Excited-state double-proton transfer dynamics of deuterated 7-azaindole dimers in a free jet studied by hole-burning spectroscopy

Abstract

Fluorescence excitation and hole-burning spectra have been measured for deuterated 7–azaindole dimers [(7AI)₂] in a free jet in order to investigate their excited-state double-proton transfer (ESDPT) dynamics. Only one transition system is observed in the S₁–S₀ region of the excitation spectrum of (7AI)₂-dd, where two hydrogen atoms of the NH groups are deuterated. Two transition systems are observed in the spectrum of (7AI)₂-hd in which one of the hydrogen atom of the NH groups is deuterated. The two systems have been ascribed to the S₁–S₀ transitions of (7AI)₂-h*d and (7AI)₂-hd*. In these molecules one monomer moiety, 7AI-h or 7AI-d, is excited in the S₁ state. The separation of the two electronic origins has been determined to be 21 cm⁻¹. In contrast to (7AI)₂-hd, two monomer moieties must be simultaneously excited in the S₁(¹B_u) states of (7AI)₂-dd and (7AI)₂-hh. These findings can be consistently explained by considering that (7AI)₂-dd and (7AI)₂-hh in the S₁ state have C_2h symmetry, whereas (7AI)₂-h*d and (7AI)₂-hd* have C_s symmetry. The bandwidth for one quantum of the intermolecular stretching vibration of (7AI)₂–h*d (4.1 cm⁻¹) in the excitation spectrum is greater than 2.4 cm⁻¹ for the stretching vibration of (7AI)₂-hd*, indicating that the rate of the ESDPT reaction depends significantly on the excited site. These results support a concerted mechanism for proton transfer in (7AI)₂-dd and (7AI)₂-hh. We will discuss the reason for the observation of bi-exponential decays detected by photo-excitation of vibronic bands of (7AI)₂ in a molecular beam with a femtoseond laser (A. Douhal, S. K. Kim and A. H. Zewail, Nature, 1995, 378, 260) on the basis of the symmetry of the 7AI dimers and vibrational mode-specific proton transfer.