Vibrational and Electronic Properties of Antiaromatic Systems: A Spectroscopic Study of 1,3,5-Tri-tert-butylpentalene

Abstract

The vibrational and electronic spectra of the stable pentalene derivative 1,3,5-tri-tert-butylpentalene (TTBP) are reported, and their properties are discussed on the basis of QCW-PI MO semiempirical calculations, which allow geometry optimization and normal mode analysis for the ground and lowest excited states. The infrared and Raman spectra have intensities mostly arising from modes of pentalene origin. The observed frequencies are compared with those obtained from vibrational calculations on the parent molecule and on several tert-butyl derivatives. The nn* electronic transitions of TTBP in the visible and UV regions are satisfactorily predicted by our calculations, including the interaction between singly excited configurations. Optimization procedures show that bond altemation, a distinctive feature of antiaromatics, is reduced in the excited states and that ground and excited state potential surfaces have minima displaced one with respect to the other. Low-temperature absorption spectra in the S2 and S3 regions are interpreted in terms of FranckCondon vibronic transitions whose strength depends directly on the structural change upon excitation. The theoretical results on SO -. S2 and SO -S3 band profiles, with consideration of the normal mode rotation in the excited state, that is, the Duschinsky effect, are in fair agreement with experiment.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Pentalene Pentalene ( Figure 16) is formally derived from the regular octagonal π perimeter of cyclooctatetraene by cross-linking opposite atomic centers [157]. Due to the reduction of molecular symmetry from D 8h to D 2h , all the π orbitals of pentalene are non degenerate and the lowest four are occupied by eight π electrons.…”

Ab Initio Calculations of Vibronic Spectra and Dynamics for Small Polyatomic Molecules: Role of Duschinsky Effect

“…In general, the Duschinsky effect scrambles the occupation of the normal modes, leading to unusual intensity distributions. The phenomenon of mode mixing has been observed in the absorption and emission spectra of many organic molecules such as benzene, naphthalene, α- and β-methylnaphthalenes, phenanthrene, pyridine, azulene and azaazulenes, − styrene, benzyl radical, N,N ‘-dicyanoquinodiimines, 1,3,5-tri- tert -butylpentalene, 2-phenylindole, o -difluorobenzene, ethynylbenzene (phenylacetylene), , tetracyanoquinodimethane anion (TCNQ - ) and naphthalene cation, anthracene, blue copper proteins, etc. For symmetry-allowed electronic transitions, the Duschinsky effect can be studied from the different intensity distributions displayed in absorption and emission and from the presence of combination bands revealed by single vibronic level excitation spectroscopy.…”

The transient absorption of 1,3,5-tri-tert-butyl-pentalene

“…4 , observed at 3.48 and 3.94 eV (B356 and 316 nm) in pentalene4 and at 3.67 and 4.42 eV (B338 and 280 nm) in TTBP. 5,12,13 The photophysical behaviour of upper excited states in antiaromatics has so far received scarce attention. To the best of our knowledge only one study on antiaromatic 1,3, 5,7-tetra-tert-butyl-indacene (TTBI) has addressed the issue, using the transient absorption technique.…”