Exploring the excited state mechanisms of a viscosity-sensitive zeromethine merocyanine dye emitter.

Merocyanine (MC) dyes are among the most versatile and tunable chromophores owing to their structural flexibility. However, their excited state dynamics, which often lead to fluorescence quenching by ultrafast non-radiative decay pathways such as photoisomerization or relaxation through conical intersections, are poorly understood for most MCs. Herein, we elucidate the excited state mechanisms of a zeromethine MC dye (PyBA), which represents a special case of the donor-acceptor architecture featuring two directly connected heterocycles. The analysis was achieved by combining steady-state measurements under varying temperature, viscosity and polarity conditions, time-resolved ultrafast spectroscopy, and quantum-chemical calculations. Our studies show that PyBA upon photoexcitation develops a twisted intramolecular charge transfer (TICT) state in dynamic environments (e.g., CH2Cl2), whereas this process is restricted in rigid environments (PMMA matrix), leading to a significant luminescence turn-on with the quantum yield (Φ PL) increasing from 0.7% to 55%. Moreover, the measurements performed in the PMMA matrix reveal room temperature phosphorescence (RTP) even under ambient conditions, i.e. even without external stimuli, which becomes more intense at cryogenic temperatures under an argon atmosphere, reaching a phosphorescence lifetime of 705 ms. Furthermore, this work introduces a simple and time-efficient strategy for screening small-molecule fluorescence probes.