Tautomerization in the ground and first excited singlet states of phenyl-lapimidazole

Ground and first excited singlet state tautomerization of phenyl-lapimidazole (PLI) was studied by absorption, fluorescence, time-resolved fluorescence, NMR spectroscopy and molecular orbital calculations. In the non-protic solvents cyclohexane and DMSO, PLI is observed by NMR, static and time-resolved fluorescence spectroscopy to exist in two different tautomeric forms, while in protic solvents a rapid equilibrium takes place above 50°C and only one spectroscopic species is observed. The overlapped fluorescence excitation and emission spectra were each decomposed into pure components by the principal component analysis-self modeling chemometric method. Ab initio (HF, CIS and CAS) and hybrid DFT (B3LYP) quantum chemical methods have been used to calculate the ground and first excited singlet state geometries, the vertical transition energies and the NMR chemical shifts were used to assign and rationalize the spectral data.