Solvent effects on ground and excited electronic state structures of p-nitroaniline

Abstract

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the Cphenyl–Nnitro, N–O, and phenyl C2–C3 bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients.

Charge Instability in Quadrupolar Chromophores: Symmetry Breaking and Solvatochromism

“…20,23,54,58,59 For elongated molecules, such as typical quadrupolar (D-π-A-π-D or A-π-D-π-A) chromophores, the shape of the cavity is hardly approximated by a sphere, so that precise estimates are difficult. In any case, our model does not rely on any specific microscopic model for ε or , but extracts its value from experimental data.…”

Electronic and vibronic properties of a discotic liquid-crystal and its charge transfer complex

“…From other studies it is apparent that considerable bond length and bond angle changes in the C-NO 2 group are involved in charge-transfer excitation of small aromatic nitro compounds. 55,59 It is therefore reasonable to expect that the localized vibrations of the nitro groups play a significant role in relaxation processes within the excited CT-band. Furthermore, it appears that the relaxation processes in the CT-band are much slower than in the higher energy band.…”

“…46 The activity of the symmetric C-NO 2 vibrations and the radial breathing mode of the HAT6 core suggest that the lowest electronic transition in the excited CT complex is due to a charge transfer from the HAT6 core to TNF, with a strong involvement of the nitro groups. Strong resonant activity of symmetric C-NO 2 stretching modes is also observed for small aromatic nitro compounds with intramolecular ground state CT. 55 In these molecules the π -π * electronic transition of the lowest excited state gives rise to significant bond length and bond angle changes in the C-NO 2 group, reflecting that the excited state wavefunction contains a large contribution from the basis functions of the nitro group. [55][56][57][58][59] For HAT6 it is well established that the HOMO is located on the aromatic core.…”

“…Strong resonant activity of symmetric C-NO 2 stretching modes is also observed for small aromatic nitro compounds with intramolecular ground state CT. 55 In these molecules the π -π * electronic transition of the lowest excited state gives rise to significant bond length and bond angle changes in the C-NO 2 group, reflecting that the excited state wavefunction contains a large contribution from the basis functions of the nitro group. [55][56][57][58][59] For HAT6 it is well established that the HOMO is located on the aromatic core. 60,61 Based on these considerations, we propose that the lowest excited state in HAT6-TNF forming the lower energy region of the excited CT-band is predominantly a π -π * type transition involving the highest occupied molecular orbitals (HOMO, HOMO-1, etc.)…”

Charge Transfer Transition: Investigation of the Effect of Solvents on Electronic Spectra of N-Alkyl/ N-Alkylphenyl Derivatives of p-Nitroaniline

“…It has been a subject of considerable experimental studies because of its non-linear optical properties [7][8][9][10][11][12][13][14] . Also it is a convenient model for theoretical studies of solvent-solute interactions and the solvation of electronic states in different environments 15 .…”