Ultrafast photoinduced processes in indole–water clusters

Abstract

Applying femtosecond laser pulses at the wavelength of 250 nm the photoinduced processes in indole(H₂O)_n and d-indole(D₂O)_n clusters have been studied. An ultrafast (∼50 fs) decay of the initially excited $\text{ππ}{}^{\mathrm{\ast }}$ state is tentatively attributed to the internal conversion to the dark $\text{πσ}{}^{\mathrm{\ast }}$ state. Subsequent processes are characterized for n=1,2 by a single-exponential signal decay on the ps time scale, while for larger clusters (n⩾3) no significant time dependence is observed. The strong differences with respect to the photophysics of indole(NH₃)_n clusters are discussed.

Introduction

The study of photoinduced intracluster reactions of the indole molecule in clusters of polar molecules such as ammonia or water allows to elucidate the elementary photochemical reactions of this biologically relevant system – indole is the chromophore of the amino acid tryptophan – in a complex surrounding more close to nature. Recent theoretical studies of indole as well as of similar aromatic molecules (e.g., phenol, pyrrole, adenine) [1] have indicated the crucial role of a dark Rydberg-like $\text{πσ}{}^{\mathrm{\ast }}$ electronic state which is populated via a closely neighbored $\text{ππ}{}^{\mathrm{\ast }}$ state and whose properties determine the character and dynamics of the subsequent photochemical processes. In this state a large amount of the electronic charge is displaced along the N–H coordinate of the indole molecule towards the H atom. In indole–water clusters the electron charge is completely separated from the indole molecule and solvated by the water molecules [2]. It is suggested that the electron transfer is accompanied by a proton transfer such that a hydrogen transfer from the indole molecule to the surrounding water cluster is the primary photochemical process. However, the reaction H + H₂O → H₃O is endoenergetic and thus, the H-transfer should be less efficient than in indole–ammonia clusters for which the H-transfer reaction is exothermic [3]. Indeed, for indole(NH₃)_n clusters a H-transfer followed by dissociation has been observed in pump–probe ionization experiments with ns laser pulses by detection of the long living neutral dissociation products (NH₃)_n−1NH₄[3], whereas the detection of analogous (H₂O)_n−1H₃O radicals as products of an H-transfer reaction in indole(H₂O)_n clusters has not been reported up to now.

However, the observation of the neutral reaction products is only one criterion for the H-transfer reaction although the most direct one. A deeper insight into the details of the dynamics and energetics of the intracluster processes is gained by time-resolved photoion and photoelectron detection in pump–probe experiments with femtosecond laser pulses. In this way we have found for indole(NH₃)_n a rather complex succession of process steps which has been described as follows [4], [5]:

$$\text{→}\text{263}\text{nm}\text{IndNH}\text{(}\text{NH}{}_3\text{)}{}_{\mathrm{n}}\text{(2)}\text{ππ}{}^{\mathrm{\ast }}\text{→}\text{τ}{}_2\text{IndNH}\text{(}\text{NH}{}_3\text{)}{}_{\mathrm{n}}\text{(3)}\text{πσ}{}^{\mathrm{\ast }}\text{→}\text{τ}{}_3\text{IndN}\text{(}\text{NH}{}_3\text{)}{}_{\mathrm{n-1}}\text{(4)}\text{πσ}{}^{\mathrm{\ast }}\text{NH}{}_4\text{→}\text{τ}{}_4\text{IndN}\text{(}\text{NH}{}_3\text{)}{}_{\mathrm{n-1}}\text{(5)}\text{NH}{}_4\text{⌋}{}_{\mathrm{<mtext>eq</mtext><mtext>.</mtext>}}$$

The reaction is initiated by the absorption of a pump photon of e.g. 4.71 eV which leads to the excitation of the $\text{S}{}_1\text{(}\text{ππ}{}^{\mathrm{\ast }}\text{)}$ state of the clusters. This state (2) decays with the time constant τ₂ to the $\text{πσ}{}^{\mathrm{\ast }}$ state (3) due to internal conversion. For small clusters IndNH(NH₃)_n, n=1–3, we found τ₂=(110±60) fs. The subsequent process with a time constant τ₃ of about 800 fs is related to the H-atom transfer on the $\text{πσ}{}^{\mathrm{\ast }}$ potential surface leading from the vertically excited state (3) to the adiabatic potential minimum (4). After the transfer of the H atom from the NH group of indole to the ammonia cluster a reorientation of the cluster geometry with the time constant τ₄ follows which ends up in the equilibrium structure (5). In particular, for larger clusters with n=4,5 the last process has been evidenced by time-dependent photoelectron spectra and is characterized by τ₄ values of 1..15 ps. For smaller clusters, n<3, τ₄ might possibly be caused by an internal conversion from the $\text{πσ}{}^{\mathrm{\ast }}$ state back to the ground state. The neutral reaction products (NH₃)_n−1NH₄ are formed with time constants around 100 ps which are determined by the IVR process of the vibrational energy in correlation to the reorientation of the cluster structure [6].

The dynamics in the similar system of phenol(NH₃)_n clusters has been studied on the ps time scale (see e.g. [7]). There the H-transfer has been clearly identified with a reaction rate which strongly depends on the excess energy in the excited $\text{S}{}_1\text{(}\text{ππ}{}^{\mathrm{\ast }}\text{)}$ state. In contrast, in analogous experiments with phenol–water clusters no signature for a H-transfer has been observed although theoretical studies [8] suggest the reaction also for this cluster type. Thus, the question arises whether the H-transfer reaction can be detected in indole(H₂O)_n clusters and which are the differences in the observed dynamics with respect to indole(NH₃)_n complexes.

In the present Letter we discuss the results of pump–probe experiments on indole(H₂O)_n clusters carried out at nearly identical experimental conditions as applied for indole(NH₃)_n clusters [4], [5], [6]. With respect to the expected somewhat higher vertical excitation energy of the $\text{πσ}{}^{\mathrm{\ast }}$ state [1] we excite the clusters with femtosecond laser pulses at the wavelength of 250 nm (4.96 eV). The spectroscopic data of the small indole(H₂O)_n clusters are well known: the $\text{S}{}_1\text{(}\text{ππ}{}^{\mathrm{\ast }}\text{)}$ state is located at 4.35 eV for n=1 and at 4.31 eV for n=2 [9], whereas the vertical ionization potential for n=1 amounts to 7.37 eV [10]. The binding energies of the heterodimer (n=1) are 0.210 eV in the electronic ground state and 0.602 eV in the ionic ground state [11]. The results of the femtosecond time-resolved experiments presented here for indole(H₂O)_n complexes are compared with those obtained for indole(NH₃)_n clusters at the identical pump wavelength. Furthermore, we study the corresponding isotope effect by measuring the dynamics of the deuterated complexes d-indole(D₂O)_n.