Chromophore-solvent clusters

In this chapter, we give a brief overview of several novel features of excited-state proton transfer in chromophore-solvent clusters which have been revealed by the interplay of computational chemistry and spectroscopy in supersonic jets. In the future, concerted efforts of theory and spectroscopy will be necessary to investigate the evolution of these phenomena with increasing cluster size towards liquid-phase photochemistry. 

We have discussed recent computational and spectroscopic results on the photoinduced hydrogen transfer and proton transfer chemistry in hydrogen-bonded chromophore-solvent clusters. The interplay of electronic spectroscopy of size-selected clusters and computational studies has led to a remarkably detailed and complete mechanistic picture... 

The 17r<7 states also dominate the photoinduced processes in hydrogen-bonded chromophore-solvent clusters. The photoinduced hydrogen transfer reaction is experimentally and computationally well documented in clusters of phenol and indole with ammonia [14,16,32], There is no clear evidence for the existence of an excited-state proton transfer process in these systems [14], The same conclusion applies to bi functional chromophores solvated in finite clusters, such as 7HQ-ammonia and 7HQ-water clusters [15]. In future work, the photochemistry of larger and biologically relevant chromophores (such as tyrosine, tryptophan, or the DNA bases) should be investigated in a finite solvent environment. 

The interpretation of the experimental data for the kinetics of photoacid-solvent clusters is complicated by the substantial fragmentation of the clusters after the excited-state reaction. The heat of reaction is often sufficient to allow the evaporation of one or several solvent molecules [14,16]. This difficulty does not arise when the H atom transfer or proton transfer occurs intramolecularly along a solvent wire attached to a bifunctional chromophore. 

The above model makes qualitative and semiquantitative predictions about the IVR and VP rates in different clusters. Of particular relevance is the comparison of the IVR and VP rates for the clusters aniline(CH4)1, aniline(N2)i, and aniline(Ar)1 aniline(Ar)1 has only the three vdW modes aniline(CH4)1 has six vdW modes and anilinefN has five vdW modes. As a result, the sums and densities of vdW states used in the calculations of fcIVR and kVP are quite different in these three clusters. Since the chromophore in these clusters remains the same, however, initial excitation to roughly the same energy in each cluster is possible. Furthermore, we expect that the extent of chromophore-solvent interaction will not be vastly different in these cases that is, the A term in eq. (5-2) will be of the same order of magnitude for each cluster. The IVR rates for the N2 cluster are expected to fall below those of the CH4 cluster but above those of the Ar cluster... 

Experiments with clusters in a supersonic jet can advantageously be employed to study the effects of solvation of a chromophore on its emission spectrum. Hence it is desirable to characterize the composition and the structure of the solute-solvent clusters as precisely as possible. The cluster size distributions can be determined by TOF-MS after resonant two-photon ionization, R2PI [84, 92a,c]. This allows for a... 

The two theories therefore predict much different appearance kinetics for individual vibronic states of solute/solvent clusters generated by IVR and the bare chromophore molecule generated by VP. While the interpretation of wavelength and time resolved measurements does not depend on the theoretical model imposed or envisioned, the interpretation of the cw experiments is indeed highly model dependent. Thus, in the absence of temporal resolution, the assumption of only parallel or only serial relaxation processes is important for the data interpretation. The question of serial yLLi parallel processes for vibronic dynamics can in any cases be uniquely answered by time resolved studies. Indeed, we have shown zpreviously for tetrazine(Ar)i,2b and herein for aniline(Ar)i, (N2)i, (CH4)i, that the serial IVR/VP process is the appropriate one. 

The isolated enantiomers S (M ) and R (Mr) of a chiral molecule M exhibit the same spectral features since their physical properties are identical. However, their aggregation with a chiral chromophore of defined configuration (Cr/s) leads to the formation of two diastereomeric complexes with different spectral properties, i.e., and [C /yM ]. The lcR2PI spectroscopy is able to discriminate between Mj and by measuring the spectral shift of the diastereomeric [C /yM ] and [Cj5/5-Mj ] complexes with respect to that of the bare chromophore Cr/s- It is convenient to define the diastereomeric clusters as homochiral when the chromophore and the solvent have the same configuration, and heterochiral in the opposite case. 

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

In this section, we discuss the photoinduced hydrogen transfer from phenol to water and ammonia in phenol-water and phenol-ammonia clusters, respectively, as a representative model of excited-state chromophore-to-solvent hydrogen transfer reactions. 

The 1 ira state is unique among the low-lying singlet states of Ph-W and Ph-A clusters insofar as spontaneous electron ejection from the chromophore to the solvent takes place. We have made this explicit by visualizations of the a orbital for representative cluster geometries. For larger clusters of phenol with water, the excess hydrogen atom is stabilized in the water network in the form of a hydronium cation (HsO+) and a solvated... 

