Influence of alkyl substituents in corrphycene on geometry, electronic structure, hydrogen bonding, and tautomerization

Geometry, electronic and vibrational structure, and relative energies of different tautomeric forms have been calculated for free base corrphycene and its five tetra-, octa-, and dodeca-alkyl-substituted derivatives, of which only one has been synthesized so far. The results demonstrate that the lowest energy structure always corresponds to the transtautomeric form. Comparison with the experimental IR and X-ray data available for 2,3,6,7,11,12,17,18-octaethylcorrphycene shows, in contrast to previous suggestions, that the transspecies is dominant both in solution and crystalline phase. Based on structural, spectral, and computational data, rates of double hydrogen transfer can be predicted. Tautomerization is expected to occur in a nanosecond or subnanosecond range. The rate should strongly decrease upon substituting parent corrphycene with alkyl groups at the pyrrolic β positions, whereas the opposite effect is expected for mesosubstitution. Depending on the position of substituent, planar or nonplanar geometries of the corrphycene chromophore are predicted. The nonplanarity leads to a substantial increase of the intensity of the low energy electronic transitions (Q-bands).