Resonance Raman and absorption spectroscopy of the lowest triplet state of 1,3,5-hexatriene and deuteriated derivatives at 183 K: molecular structure in the T1 state

Abstract

Resonance Raman and absorption spectroscopy of the lowest triplet state of 1,3,5-hexatriene and deuteriated derivatives at 183 K : molecular structure in the T1 state Langkilde, F.W.; Wilbrandt, R.; M?ller, S.; Brouwer, A.M.; Negri, F.; Orlandi, G. Published in:Journal of Physical Chemistry DOI:10.1021/j100171a028Link to publication Citation for published version (APA):Langkilde, F. W., Wilbrandt, R., M?ller, S., Brouwer, A. M., Negri, F., & Orlandi, G. (1991). Resonance Raman and absorption spectroscopy of the lowest triplet state of 1,3,5-hexatriene and deuteriated derivatives at 183 K : molecular structure in the T1 state. Journal of Physical Chemistry, 95, 6884-6894. DOI: 10.1021/j100171a028 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Time-resolved resonance Raman (RR) spectra of the lowest excited triplet state TI of (E)-1,3,5-hexatriene, (E)-3-deuterio-l,3,5-hexatriene, and (E)-l,l-dideuterio-l,3,5-hexatriene are obtained at 183 K. The TI potential energy surface (PES), determining energy minima, equilibrium geometries, frequencies, and normal modes of vibration, is calculated, and RR intensities are obtained on the basis of the Franck-Condon mechanism. By comparison with the calculated spectra, the observed RR spectra are assigned to an equilibrium mixture of planar E and Z species in the TI state. The relative intensity of the E and Z bands at temperatures between 293 and 183 K gives an upper limit of 0.22 kcal/mol for the energy difference between the planar E and 2 forms. Time-resolved triplet-triplet absorption measurements as a function of temperature between 296 and 183 K yield a frequency factor A of 1.6 X lo7 s-I and an apparent activation energy E, of 0.7 kcal/mol for the decay of the TI state. Consequently, an upper limit of 0.7 kcal/mol above the planar E and 2 geometries is established for the possible local energy minimum at the twisted geometry. On the basis of these data, a PES is proposed for the TI state of 1,3,5-hexatriene with respect to torsion around the central CC bond and the C2C3 and C4Cs bonds.

Potential energy surfaces of short polyenes in the state T1: analysis of time resolved resonance Raman spectra

“…The transient RR spectra of a few deuterated Fu% envauves of hexatriene have been studied and their analysis confirms the results obtained from the undeuterated hexatriene [ 1,2].…”

“…This, together with the observed spectrum, is compatible with a transtwisted form energy difference less than 2.7 kcal/mol. This conclusion is confirmed by the observation of Tl lifetime and RR spectra at different temperatures [2]. Upon lowering the temperature to 183 K the RR spectrum remains unchanged except for the sharpening of vibrational bands.…”

“…At the twisted geometry the energy gap between So and Tt is small, roughly a few tenths of eV, and thus the radiationless decay from Tt is expected to occur at this geometry and to be fast in view of the energy gap law obeyed by radiationless transitions. This is confirmed by the measured lifetime of Tt, 200 ns [2], which is much shorter than Tt lifetimes of conjugated hydrocarbons without a torsional degree of fmedom.…”

“…[l3] The unscaled and scaled vibrational frequencies ( the scaling factors amthesameusedinthelitemture [14]forgroundstatefmquencies)areteportedinTable1,in which for comparison, am listed also the QCFFPI [l] and the experimental frequencies [2]. From the data presented in Table 1 it appeats that scaled ab initio frequencies am in agreemnt with the observed frequencies and the analysis of transient RR spectra based on the QCFF/PI hamiltonian is confumed.…”

Triplet State Z/E-Photoisomerizations of Polyenes: A Comparison of ab Initio and Density Functional Methods

“…With regard to 1,3,5-hexatriene ( HT ) it was found that twisting around the central CC bond is responsible for the decay to S 0 from the T 1 PES. − The vertical excitation energies for S 0 → T 1 are 59.4 and 60.8 kcal/mol for E - HT and Z - HT , and the 0−0 energy gaps, derived from the first feature of the S 0 → T 1 absorption spectra, are 46.9 and 47.7 kcal/mol, respectively . However, the uncertainty with regard to the 0−0 gaps is at least 0.5 kcal/mol.…”