Why is 1 photostable? Many other stilbazolium compounds, including protonated 1, undergo isomerization to the cis form upon irradiation. This isomerization is forestalled, however, when a quinoid resonance structure of type 1a is favored. The cisoid conformer of 1a is thermodynamically unstable compared with 1a (transoid).
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George L. Gaines (1987) studied this question.
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