The approaches based on explicit representations of the environment molecules include full quantum mechanical (QM) and hybrid QM/MM methods. In the former, the supramolecular system that is the object of the calculations cannot be very large for instance, it can be composed of the chromophore and a few solvent molecules ( cluster or microsolvation approach). A full QM calculation can be combined with PCM to take into account the bulk of the medium [5,13], which is also a way to test the accuracy of the PCM and of its parameterization, by comparing PCM only and PCM+microsolvation results. The full QM microsolvation approach is recommended when dealing with chromophore-environment interactions that are not easily modelled in the standard ways, such as those involving Rydberg states. An example is the simulation of the absorption spectrum of liquid water, by calculations on water clusters (all QM), clusters + PCM, and a single molecule + PCM only the cluster approach (with or without PCM) yielded results in agreement with experiment [13] (but we note that this example does not conform to the above requirement for a clear distinction between chromophore and environment). 

In the following, we will focus our discussion on reactions occurring in clusters in which one chromophore is surrounded by the solvent molecules the reactions occur when the chromophore is excited in its first excited state or ionized. The interest of using a, chromophore within the cluster is that multiphoton ionization can be used in connection with mass spectrometry. In this case, the ionization is a soft process and the spectroscopy of the cluster can give information on the size of the cluster, which is excited and responsible for the reactive event. This assigment can be difficult in other methods (electron bombardment) due to fragmentation processes associated with ionization. 

Studies of this nature are just at their inception, with solvent variation and different chromophoric species to be explored. This form of ionic fragmentation chemistry is quite interesting and is highly dependent on solvation structure dependence of these fragmentations can also be investigated in small clusters (n < 6). The opportunity for more experiments and theory here is quite clear. These are perhaps the most solvent rich and dependent processes thus far characterized in clusters. 

Solute-solvent complexes of different stoichiometry have been observed between all the D-A compounds under consideration and various solvent molecules. Some of the clusters show structured excitation spectra and a narrow short-wave emission that has been assigned to the primary excited state of the vdW complex. The broad, red-shifted emission of other clusters can be explained in terms of the transformation of the vdW complexes of stoichiometry l n (n > 0) into excimers or the transition into an intramolecular CT state of the D-A chromophore which is induced by its polar partner(s) (for the complexes of stoichiometry 1 ). The main conclusion from the fluorescence behaviour of the jet-cooled vdW clusters is that dual luminescence is obviously connected with the preference of specific solute-solvent geometries. 

The SCRF model reproduces very well the shifts observed in aptotic solvents. In order to simulate an aqueous solution, calculations on supermolecules containing the chromophore and specifically bound water molecules, all embedded in the dielectric continuum, are required even for merely qualitative agreement. Supermolecule calculations with several water molecules treated as a gas-phase cluster often give qualitatively incorrect results (Karelson and Zerner, 1992). 

It is well known that water-mediated interaction stabilizes structure of biomolecules [1, 138, 247-250]. Therefore, several model molecular systems have been chosen to probe the water-mediated interactions in biomolecules and a large amount of experimental and theoretical work has been published over the years on this subject [78, 138, 251-258]. Since phenol is the simplest aromatic alcohol resembling chromophore of an aromatic amino acid, hydration of phenol molecules has been studied to understand H-bonding and solute-solvent interaction in biological systems. Several experimental and theoretical calculations have been made on the phenol-water clusters [259-273]. Recently, we have made a comprehensive analysis on structure, stability, and H-bonding interaction in phenol (P1-4), water (W1-4), and phenol-water (PmW (w = 1-3, n = 1-3, w + n < 4)) clusters using ab initio and DFT methods [245]. In this section, electronic structure calculations combined with AIM analysis on phenol-water clusters are presented. 

In recent years, there have been many attempts to combine the best of both worlds. Continuum solvent models (reaction field and variations thereof) are very popular now in quantum chemistry but they do not solve all problems, since the environment is treated in a static mean-field approximation. The Car-Parrinello method has found its way into chemistry and it is probably the most rigorous of the methods presently feasible. However, its computational cost allows only the study of systems of a few dozen atoms for periods of a few dozen picoseconds. Semiempirical cluster calculations on chromophores in solvent structures obtained from classical Monte Carlo calculations are discussed in the contribution of Coutinho and Canuto in this volume. In the present article, we describe our attempts with so-called hybrid or quantum-mechanical/molecular-mechanical (QM/MM) methods. These concentrate on the part of the system which is of primary interest (the reactants or the electronically excited solute, say) and treat it by semiempirical quantum chemistry. The rest of the system (solvent, surface, outer part of enzyme) is described by a classical force field. With this, we hope to incorporate the essential influence of the in itself uninteresting environment on the dynamics of the primary system. The approach lacks the rigour of the Car-Parrinello scheme but it allows us to surround a primary system of up to a few dozen atoms by an environment of several ten thousand atoms and run the whole system for several hundred thousand time steps which is equivalent to several hundred picoseconds. 

---