Photophysical Properties of Biphotochromic Dihydroindolizines. Ring-Opening into Extended Bis-Betaines

• Published in: Journal of organic chemistry 1998-02, Vol.63 (4), p.990-1000
• Main Authors: Bleisinger, Heike, Scheidhauer, Patrick, Dürr, Heinz, Wintgens, Véronique, Valat, Pierre, Kossanyi, Jean
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo971182r

New photochromic spirodihydroindolizines (DHI) that are mono- (1a−e) or biphotochromic (3a−e) have been prepared. The monophotochromic DHIs 1d and 1e contain sterically highly demanding substituents. Photophysical studies, both static and time-resolved, were carried out with 1 and 3. Monophotochromic DHIs 1a−e show fluorescence and phosphorescence. Temperature decrease produces a hypsochromic shift in the fluorescence spectra. Time-resolved studies demonstrate that the photochromism is based on a one-photon electrocyclic photoreaction of 1 to the colored betaine. Only one transient of 0.6−6 μs lifetime could be detected for the transformation of 1a−e. For the bichromophoric DHIs 3a−e, two different transients were recorded with lifetimes of 1 and 45−680 μs. It was proved that the ring opening 1 → 4 (as well as 3 → 9) results from an excited singlet species, as demonstrated by the absence of an oxygen effect and by sensitization experiments. Viscosity effects underline the conformational transformations involved in the sequence 1 → T → 4 and 3 → T 1 → T 2 → 9 to occur in distinct steps, thus showing the detailed reaction paths of photochromic molecules 1 and 3.