Electronic and vibrational spectroscopy of jet-cooled m-cyanophenol and its dimer: laser-induced fluorescence and fluorescence-dip IR spectra in the S0 and S1 states

m-Cyanophenol and its dimer have been studied by laser-induced fluorescence and infra-red fluorescence-dip spectroscopy, in the region of the hydride stretch. DFT calculations have been carried out in conjunction with the experimental work. The bare molecule exists under two isomeric forms, with very similar spectroscopic properties. The two forms are assigned to a cis or a trans orientation of the hydroxy group relative to the cyano substituent. The dimer containing two cis monomers in a head-to-head arrangement shows a large red-shift (1098 cm-1) of its S0-S1 transition relative to the monomer. The ground state vibrational spectrum of the m-cyanophenol dimer is dominated by an intense band assigned to the out-of-phase combination of the hydride stretch modes, followed by lower intensity bands. The nature of these bands has been discussed by comparison with the o-cyanophenol dimer, which exhibits a very similar vibrational spectrum. The vibrational spectrum of the m- and o-cyanophenol dimers in the electronic excited state shows a pronounced decrease of the ν(OH) stretch frequency relative to the ground state. Limited anharmonic coupling between ν(OH) and intermolecular motions has been evidenced. Despite both dimers showing equivalent red-shifts of their electronic transitions, they differ in the shift of the frequency of the ν(OH) stretch relative to the monomer, and its modification upon electronic excitation, which are more pronounced in the case of the o-cyanophenol dimer. This difference in behaviour has been discussed in terms of electron density transfer within the dimer, as obtained by a natural bond analysis (NBO)