Tailoring of spectral response and intramolecular charge transfer in β-enaminones through band gap tuning: synthesis, spectroscopy and quantum chemical studies

Abstract

In this paper, we investigate the synthetic tailoring of the spectral response and intramolecular charge transfer (ICT) of b-enaminones through bandgap modulation. Two donor/acceptor substituted benaminones, namely, 3-(4-methoxyphenylamino)-2-cyclohexen-1-one (OACO) and 3-(4nitrophenylamino)-2-cyclohexen-1-one (NACO) have been synthesized along with their unsubstituted counterpart, 3-(phenylamino)-2-cyclohexen-1-one (PACO). Steady state as well as time resolved spectroscopic techniques with picosecond resolution are used to record their spectral responses. Substitution of the donor group (-OCH 3 ) mildly enhances the charge transfer from the phenyl ring to the enaminone moiety, while substitution of the acceptor group (-NO 2 ) jeopardizes the charge transfer through mutual electron withdrawing effects of PNA and enaminone moieties. Combined experimental and quantum chemical investigations reveal that the ground state photophysics of OACO and NACO in water are controlled by both microscopic and macroscopic solvation with dominant contribution from the former. Time dependent density functional theory (TDDFT) calculations predict that the HOMO to[LUMO+1] transition gives rise to the absorption spectra of OACO in water, while the absorption by the enaminone moiety of NACO arises as a result of a HOMO to LUMO transition. A crossing between the first (S 1 ) and the second excited (S 2 ) states takes place in the microclusters of PACO, OACO and NACO with water. The intersystem crossing (ISC) has been found to be the major reason for low quantum yields in these molecules. The band gap modulation through waxing and waning of the conjugation strength is expected to throw light on many ICT-driven processes and provides means of tuning the properties depending on it.

Tuning of Second-Order Nonlinear Optical Response Properties of Aryl-Substituted Boron-Dipyrromethene Dyes: Unidirectional Charge Transfer Coupled with Structural Tailoring

“…The geometry optimizations are followed by calculations of components of polarizability (α) and the first hyperpolarizability (β) at the respective optimized structures at the same level of theory. The effect of electron-correlation, which is reported to be an important parameter in polarizability calculations, has been taken care of using second-order Møller–Plesset perturbation theory (MP2) calculations. , In every case, the lack of imaginary frequencies in the optimized structures confirmed the optimization of the geometries of the molecules by reaching at least one local minimum. Time-dependent density functional theory (TDDFT) calculations were performed using CAM-B3LYP functional and cc-pVTZ basis set to get the first 10 vertical excited states of the molecules.…”

“…Lager et al 21 have reported the synthetic methodologies of several arylsubstituted BODIPY dyes. Incorporation of donor/ acceptor group in the phenyl ring in conjugation with the BODIPY can be used to affect the ICT process, ultimately altering the NLO response [22][23][24] . The donor/ acceptor substitution also helps to tune the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) 22,[25][26] .…”

“…Incorporation of donor/ acceptor group in the phenyl ring in conjugation with the BODIPY can be used to affect the ICT process, ultimately altering the NLO response [22][23][24] . The donor/ acceptor substitution also helps to tune the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) 22,[25][26] . Incorporation of the donor/ acceptor group is sometimes used to achieve non-centrosymmetry in several molecules 27 , a criterion for obtaining non-vanishing first hyperpolarizability.…”

“…Very recently, some experimental studies on the NLO response of fluoroalkylated and dimethylaminostyryl substituted BODIPY dyes have been reported literature. , Lager et al have reported the synthetic methodologies of several aryl-substituted BODIPY dyes. Incorporation of donor/acceptor group in the phenyl ring in conjugation with the BODIPY can be used to affect the ICT process, ultimately altering the NLO response. − The donor/acceptor substitution also helps to tune the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). ,, Incorporation of the donor/acceptor group is sometimes used to achieve noncentrosymmetry in several molecules, a criterion for obtaining nonvanishing first hyperpolarizability. As the NLO response properties of a material is ultimately shaped by the properties of the individual chromophores, tuning of the NLO response at the molecular level using the aforesaid strategies could be a key to develop novel NLO materials.…”

“…28−30 The donor/acceptor substitution also helps to tune the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). 28,31,32 Incorporation of the donor/acceptor group is sometimes used to achieve noncentrosymmetry in several molecules, 33 a criterion for obtaining nonvanishing first hyperpolarizability. As the NLO response properties of a material is ultimately shaped by the properties of the individual chromophores, tuning of the NLO response at the molecular level using the aforesaid strategies could be a key to develop novel NLO materials.…”