One- and two-photon fluorescence excitation spectra of the 2 1A g states of linear tetraenes in free jet expansions

Abstract

One-and two-photon fluorescence excitation spectra of the S 1 ←S 0 transitions of the all-trans isomers of 1,3,5,7-octatetraene, 1,3,5,7-nonatetraene, and 2,4,6,8-decatetraene have been obtained in free jet expansions. Comparison of the one-and two-photon spectra allows the unambiguous identification of electronic and vibronic origins and, for octatetraene and decatetraene, provides clear evidence for molecular inversion symmetry. One-photon spectra show a g progressions built on Herzberg-Teller, b u promoting modes, while two-photon spectra are built on progressions of a g modes starting from the 2 1 A g ←1 1 A g electronic origins. In nonatetraene, the absence of inversion symmetry results in an allowed electronic origin in both the one-and two-photon spectra. Nevertheless, bands built on vibronic origins dominate the one-photon spectrum. The S 1 ←S 0 spectra of nonatetraene and decatetraene exhibit characteristic splittings of vibronic bands that can be quantitatively explained by the tunneling of the methyl groups through low energy, torsional barriers in the S 1 states. Couplings between methyl torsions and low frequency skeletal modes further complicate the optical spectra. Fluorescence lifetimes indicate abrupt onsets of nonradiative decay processes ͑tentatively attributed to trans→cis isomerization͒ at ϳ2100 cm Ϫ1 excess energy. Systematic differences in the energy dependencies of S 1 nonradiative decays in the three polyenes can be explained by the higher densities of vibronic states in the methyl-substituted compounds.

Spectroscopic and Photochemical Properties of Open-Chain Carotenoids

“…59 For symmetric polyenes under conditions (e.g., in supersonic jets or in mixed-crystal environments) that retain their centers of inversion, S 1 T S 0 (0-0)'s are missing with the electronic transitions being built on b u false origins. 60 However, the most prominent b u promoting modes are in-plane bending modes with very low frequency, <100 cm -1 in long polyenes. The distinction between (0-0) bands and b u false origins thus will not be detectable in the relatively low-resolution spectra (vibronic bandwidths >500 cm -1 ) presented here.…”

“…Furthermore, solvent perturbations and asymmetric substitutions result in (0-0) intensities that are comparable to those of false origins. 60,61 The electronic origins of S 1 T S 0 spectra in solution thus should be viewed as a complicated superposition of spectra of distorted and undistorted molecules, all of which contribute to the (0-0) line shapes in typical condensed phase spectra.…”

Optical Spectroscopy of Long Polyenes

“…One-and two-photon absorption, fluorescence, and fluorescence excitation experiments have allowed the mapping of polyene electronic, vibronic, and rovibronic states in considerable detail in these prototypical linearly conjugated systems. 6,7 This has led to a deeper understanding of the electronic properties of one-dimensional π-electron systems on a framework of alternating double and single bonds. 8 One of the most important outcomes of the work on simple polyenes was the discovery of a low-energy, symmetry-forbidden (1 1 A g -f 2 1 A g -) transition that lies below the strongly allowed 1 1 A g -f 1 1 B u + transitions associated with the elevation of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in simple versions of molecular orbital theory.…”

Tetraalkoxylated-PPV derivative: An efficient and pure green electroluminescent polymer with good solution processability

“…This absorption originates from the p-p * transition of the conjugated polymer backbone. The PL emission spectrum showed a relatively well-defined vibronic structures originated by symmetric C@C stretching modes in the conjugated polymer backbone [29][30][31][32][33]. The PL maximum emission peak of TALK-PPV was found to be at 554 nm, which corresponds well to its absorption onset.…”