Calorimetric and quantum chemical studies of some photodimers of anthracenes

By differential scanning calorimetry (DSC) the reaction enthalpies of the thermal splitting of the following photochemically produced dimers of meso-substituted anthracenes in fluid solutions were determined: the head-to-tail (ht) dimers of 9-decylanthracene (DEA) and 9-decyloxyanthracene (DOA) in hydrocarbon solvents, and the head-to-head (hh) dimer of 9-methoxyanthracene (MOA) and the mixed hh dimer of 9-methoxyanthracene and 9-methoxy-10-methylanthracene (MMOA) in chlorinated solvents. The molar reaction enthalpy (in kJ mol(-1)) is found to be lower for the ht dimers (-28 kJ mol(-1), DEA and -29 kJ mol(-1), DOA) than for the hh dimers (-43 kJ mol(-1), MOA and -44 kJ mol(-1), MMOA) with an estimated precision of +/-3 kJ mol(-1) each. Quantum mechanical computations give energy differences for the thermal splitting of dianthracene and the two dimers of 9-methoxyanthracene in reasonable agreement with the reaction enthalpies observed. An analysis of the change in bond energies caused by the dimerization reveals that the relative stability of the dimer is mainly caused by a large increase in bond strength of the lateral benzene rings on dimerization.