Dihydroazulene/Vinylheptafulvene Photochromism: Effects of Substituents, Solvent, and Temperature in the Photorearrangement of Dihydroazulenes to Vinylheptafulvenes

Article abstract of DOI:10.1021/j100118a030

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 (Φ_D->V) at room temperature ranges from 0.1 to 0.6.The VHFs (λ_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) * 10¹⁰ 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., λ_f ca. 480 nm for the parent compound).The quantum yield (Φ_f) at -196 deg C ranges from 0.15 to 0.9.A transient (λ_max ca. 450 nm, τ_T D->V is strongly retarded, the triplet is observable on direct excitation with a lifetime of less than 10 μs even at -196 deg C.The increase of Φ_f and the reverse effect for Φ_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.