Novel insights on the vibronic transitions in free base meso-tetrapyridyl porphyrin

Abstract We present novel results on the free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H 2 TPyP). This molecule presents complex electronic and vibrational properties and despite the vast literature reporting the transitions observed in its absorption and fluorescence spectra, a more accurate interpretation has been kept elusive. In particular, we show that the molecule's Q -band develops into many electronic and vibronic transitions, whose the well-known “four orbital model” finds it difficult to reconcile. Using distinct spectroscopy techniques, we conclude that both Q x - and Q y -bands comprise, in fact, two quasi-degenerated electronic states together with their respective vibronic progressions each. The analysis of the Huang-Rhys factors and complementary time- and polarization-resolved measurements reinforce the need for the proposed Q -band multi features remodeling.