Substantial Intramolecular Charge Transfer Induces Long Emission Wavelengths and Mega Stokes Shifts in 6-Aminocoumarins

Coumarins are deployed in numerous bioimaging and biosensing applications. Among various coumarin derivatives, 6-aminocoumarins attract increasing attention for their red-shifted emissions, mega Stokes shifts, and significant solvatochromism. These spectral characteristics together with weak emission intensities have historically been ascribed to the formation of the twisted intramolecular charge transfer (TICT) state in 6-aminocoumarins. In this work, we demonstrate that it is actually substantial intramolecular charge transfer (ICT) that is responsible for these fluorescent properties. Based on this new understanding, we reanalyzed the sensing mechanism of a 6-aminocouarmin based fluorescent probe and obtained close agreement with experimental data. Our results lead to a deeper understanding of the photophysics of 6-aminocoumarins and will inspire the rational development of novel fluorescent probes.