Electronic and vibrational spectroscopic studies of jet-cooled 5-cyanoindole and its water clusters, 5CI–(H2O) , (n= 0–2)

Excited state hydrogen or proton transfer pathways in microsolvated n-cyanoindole fluorescent probes

The sensitivity of the fluorescence properties of n-cyanoindole (n-CNI) fluorescent probes to the microenvironment makes them potential reporters of protein conformation and hydration. The fluorescence intensity of 5-CNI, 6-CNI, and 7-CNI is quenched when exposed to water solvent whereas substitution on position 4 of indoles dramatically increases it. A potential mechanism for this sensitivity to water may be similar to that found in indole. The fluorescence of indole is found to be quenched when interacting with water and ammonia solvent molecules via radiationless decay through an S1 (πσ*)/S0 conical intersection caused by excited state proton or hydrogen transfer to the solvent molecules. In this study we examine this fluorescence quenching mechanism along the N-H bond stretch of n-CNI probes using water cluster models and quantum mechanical calculations of the excited states. We find that n-CNI-(H2O)1-2 clusters form cyclic or non-cyclic structures via hydrogen bonds which lead to different photochemical reaction paths that can potentially quench the fluorescence by undergoing internal conversion from S1 to S0. However, the existence of a high energy barrier along the potential energy surface of the S1 state in most cases prevents this from occurring. We show that substitution on position 4 leads to the highest energy barrier that prevents the fluorophore from accessing these non-radiative channels, in agreement with its high intensity. We also find that the energy barrier in the S1 state of non-cyclic 5-CNI-(H2O)1-2 excited complexes decreases as the number of water molecules increases, which suggests great sensitivity of the fluorescence quenching on the aqueous environment.

Excited state dipole moments and lifetimes of 2-cyanoindole from rotationally resolved electronic Stark spectroscopy

The permanent dipole moments of 2-cyanoindole (cyanoindole = CNI) in its ground and lowest excited singlet states have been determined from rotationally resolved electronic Stark spectroscopy under jet-cooled conditions. From the orientation of the transition dipole moment and the geometry changes upon electronic excitation the lowest excited singlet state could be shown to be of L_b-symmetry. The general statement, that the L_a-state has the larger permanent dipole moment of the two lowest excited singlet states, will be challenged in this contribution. On the basis of the different electronic nature of the first excited singlet state the behavior of 2-, 3-, 4- and 5-CNI is discussed. The excited state lifetime of isolated 2-CNI in the gas phase has been determined to be 9.4 ns. This value is compared to the excited state lifetime in ethyl acetate solution of 2.6 ns, which was quantified with a Strickler-Berg analysis. Using water as solvent shortens the 2-CNI lifetime to <40 ps. The reason for this drastic shortening is discussed in detail. Additionally, the rotationally resolved electronic spectrum of 2-CNI(1-d₁) has been measured and analyzed.

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.

Solvent Dependence of Cyanoindole Fluorescence Lifetime

Several cyanotryptophans have been shown to be useful biological fluorophores. However, how their fluorescence lifetimes vary with solvent has not been examined. In this regard, herein we measure the fluorescence decay kinetics as well as the absorption and emission spectra of six cyanoindoles in different solvents. In particular, we find, among other results, that only 4-cyanoindole affords a long fluorescence lifetime and hence high quantum yield in H₂O. Therefore, our measurements provide not only a guide for choosing which cyanotryptophan to use in practice but also data for computational modeling of the substitution effect on the electronic transitions of indole.