Structures, dipole moments and excited state lifetime of isolated 4-cyanoindole in its ground and lowest electronically excited singlet states†
The rotationally resolved electronic spectrum of 4-cyanoindole and some N-D and C-D deuterated isotopologues has been measured and analyzed. Dipole moments in the ground and electronically excited state have been determined, using electronic Stark spectroscopy. From the geometry changes upon excitation, orientation of the transition dipole moment, and the values for the permanent dipole moments, the lowest excited singlet state could be shown to be of La symmetry. The excited state lifetime of isolated 4-cyanoindole has been determined to be 11 ns, while for the ringdeuterated isotopologues lifetimes between 5 and 6 ns have been found. The different behavior of 3-, 4-, and 5-cyanoindole is discussed on the basis of the different electronic nature of the electronically excited singlet states.