Relaxation Properties of Porphyrin, Diprotonated Porphyrin, and Isoelectronic Tetraoxaporphyrin Dication in the S₂ State

Abstract

The fluorescence spectra of unsubstituted porphyrin (H2P), diprotonated porphyrin (H4P2+), and isoelectronic tetraoxaporphyrin dication (TOxP2+) have been measured in solution at room temperature. The S2-->S0 fluorescence has been observed, much more intense for TOxP2+ than for H4P2+ and H2P. In the TOxP2+ case, the S2-->S0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respect to S0-->S2 absorption. On the contrary, the spectra of H4P2+ and H2P are shifted and more extended with respect to the absorption counterparts. The differences have been attributed primarily to the change of the equilibrium geometry upon excitation, larger in H2P and H4P2+ than in TOxP2+ and in the case of H4P2+ to the nonplanar conformation of the macrocycle. Also the S1-->S0 spectra of H2P, H4P2+, and TOxP2+ have been measured and more qualitatively discussed. The S1 and S2 fluorescence decays have been observed for H4P2+ and TOxP2+ exciting with ultrashort pulses. The S2 lifetime of TOxP2+ is of the order of the temporal resolution of our experimental apparatus, whereas that of H4P2+ is shorter. The S2-->S0 quantum yield of TOxP2+ has been estimated to be 0.035, approximately 3 orders of magnitude higher than that of H4P2+. It is proposed on the basis of ab initio model calculations that excited states of the H4P2+(CF3COO-)2 complex with charge-transfer character are responsible of the increased extension of the S2-->S0 spectrum with respect to that of H2P.

Effect of protonation on the singlet–singlet excited-state absorption of meso-tetrakis(p-sulphonatophenyl) porphyrin

“…3a. This fact, together with the small fluorescence Stokes shift observed previously [4,17], indicates that the TPPS 4 vibronic structure is preserved upon excitation [18,19]. In other words, the molecular structure is the same for all singlet states participating in the absorption process and the D 2h molecular symmetry, characteristic for the ground state other hand, the ratio r 1n (k)/r 01 (k) % 2, observed in the entire Q-band region shows that the S 1 ?…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…The results from three recent calculations relative to porphyrin and presented in Table 13 differ from experiment [299,304] by no more than 0.2 -0.3 eV not only for the Q x , Q y and B x , B y pairs but also for the weak transitions, usually denoted N x , N y and L y , and observed at higher energies with respect to the Soret band. Exciting into the Soret absorption band of tetraoxaporphyrin as indicated in Figure 48 radiative and non radiative decay processes are promoted [305,306]. The fluorescence emissions from the first and second excited state, Q → S 0 and B → S 0 , are easily observed with quantum yield 0.11 and 0.035, respectively [305].…”

“…Exciting into the Soret absorption band of tetraoxaporphyrin as indicated in Figure 48 radiative and non radiative decay processes are promoted [305,306]. The fluorescence emissions from the first and second excited state, Q → S 0 and B → S 0 , are easily observed with quantum yield 0.11 and 0.035, respectively [305]. The upper state fluorescence is favored by the sharpness of the Soret band having a considerably high extinction coefficient and by the absence of intermediate states in the large energy gap between B and Q.…”

meso-Aryl-substituted free-base porphyrins: formation, structure and photostability of diprotonated species

“…Stability of the modified macrocycle has been related directly to the bonding stress resulting from hosting one or two electron acceptor species at the porphyrin core. In consequence, the symmetry of the initially quasiflat molecule considerably changes [34][35][36][37][38]. Due to redistribution of electronic density within the bonding system of the core, the newly created adducts may reveal unique chemical and photochemical properties, unlike typical porphyrins, first reflected in their absorption and fluorescence spectra [20].…”