Intermolecular Excited‐State Proton Transfer in Clusters of 1‐Naphthol with Water and with Ammonia

Abstract

The excited‐state proton transfer reactions of 1‐naphthol(H2O)n and 1‐naphthol(NH3)n clusters are compared. Both exisiting and new data suggest that the two systems may react in strikingly different fashions, roughly corresponding to non‐adiabatic and adiabatic limiting cases, respectively. In water clusters, the reaction is kinetically limited by solvent motion, and there is a strong dynamic interaction involving the water and two lowest electronic excited states, which thereby invert. In ammonia clusters, the excited states appear to be premixed by the the stronger base, and the reaction may proceed in a rapid manner, resembling intracluster vibrational relaxation, with relatively little solvent reorganization.

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…We thus feel that an electronic state description in terms of pure LE( 1 L b ) and CT( 1 L a ) configurations is less appropriate. In the next section we propose that the strongly mixed 1 L b -1 L a model as suggested by Knochenmuss et al [9,38] is a better description for HPTS and MPTS.…”

“…Originally proposed [9,38] to explain the spectra of 1-naphthol in small ammonia clusters where excited-state proton transfer from 1-naphthol was observed, this model describes the S 1 and S 2 states of 1-naphthol in ammonia as being made of 1 L b and 1 L a states that are strongly mixed with each other. This means that the original orthogonallity of the two L states found in the parent molecule naphthalene is lost by the combined action of the OÀH substituent at the alpha position of the naphthalene ring and by strong-polar interactions with the solvent.…”

“…For 1-naphthol [36,37] the 1 L b ! 1 L a transition is considered to be the ESPT-determining step [9,38,39] In contrast, in a recent combined experimental/theoretical study of ESPT of pyranine [40][41][42] the rate determining step is not the conversion from the optically accessible locally excited (LE) state (or alternatively the 1 L b state), to the second photoacid electronic excited n-p* CT state (or 1 L a state), but the transition of the photoacid CT to photobase CT states.…”

Bimolecular Proton Transfer in Solution

“…A strong solvent dependent coupling between the energetically nearby lying 1 L b and 1 L a -states of HPTS ( Fig. 14.2(d)), in similar fashion as in the description of gas phase 1-naphthol-ammonia clusters [56,58], has been postulated as the underlying reason for these observations. Future developments in comparison of experimental vibrational mode patterns with quantum chemical calculations may reveal the molecular origins of photoacidity.…”

“…Thus, a singlet-singlet level crossing may occur in polar solvents like water when the vertical excitation is to the 1 L b state and the ensuing solvent relaxation process is capable of shifting the excited-state population of the photoacid to the 1 L a state. For 1-naphthol the 1 L b fi 1 L a transition has been considered to be the proton transfer rate determining step [56][57][58]. In contrast, in a recent combined experimental/theoretical study of pyranine (8-hydroxy-1,3,6-trisulfonate-pyrene; HPTS) [59][60][61] it has been concluded that the rate determining step in the excited state proton dissociation reaction is not the conversion from the optically accessible locally excited (LE) state (resembling the properties of the 1 L b -state of naphthalene) to the electronically excited CT state (resembling the 1 L a -state character of naphthalene), but the transition of the photoacid CT to photobase CT states.…”