On the Influence of Higher Excited States on the ISC Quantum Yield of Octa-aL-alkyloxy-substituted Zn-Phthalocyanine Molecules Studied by Nonlinear Absorption

Abstract— Octa-aL-alkyloxy-substituted Zn-phthalocyanines are an interesting class of far red-absorbing photosensitizers. The chemical structure, the calculated steric conformation, the observed linear optical properties and an anomalous luminescence from a higher than S, excited state are reported. To study the optical properties of higher excited states and their occupation dynamics up to delay times of 15 ns we have carried out measurements of transient absorption spectra after 14 ps pulsed, resonant B-band and Q-band excitation. From these measurements the excited state singlet-singlet and triplet-triplet spectra as well as the intersystem crossing (ISC) quantum yields are obtained. The main result is an excitation wavelength-dependent ISC quantum yield that can be explained by an additional ISC channel between higher excited singlet and triplet states. The large rate of this channel is justified by the resonance between higher triplet states, observed in the triplet-triplet spectrum and the B, absorption band. Using kinetic model calculations, a lifetime of the higher excited singlet state of some picoseconds is predicted and the influence of a two-step absorption process on the population density of this higher excited singlet state is discussed.