Aggregation-Induced Emission in Phenothiazine–TPE and −TPAN Polymers

Abstract

Two phenothiazine-based polymers—tetraphenylethylene- (PTzTPE) and triphenylacrylonitrile-substituted (PTzTPAN) polyphenothiazines—were studied in organic solvents, solid state, and THF:water and dioxane:water mixtures to investigate the occurrence of aggregation-induced emission (AIE). It is shown that AIE is present for the PTzTPE and PTzTPAN polymers in THF:water mixtures, although to a lesser extent in the latter case. The emission of PTzTPE was found to display the simultaneous emission of locally excited (LE) and charge transfer (CT) states with emission maxima at ∼480 and ∼640 nm, respectively. Dynamic light scattering measurements in THF:water mixtures point out to the formation of small nanoaggregates in which the polymers likely adopt a collapsed structure. The overall effect of the restriction of molecular movements of TPE or TPAN units and the poly(phenothiazine) backbone (thus leading to the enhancing of the excited state radiative channel over the internal conversion deactivation channel through the reduction of the “loose bolt” or free rotor effect), together with the decrease of the CT contribution with the increase of the water fraction was associated with the AIE effect observed in THF:water mixtures. For PTzTPE in dioxane:water mixtures an opposite effect, i.e., an aggregation caused quenching (ACQ), is observed with the increment of water fraction, f w, in the mixture (ϕ F decreasing from 0.14 in pure dioxane to 0.042 for f w = 90%). This selective AIE behavior in specific solvents was attributed to the conversion of emissive LE states into dark CT states. DFT calculations of the phenothiazine-TPE and -TPA trimer units confirm the bent butterfly shape generally adopted by the phenothiazine moiety and the excited state charge transfer from the phenothiazine donor to the side-chain acceptor units.