Design, synthesis and photophysics of boron(III)-, aluminium(III)- and copper(I)-containing dithienylethenes : from photochromism to supramolecular assembly

Abstract

A library of dithienylethene-containing beta-ketoimines and beta-diketones with long alkoxy chains as well as their boron(III)-containing complexes has been rationally designed and synthesized. The photophysical properties of the compounds have been examined. Upon photoirradiation, reversible photochromism with drastic color changes has been observed for all the beta-ketoimines and their boron(III)-containing derivatives, with the exception of DTE-Th-ac-NPh-ac and its difluoroboron complex. Photocyclization has been found to be suppressed upon introduction of a phenyl substituent on the nitrogen atom of the beta-ketoiminate core, whereas photochromism could be observed by replacement of the phenyl substituent with a bulky mesityl group or by attachment of pentafluorophenyl units to the periphery of the boron(III) center. The steric effects of the mesityl unit and the pentafluorophenyl moeties have been suggested to restrict the molecular rotation that would quench the photochromic behavior. In the case of beta-diketones and their boron(III) complexes, gelation properties have been observed for derivatives containing alkyl chains of 20 carbons with gel-to-sol transition temperature at ca. 40 °C. Similar to their beta-ketoiminate counterparts, photocyclization reactions have been found to occur in all the compounds under photo-excitation and a gradual reduction in luminescence intensity has also been observed. The photo-responsive behavior has also been found to occur not only in solution but also retained in the gel state. Furthermore, reversible photo-induced color changes upon photoirradiation of the organogels of DTE-Th-acac-C6H2(OC20H41)3 has been demonstrated.

Apart from the boron(III) center, the study has been extended to the heavier Group 13 congener – the aluminium(III) center. A novel class of dithienylethene-containing 8-hydroxyquinolines and their aluminium(III) complexes has been prepared and characterized. Coordination of the aluminium(III) centers to the hydroxyquinolinate ligands has been shown to exert influence on their photophysical properties. Upon photo-excitation, the compounds have been found to exhibit reversible photochromism with distinctive color changes which showed dependence on the nature of the heterocyclic bridges of the dithienylethene moieties as well as the introduction of the aluminium(III) center. The emission intensity has found to be progressively decreased upon photocyclization and the thermal stability together with the fatigue resistance of the closed forms of selected compounds have been examined. Moreover, the multi-addressable photochromic reactivity of 5-(DMT-BenzoSulfone-MeTh)-Q (Q = 8-hydroxyquinoline) upon addition of hydroxide ion has also been explored.

In addition, a series of dithienylethene-containing copper(I) diimine complexes has been synthesized and characterized. Their photophysical and electrochemical properties have been shown to be affected by the phosphine and diimine ligands. On the other hand, the photochromism of the complexes has been found to show dependence on the molecular structures of the dithienylethene-containing 1,10-phenanthroline ligands as well as the nature of the ancillary ligands. The possible mechanism for the quenching pathway has also been discussed. The thermal-induced ring-opening reaction of one of the complexes has been studied at various temperatures and the corresponding photo-reversibility has also been determined.