Infra-red and Raman spectroscopy of free-base and zinc phthalocyanines isolated in matrices

Abstract

The infrared absorption spectra of matrix-isolated zinc phthalocyanine (ZnPc) and free-base phthalocyanine (H₂Pc) have been recorded in the region from 400 to 4000 cm⁻¹ in solid N₂, Ar, Kr and Xe. Raman spectra have been recorded in doped KBr pellets. The isotopomers HDPc and D₂Pc have been synthesised in an attempt to resolve the conflicting assignments that currently exist in the literature for the N–H bending modes in H₂Pc spectra. A complete correlation between the vibrational modes of the three free-base isotopomers and ZnPc has been achieved. Comparison of the IR and Raman spectroscopic results, obtained with isotopic substitution and with predictions from large basis set ab initio calculations, allows identification of the in-plane (IP) and out-of-plane (OP) N–H bending modes. The largest IP isotope shift is observed in the IR at 1046 cm⁻¹ and at 1026 cm⁻¹ in Raman spectra while the largest effect in the OP bending modes is at 764 cm⁻¹. OP bending modes are too weak to be observed in the experimental Raman data. The antisymmetric N–H stretching mode is observed at ∼3310 cm⁻¹ in low temperature solids slightly blue shifted from, but entirely consistent with the literature KBr data. With the exception of the N–H stretches, the recorded H/D isotope shifts in all the N–H vibrations are complex, with the IP bending modes exhibiting small ν_H/ν_D ratios (the largest value is 1.089) while one of the observed OP modes has a ratio < 1. DFT results reveal that the small ratios arise in particular from strong coupling of the N–H IP bending modes with IP stretching modes of C–N bonds. The unexpected finding of a ν_H/ν_D ratio smaller than one was analysed theoretically by examining the evolution of the frequencies of the free base by increasing the mass from H to D in a continuous manner. A consequence of this frequency increase in the heavier isotopomer is that the direction of the N–D OP bend is reversed from the N–H OP bend.