Franck–Condon analysis of the 1 1A g→1 1B u absorption in linear polyenes with two through six double bonds

Abstract

The vibronic intensity distribution at room temperature and 77 °K of the 1 1Ag→1 1Bu transition for the series of linear polyenes with two through six conjugated double bonds has been measured and analyzed. These inhomogeneously broadened solution spectra can be described reasonably using only two harmonic normal modes of vibration: single and double bond stretches at ∼1200 and ∼1600 cm−1, respectively. Excited state normal coordinate displacements were determined by a least-squares fitting procedure. For both the double and single bond modes the estimated change in bond length upon excitation decreases as the length of the polyene chain increases. The bandwidth of these spectra decreases with increasing polyene length. This is well described by assuming that in solution a range of conformations differing by very small torsions about the terminal single bonds has been frozen in. The Franck–Condon analysis indicates that under these conditions all of the polyenes are approximately planar in the 1Bu state, ruling out the argument that the lack of fluorescence in butadiene and hexatriene is due to the polyene framework being significantly twisted in the lowest energy 1Bu state.

Semiclassical time‐dependent theory of two‐photon spectroscopy. The effect of dephasing in the virtual level on the two‐photon excitation spectrum of isotachysterol

“…4)]. The one-photon absorption spectrum of the "'B,*"' + So transition can be modeled to excellent precision us- ing only two Franck-Condon active modes [1670 cm-' (A = 1.33) and 1250 cm-' (A = 1.05)l. Our results agree well with previous studies of all-trans hexatriene [16,17], although the system origin of isotachysterol is red shifted relative to alltrans hexatriene due to substituent effects. As can be seen by reference to Figure 5, the assumption of T, and T; dephasing processes and a virtual-state population time of 4-6 fs significantly improve the fit of the simulation to the observed spectrum [e.g., compare Fig.…”

Excitation energies of π-conjugated oligomers within time-dependent current-density-functional theory

“…1 we plot the ALDA and VK results for the ͑dipole allowed͒ 1 B u excitation energies and corresponding oscillator strengths against the number of oligomer (C 2 H 2 ) units. We compare our data with experimental results 40 and CCSD-EOM results. 41 The 1 1 B u excitation energies obtained within the ALDA are close to the CCSD results for the small chain lengths ͑3-4 U͒, but for the longer chains the excitation energies are underestimated.…”

Raman investigation of pigmentary molecules in the molluscan biogenic matrix

“…R (the ECC) is pivotal in expressing the state of conjugation. 27,42 Thus, a much better description of the conjugation state can be obtained by comparing the force constant F R and the wavenumber of the a g modes (e.g. 1 or 2 ).…”