From stimuli-responsive emission to a rare aggregation induced emission effect in platinum(II) terpyridyl complexes

Controlling the spectral properties by changing external factors is a key ingredient in the search for new sensing materials. This work is focused on the photophysical response of [PtCl(tBuTPAterpy)] (where tBuTPAterpy = 4'-(4-(di(4-tert-butylphenyl)amino)phenyl)-2,2',6',2''-terpyridine) (C). The examined compound was synthesized and characterized by acquiring UV-vis and photoluminescence (PL) spectra in solutions and in the solid state at room and low temperatures. Its photophysical properties were discussed in relation to those of [PtCl(4'-(CH₃)₂NPh-terpy)]CF₃SO₃ (A) and [PtCl(4'-Ph₂NPh-terpy)]CF₃SO₃ (B), which have a similar donor-acceptor structure, in order to emphasise the beneficial features of the studied compound. At room temperature, its photoluminescence is 'ON' in a viscous medium (triacetine), whereas in non-viscous media, no emission signal was observed. The rotation of the substituent may be the factor responsible for the lack of emission in dilute solutions. To support this hypothesis, a series of experiments was performed, which showed that the photoluminescence is switched on under conditions of low temperature, acid-base equilibrium, viscosity and aggregation. Compound C exhibits reversible mechanochromism with a colour change from red to brown and an emission shift from 708 nm to 783 nm. PXRD and SEM techniques were employed to support the observation of mechanochromic changes. The attractive solid-state photophysical properties, including reversible mechanochromism and tunable emission, indicate that compound C is a promising candidate for exploration for OLED applications.