Zwitterionic Ladder Stilbenes with Phosphonium and Borate Bridges: Intramolecular Cascade Cyclization and Structure–Photophysical Properties Relationship

The synthesis and properties of the phosphonium- and borate-bridged stilbenes are reported. The zwitterionic ladder stilbenes were synthesized by the intramolecular cascade cyclization from the phosphanyl- and boryl-substituted diphenylacetylenes <b>1</b>. The study of the substituent effects of the phosphanyl and boryl groups revealed the significant dependence of the reactivity on both the nucleophilicity of the phosphanyl group and the electrophilicity of the boryl group. Theoretical calculations indicated that this reaction is initiated by the nucleophilic attack of the phosphanyl group, irrespective of the degree of the electrophilicity of the boryl group, in contrast to the analogous intermolecular reaction promoted by the frustrated Lewis pairs of R<sub>3</sub>P and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. Moreover, even in the case of compound <b>1c</b>, which does not undergo cascade cyclization under thermal conditions, photoexcitation promoted the cyclization to produce the corresponding zwitterionic stilbene <b>2c</b>, indicative of the potential use as a photoresponsive material. The photophysical properties of a series of zwitterionic stilbenes, <b>2a</b>–<b>e</b>, also display dependence on the substituents. The fluorescence quantum yields (Φ<sub>F</sub>) of the stilbenes <b>2d</b> and <b>2e</b>, with electron-withdrawing diarylboryl groups, were an order of magnitude higher than those of <b>2a</b>–<b>c</b>, with a dimesitylboryl group. Time-resolved fluorescence spectroscopy as well as the measurement of Φ<sub>F</sub> in the polymer matrices revealed that the electron-withdrawing diarylboryl groups significantly retarded the nonradiative decay process from the singlet excited state, resulting in a higher Φ<sub>F</sub>.