Acid−Base Equilibria in 5,10,15,20-Tetrakis(4-sulfonatophenyl)chlorin:  Role of Conformational Flexibility

Abstract

The acid−base equilibria in 5,10,15,20-tetrakis(4-sulfonatophenyl)chlorin were studied in aqueous solution and compared with the respective data for the corresponding porphyrin. The reduction of the pyrrole ring in the tetrapyrrolic macrocycle noticeably influences both free base/monoprotonated and mono-/diprotonated species equilibria. In strong acidic solutions protonation of 4-sulfonatophenyl groups takes place in addition to protonation of the macrocycle core. The photophysical properties of all ionic forms are influenced by an enhanced rate of internal S1 → S0 conversion, leading to about 50% and 90% deactivation through this channel for the free base and diprotonated species, respectively. The enhancement of the rate of the radiationless transitions is explained by an increased conformational flexibility of the chlorin macrocycle with respect to that of a porphyrin. Structural volume change measurements with laser-induced optoacoustic spectroscopy support this explanation. The contraction upon triplet state formation of the free base is about one-half of that measured for the corresponding porphyrin. This contraction should be due to intramolecular structural rearrangements of the macrocycle to adopt a minimum energy conformation in case of the chlorin. On the contrary, for the more rigid porphyrin macrocycle the interactions of the molecule with the solvent environment play a more important role. The diprotonated forms of both porphyrin and chlorin show a high radiationless S1 → S0 conversion rate and seem to have a similar conformational flexibility. In agreement with previous calculations, the conformational flexibility of the diprotonated forms appears to be higher than that of the free base molecule.

Influence of Macrocycle Protonation on the Photophysical Properties of Porphyrins

“…[6][7][8][9] In this framework, essential progress has been achieved in the elucidation of the molecular conformation of diprotonated forms, [4][5][6][7][8][9][10][11][12][13][14][15] and the type and extent of macrocycle nonplanar distortions have been extensively discussed in terms of molecular flexibility, [7,9,15] the peripheral substitution pattern, [6,7,9,14,15] and the strength of intermolecular interactions with acid residues and other anionic species in solution. [14,16,17] The relationship between the molecular structure and macrocycle core acid-base equilibrium has been studied. [16,[18][19][20][21][22] Studies of the excited states properties of diprotonated porphyrins have revealed dramatic changes in the rates and channels of the excitation energy deactivation as compared with those for the free bases, and different mechanisms involved in these changes have been proposed.…”

“…[14,16,17] The relationship between the molecular structure and macrocycle core acid-base equilibrium has been studied. [16,[18][19][20][21][22] Studies of the excited states properties of diprotonated porphyrins have revealed dramatic changes in the rates and channels of the excitation energy deactivation as compared with those for the free bases, and different mechanisms involved in these changes have been proposed. [6,15,16,[23][24][25][26] Much less is known about the molecular conformation and optical properties of monoprotonated forms of porphyrins.…”

“…[16,[18][19][20][21][22] Studies of the excited states properties of diprotonated porphyrins have revealed dramatic changes in the rates and channels of the excitation energy deactivation as compared with those for the free bases, and different mechanisms involved in these changes have been proposed. [6,15,16,[23][24][25][26] Much less is known about the molecular conformation and optical properties of monoprotonated forms of porphyrins. [16,21,[26][27][28][29] Mainly, this is because, in contrast with diprotonated species, it is not so easy to observe monoprotonated porphyrins in solution.…”

“…[6,15,16,[23][24][25][26] Much less is known about the molecular conformation and optical properties of monoprotonated forms of porphyrins. [16,21,[26][27][28][29] Mainly, this is because, in contrast with diprotonated species, it is not so easy to observe monoprotonated porphyrins in solution. Starting from the early titration studies, [30] the free base and diprotonated molecules have been observed only in most cases.…”

Spectral-Luminescent Properties of meso-Tetraarylporphyrins Revisited: The Role of Aryl Type, Substitution Pattern and Macrocycle Core Protonation

“…The fluorescence quantum yields of the monoprotonated forms were determined with an indirect procedure developed earlier for the intermediate protonation species. [47] All Φ fl values are summarized in Table 2. The obtained Φ fl values are also plotted as a function of the number of attached unhindered meso-aryl substituents (Figure 7).…”