Direct observation of the twisted intramolecular charge-transfer state in polycyclic purinium salts

Based on femtosecond transient absorption spectroscopy (fs-TAS) of 3-methyl-7,8-diphenyl-3H-purino[6,1-a] isoquinolin-6-ium trifluoromethanesulfonate in dimethyl sulfoxide (DMSO), the stimulated emission (SE) signal redshifts from 400 to 500 nm in 4.8 ps. In a glycerol/DMSO mixed solution, when the glycerol content is 25% and 50%, the delay is extended to 6.9 and 68 ps, respectively. The theoretical emission spectrum in the scanning of potential energy curves demonstrates that the rotation of the φ₂ with a lower energy barrier (16.1 kcal/mol) occurs prior to that of φ₁. In addition, the purinium salt molecule has a twisted intramolecular charge transfer (TICT) process occurring from the locally excited (LE) state to the charge transfer (CT) state, and torsion of φ₂ is restrained in high-viscosity environments, which induces the redshift of SE and its increasing lifetime.