Free base phthalocyanine: Influence of thermal effects and dimerization on the electronic absorption spectrum

Theoretical results for the electronic properties of the free base phthalocyanine monomeric and dimeric species are presented. The binding energy for the most stable dimer (a parallel-displaced structure) is 12 kcal/mol. The calculations of the electronic excitation spectrum rely on the analysis of structures generated by Born–Oppenheimer Molecular Dynamics at 500 ± 4 K (monomer) and 700 ± 4 K (dimer). The results for the absorption spectrum illustrate the importance of taking into account thermal broadening and dimerization. The Qx–Qy band splitting is ∼40 nm in reasonable agreement with the experimental value of 64 nm at T = 783 K.