Rotationally resolved electronic spectroscopy of 3-cyanoindole and the 3-cyanoindole–water complex

Abstract

The rotationally resolved electronic spectra of the origin bands of 3-cyanoindole, 3-cyanoindole(d1), and the 3-cyanoindole-(H2O)1 cluster have been measured and analyzed using evolutionary algorithms. For the monomer, permanent dipole moments of 5.90 D for the ground state, and of 5.35 D for the lowest excited singlet state have been obtained from electronic Stark spectroscopy. The orientation of the transition dipole moment is that of an 1Lb state for the monomer. The water moiety in the water cluster could be determined to be trans-linearly bound to the NH group of 3-cyanoindole, with an NHO hydrogen bond length of 201.9 pm in the electronic ground state. Like the 3-cyanoindole monomer, the 3-cyanoindole-water cluster also shows an 1Lb-like excited singlet state. The excited state lifetime of isolate 3-cyanoindole in the gas phase has been determined to be 9.8 ns, and that of 3-cyanoindole(d1) has been found to be 14.8 ns, while that of the 1 : 1 water cluster is considerably shorter (3.6 ns). The excited state lifetime of 3-cyanoindole(d1) in D2O solution has been found to be smaller than 20 ps.

Structures, dipole moments and excited state lifetime of isolated 4-cyanoindole in its ground and lowest electronically excited singlet states

“…Such an increase of nearly 2 D is not observed for excitation to 1 L b states, in which the permanent dipole moment in general is equal or even smaller than in the electronic ground state. For 3-cyanoindole, for which the lowest state is of 1 L b character, we found a decrease from 5.89 D to 5.39 D upon electronic excitation, 40 while for 5-cyanoindole (lowest excited singlet state is 1 L a ) an increase of 1 D is found. 66 Thus, the change of permanent dipole moment upon excitation of the singlet state can be used as a sensitive indicator of the electronic nature of this excited state.…”

“…For 3-cyanoindole a similar value of 0.43 has been found using the parameters from ref. 40, while for 5-cycanoindole a substantially smaller quantum yield of 0.10 is found with the parameters from ref. 34.…”

“…62 In isolated 3-cyanoindole an increase from 9.8 ns to 14.8 ns is observed. 40 For aniline it has been found that the excited state lifetime of 8.51 ns decreases to 8.48 ns upon N-d 2 substitution, but increases to 9.13 ns upon C-d 5 substitution. 70 In phenol, the effect is more pronounced: OH-deuteration increases the excited state lifetime from 2.4 ns to 13.3 ns.…”

“…This structure was confirmed by the rotational constants from rotationally resolved electronic spectroscopy. 40 The excited state lifetime of 3-cyanoindole has been determined to be 9.8 ns, while that of the 3-cyanoindole(H 2 O) 1 cluster shows a considerably shorter lifetime of 3.6 ns. Interestingly, the fluorescence lifetime of 3-cyanoindole in water solution is the shortest of all cyanoindoles.…”

“…34 In 3-cyanoindole, the order of the 1 L a and 1 L b states is reversed again, the 1 L b state being the lower one as determined by rotationally resolved electronic Stark spectroscopy. 40 The water clusters of 3-cyanoindole(H 2 O) n with n = 1, 2 have been studied in the group of Choi by UV-UV hole burning and IR-dip spectroscopy. 38,39 Based on the comparison of the IR-dip spectra with the results of normal mode analyses and a Franck-Condon analysis of the mass-selected R2P spectra of 3-cyanoindole(H 2 O) n with n = 1, 2 they deduced a linearly bound water moiety with an N-HÁ Á ÁOH 2 binding motif.…”