GORNER, H and FISCHER, C and GIERISCH, S and DAUB, J (1993) DIHYDROAZULENE VINYLHEPTAFULVENE PHOTOCHROMISM - EFFECTS OF SUBSTITUENTS, SOLVENT, AND TEMPERATURE IN THE PHOTOREARRANGEMENT OF DIHYDROAZULENES TO VINYLHEPTAFULVENES. JOURNAL OF PHYSICAL CHEMISTRY, 97 (16). pp. 4110-4117. ISSN 0022-3654,
Full text not available from this repository.Abstract
1,1-Dicyano-2-phenyl-1,8a-dihydroazulene (H-D) and a series of p-phenyl-substituted derivatives (R-D, R; NO2, CN, Br, Cl, CH3, OCH3, and NH2) were investigated by time-resolved and steady-state photochemical methods. The dihydroazulenes (DHAs) undergo an efficient photoreaction to the corresponding vinylheptafulvenes (VHFs), the quantum yield (PSI(D --> V) at room temperature ranges from 0.1 to 0.6. The VHFs (lambda(v) = 440-470 nm) are nonemitting and photochemically nonreactive and undergo a thermal rearrangement. The activation energy for this back-reaction is 18-21 kcal/mol. The A factor is in the (0.01-5) x 10(10) s-1 range; it varies with substituent and increases with increasing solvent polarity. The photoproduct is formed via a singlet pathway, 1DHA* --> VHF; triplet states are not involved in this reaction. Fluorescence from the DHAs was observed, weakly in fluid solution and most efficiently in glasses at low temperatures (e.g., lambda(f) almost-equal-to 480 nm for the parent compound). The quantum yield (PSI(f)) at -196-degrees-C ranges from 0.15 to 0.9. A transient (lambda(max) almost-equal-to 450 nM, tau(T) less-than-or-equal-to 5 mus), detectable at room temperature only on excitation of a sensitizer (e.g., xanthone in acetonitrile), is assigned to the lowest DHA triplet. In viscous media, where PSI(D --> V) is strongly retarded, the triplet is observable on direct excitation with a lifetime of less than 10 mus even at -196-degrees-C. The increase of PSI(f) and the reverse effect for PSI(D --> V) with decreasing temperature indicates competition of these processes due to an activation barrier (<5 kcal/mol) along the 1DHA* --> VHF pathway. Characteristic features of the ground-state and first excited singlet energy surfaces are presented.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | EXCITED-STATES; REVERSIBLE PHOTOCHEMISTRY; CHARGE SEPARATION; FLUORESCENCE; TICT; PHOTOISOMERIZATION; MOLECULES; SYSTEM; RING; |
| Depositing User: | Dr. Gernot Deinzer |
| Last Modified: | 19 Oct 2022 08:42 |
| URI: | https://pred.uni-regensburg.de/id/eprint/54002 |
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