On the ground and excited state dipole moments of planar vs. twisted nitroaniline analogues

Abstract

. 69,550 (1991). Planar and conformationally twisted p-nitroaniline (PNA) and N,N-dimethyl-p-nitroaniline (DMPNA) systems have been subjected to electric field study (electrochromism) to understand their charge transfer characteristics in the ground and excited states. The observed values of ground and excited state dipole moments suggest that twisting of either the donor (D) with respect to the acceptor (A) or vice versa can have a large impact on the resultant charge distribution, particularly on the excited state charge distribution. These observations remarkably support the TICT hypothesis and minimum overlap rule, as proposed by Grabowski et al. Simple molecular orbital calculations provide adequate explanation for the observed changes in the dipole moment and reasonable agreement has been found in each case studied.

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

“…Photoexcitation of PNA to the S 1 state has been interpreted in terms of CT from the electron-donating NH 2 group to the electron-withdrawing NO 2 group (zwitterionic form 2). [4,[27][28][29][30][31] It would be natural to expect that the T 1 state would be of similar character as well. However, our interpretation of the IR spectra of S 0 and T 1 PNA does not fully confirm this hypothesis.…”

“…As a model system, PNA has long been studied both experimentally [4,[12][13][14][15][16][17][18][19][20][21][22] and theoretically. [23][24][25][26][27] A number of ultrafast spectroscopic studies [28][29][30][31] have been done to understand the dynamics of PNA after photoexcitation. The lowest excited singlet (S 1 ) state of PNA is well-known to have CT character with a large dipole moment [27] of about 15 D, and the S 1 state undergoes very Abstract: Low-lying excited electronic states of an important class of molecules known as push-pull chromophores are central to understanding their potential nonlinear optical properties.…”

“…[23][24][25][26][27] A number of ultrafast spectroscopic studies [28][29][30][31] have been done to understand the dynamics of PNA after photoexcitation. The lowest excited singlet (S 1 ) state of PNA is well-known to have CT character with a large dipole moment [27] of about 15 D, and the S 1 state undergoes very Abstract: Low-lying excited electronic states of an important class of molecules known as push-pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high-sensitivity nanosecond time-resolved dispersive IR spectroscopy and DFT calculations on p-nitroaniline (PNA), a prototypical push-pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure.…”

“…[29,31] In a conventional twostate valence-bond model, the ground state is dominated by neutral form 1, whereas the S 1 state has a considerable contribution of zwitterionic form 2, which explains the large change in dipole moment from about 6 D in the ground state [32] to about 15 D in the S 1 state. [27] In light of the fact that the T 1 state is a primary channel for the photophysical and photochemical processes in the nanosecond to microsecond time region, it is desirable to obtain detailed molecular information on this state as well. Schuddeboom et al [30] used time-resolved microwave conductivity to determine the lifetime and dipole moment of the T 1 state in benzene and dioxane.…”

Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline

“…37,38 This is accompanied by an intramolecular charge transfer resulting in dipole moments of 6 D in the electronic ground state and 14 D in the first excited singlet state. 39,40 From results of semiempirical quantum chemical calculations it has been suggested that IC occurs subsequent to twisting of the nitrogroup. 41 Experimentally, in-plane structural changes have been derived from origin shifts obtained from the resonance Raman spectrum of PNA together with normal coordinate calculations resulting in most pronounced changes within the amino group and between the amino group and the neighboring phenyl-carbon.…”