Modulation of the photochromic properties of TPE and other tetrasubstituted olefins by the “amino conjugation effect”: a quantitative study

Tetraphenylethylene (TPE) derivatives have been a key building block in the development of solid-state fluorophores with tunable emission wavelength and large quantum efficiencies. Recent literature has brought an array of evidence that rotation around the central C=C bond constitutes main route of deexcitation in solution, and that its restriction in solid-state leads to aggregation-induced emission (AIE) in this family of derivatives. However, the influence of substitution on the dynamics of TPEs in solution has so far received little attention, probably because of the difficulty in efficiently separating (E) and (Z) isomers. Here we report the photophysical properties in solution of extended stereopure TPE derivatives. The introduction of triphenylamine (TPA) substituent results in differences in the spectral properties between the (E) and (Z) isomers, thereby allowing modulation of the photostationary state (PSS) and thus switching in the equilibrium between these two forms depending on the irradiation wavelength. Importantly, we show that this photochromism is observed with a very marked decrease in the photoisomerization quantum yields (Φiso) of TPE-TPA, compared to non-extended TPE derivatives, which we attribute to the so called “amino conjugation effect” already reported for several stilbene derivatives. As a generalization of this mechanism, we show that TPA substitution provides similar effect on other stilbenoid derivatives. These results provide important new insight into the photophysics of electron donor substituted TPEs, and their possible use as photochromic materials.