Relaxation Properties of Porphyrin, Diprotonated Porphyrin, and Isoelectronic Tetraoxaporphyrin Dication in the S 2 State Agnese Marcelli Laboratorio Europeo di Spettroscopie non Lineari (LENS), UniVersita ´ di Firenze, Via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy Paolo Foggi † Dipartimento di Chimica, UniVersita ´ di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy Laura Moroni, Cristina Gellini, and Pier Remigio Salvi* Dipartimento di Chimica, UniVersita ´ di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy Ivana Jelovica Badovinac Faculty of Arts and Sciences, Physics Department, UniVersity of Rijeka, Omladinska 14, Rijeka, Croatia ReceiVed: December 18, 2006; In Final Form: January 25, 2007 The fluorescence spectra of unsubstituted porphyrin (H 2 P), diprotonated porphyrin (H 4 P 2+ ), and isoelectronic tetraoxaporphyrin dication (TOxP 2+ ) have been measured in solution at room temperature. The S 2 f S 0 fluorescence has been observed, much more intense for TOxP 2+ than for H 4 P 2+ and H 2 P. In the TOxP 2+ case, the S 2 f S 0 fluorescence spectrum is remarkably sharp and shows an excellent mirror symmetry with respect to S 0 f S 2 absorption. On the contrary, the spectra of H 4 P 2+ and H 2 P 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 H 2 P and H 4 P 2+ than in TOxP 2+ and in the case of H 4 P 2+ to the nonplanar conformation of the macrocycle. Also the S 1 f S 0 spectra of H 2 P, H 4 P 2+ , and TOxP 2+ have been measured and more qualitatively discussed. The S 1 and S 2 fluorescence decays have been observed for H 4 P 2+ and TOxP 2+ exciting with ultrashort pulses. The S 2 lifetime of TOxP 2+ is of the order of the temporal resolution of our experimental apparatus, whereas that of H 4 P 2+ is shorter. The S 2 f S 0 quantum yield of TOxP 2+ has been estimated to be 0.035, ∼3 orders of magnitude higher than that of H 4 P 2+ . It is proposed on the basis of ab initio model calculations that excited states of the H 4 P 2+ (CF 3 COO - ) 2 complex with charge- transfer character are responsible of the increased extension of the S 2 f S 0 spectrum with respect to that of H 2 P. I. Introduction Porphyrins are fundamental protein components for a number of naturally occurring processes such as oxygen transport and storage in aerobic organisms, electron transfer in the intracellular respiratory chain, and electron and energy transfer in photo- synthesis. 1 In the last years, several distorted porphyrins have been synthesized by substituent crowding at the periphery of the macrocycle 2,3 with the purpose of mimicking the modulation of reactivity in biological systems. 4,5 This has led us to study the correlation between conformational flexibility and photo- physical properties in nonplanar porphyrinic prosthetic groups. 6,7 It was found that the S 1 (ππ*) lifetimes of distorted dodecaphe- nyl- and of tetra-tert-butylporphyrin at room temperature in nonpolar solvents are strongly reduced with respect to those of planar tetraphenyl- and octaethylporphyrin. 6,7 Correspondingly, a model was proposed where the nonplanar macrocycle, following the photoexcitation and the increase of conformational freedom into the S 1 state, shifts to funnel geometries where S 1 and S 0 are close in energy, thus enhancing the deactivation channel of internal conversion. 6,7 Alternatively, diprotonated porphyrins may serve as simple yet attractive systems to investigate the influence of nonplanarity on the macrocycle photophysics. In this respect, work on substituted nonplanar porphyrin diacids has already appeared. 8-10 For instance, in diprotonated octaethylporphyrin (H 4 OEP 2+ ) and tetraphenylporphyrin (H 4 TPP 2+ ), the increase of the S 1 ' S 0 nonradiative decay relative to the neutral species was observed. 8 It seems however that little attention has been devoted to the diprotonated species of unsubstituted porphyrin (H 4 P 2+ ; 2 in Figure 1). Although no direct structural information is available, X-ray results on closely related systems 11 and recent MO/DF ab initio calculations 10,12,13 on H 4 P 2+ give the common indication that the lowest ground state conformation is nonplanar, i.e., saddle-shaped with D 2d symmetry. Due to the absence of peripheral substituents, 2 is recommended to study the effect of macrocycle bare nonplanarity. A previous study 14 has reported on the shortening of the S 1 lifetimes from the parent planar * To whom correspondence should be addressed. E-mail: piero.salvi@unifi.it. † Also at Laboratorio Europeo di Spettroscopie non Lineari (LENS), Universita ´ di Firenze, via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy and at INOA-CNR, Largo E. Fermi 6, 50125 Firenze, Italy. 2276 J. Phys. Chem. A 2007, 111, 2276-2282 10.1021/jp0686846 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007