necessary to understand the electronic and photophysical properties of the dye systems. In this work, we report a detailed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) investigations of the excited state characteristics and the influence of various groups (-HCO, =CH2, (-CH3)(2), (HCO)(2), and (-OCH3)(2)) attached to the donor group (-NH2) in a p-nitroaniline D-pi-A system which are symbolized respectively as p-nitroaniline (A), N,N-dimethylnitroaniline (A2), N,N-dicarbonylnitroaniline (A3), N-methylenenitroaniline (A4), and N,N-dimethoxynitroaniline (A5). The first principles DFT and TD-DFT calculations from the ground state -(S0) to the first five excited states: -(S-0 -> S-1), (S-0 -> S-2), (S-0 -> S-3), (S-0 -> S-4), and (S-0 -> S-5) were utilized to explore the reactivity of D-pi-A system using the conceptual DFT approach, characterization of electron excitation using the hole-electron analysis, visual study of the various real space functions in the hole-electron framework, density of states (DOS), measurement of charge transfer (CT) length of electron excitation (Delta r), measurement of the overlapping degrees of hole and electron of electron excitation (Lambda), interfragment charge transfer (IFCT) during electron excitation, and the secondorder perturbation energy analysis from the natural bond orbitals (NBO) computation. Results of the excitation studies show that all the studied compounds exhibited an n ->pi* localized type for first excitations (S-0 -> S-1) on -NO2 group in A, A2, A4, and A5 and NCl2 in A3. pi ->pi* charge transfer excitations were confirmed for S-0 -> S-2/S-4/S-5 in A and A2, S-0 -> S-3/S-4/S-5 in A3 and A5, and S-0 -> S-4/S-5 in A4. The NBO second-order perturbation energy analysis suggest that the most significant hyperconjugative interactions were pi(C1 - C2)-> LP * (1)C6 (54.43kcal/mol), pi C1 - C2 -> LP(1)C3 (40.82kcal/mol), pi N14 - O16 -> LP(3)O15 (11.67kcal/mol), pi(C1 - C6)->pi* (N12 - O13) (29.52kcal/mol), pi(N12 - O13). LP(3)O14 ( 11.55kcal/mol), pi(C3 - C4). pi * (C5 - C6) (23.40kcal/mol), and pi(C5 - C6)->pi* (N12 - O14) ( 24.88kcal/mol) pi(C3 - C4) -> pi* (C5 - C6)(24.64kcal/mol), which respectively corresponds to the A, A2, A3, A4, and A5 D-pi-A systems under investigation, and these strong interactions stabilize the systems.