Ground and Excited States of Zinc Phthalocyanine Studied by Density Functional Methods

The first time-dependent DFT study of the excited states of ZnPc is presented. The theoretical results provide an accurate description of the UV−vis and vacuum−UV spectra and prove to be in excellent agreement with gas-phase spectra and generally in line with deconvolution analyses of solution and Ar/matrix absorption and MCD spectra. The nature and intensity of the main spectral features are highlighted and interpreted on the basis of the ground state electronic structure of the complex. A fragment approach where the four benzopyrrole rings and the aza bridges are taken as building blocks has proven to be a very important tool to fully understand the energy and composition of the MOs involved in the transitions and, from these, the excitation energies and intensities. The Gouterman a1u orbital is the HOMO and the assignment of the Q band is conventional and uncontroversial. The B band comprises five Eu excitations, whose positions and intensities are in very good accordance with the deconvolution of the experimental absorption band performed with the help of MCD spectra. However, this deconvolution invokes Jahn−Teller splitting of the Eu states which we have not calculated. We find at the red edge of the B band the weak 2nd π→π* transition and at the blue edge the weak n→π* transition which have been identified in the experiments. We do not confirm at low energy, in the tail of the Q band (the “Q02” region) an electronic origin for the band which has been suggested to arise from the lowest z-polarized n→π* transition. This transition is predicted by our calculations to be very weak and to lie in the B band region. The energies and intensities of the higher excitations are in excellent agreement with the UV N and L bands and with the far UV C and X bands. The predicted level pattern of the lowest triplet excited states fits in with phosphorescence data available and excited-state absorption spectra.