Optical Properties and Tautomerism of 2-Carbamido-1,3-Indandione in Ground and Excited States

Abstract

2-Carbamido-1,3-indanedione (CAID) is a fluorescent compound, existing in two tautomeric forms: A and B. Spectroscopic features of CAID suggest that the tautomeric equilibrium is possible both in ground and excited state. To further elucidate this, measurements of UV–Vis absorption and fluorescence spectra were recorded in various solvents. The ground and low-lying excited states of CAID are modeled at SOS-MP2/ADC(2) computational level. The two tautomers A and B coexist in ground state with the amount of A amounting to 90% and a dynamic equilibrium is established. However, in all excited states the tautomer B is more stable and its amount is almost 100%. Along with fluorescence from the first excited state of both tautomers, a rare case of breaking Kasha's rule—fluorescence from the third excited state of the two tautomers—has been observed. A scheme is proposed to explain the photophysical processes and photoreactions of CAID in ethanol solution.