The role played by solvent polarity in regulating the competitive mechanism between ESIPT and TICT of coumarin (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one)

Excited-state intramolecular proton transfer (ESIPT) and twist intramolecular charge transfer (TICT) are the two most fundamental dynamic processes, ubiquitous in biological and chemical reactions. The excited-state properties of (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CDPA) in various solvents with different polarities were investigated by using steady-state and femtosecond transient absorption spectroscopy combined with DFT/TDDFT calculations. The results demonstrated that CDPA exhibited low fluorescence in polar acetonitrile (ACN) due to ESIPT but high fluorescence in nonpolar n-Hexane was attributed to intramolecular rotation blocking ESIPT. TDDFT calculations confirmed that the dramatic phenyl group torsional of CDPA in Hexane, whereas a near planar conformation in ACN solvent. The ESIPT barrier decreases regularly with the increase of solvent polarity from n-Hexane, tetrahydrofuran to ACN solvent. These results demonstrated that the ESIPT and TICT processes of CDPA are competitive mechanisms. Our work revealed the effect solvent polarity on the emission behavior and excited-state deactivation mechanism of CDPA, which could help to design and develop new polarity probe in the microenvironments. The competitive mechanism between ESIPT and TICT processes was revealed by varied solvent polarity in the studied CDPA system. [Display omitted] •We studied excited-state dynamics of CDPA in different polar solvents using femtosecond transient absorption and DFT/TDDFT.•Increased solvent polarity contributes to accelerating the occurrence of ESIPT and TICT processes.•Dramatic structural torsion of CDPA in n-Hexane changes the charge distribution and increases the ESIPT barriers.•The competitive mechanism between ESIPT and TICT processes in different polar solvents was revealed in the CDPA system.