We study the enhancement of two-photon absorption (TPA) in a series of porphyrins and tetraazaporphyrins by measuring the absolute TPA cross sections with 100-fs-duration pulses in two ranges of laser wavelengths, from 1100 to 1500 and from 700 to 800 nm. The cross section in the Q transition region is 2 ϳ 1 -10 GM (where 1 GM ϭ 10 Ϫ50 cm 4 s Ϫ1 photons Ϫ1 ), a value that is explained by partial lifting of the prohibition that is due to a parity selection rule. In the Soret transition region we find 2 enhancement by ϳ1 order of magnitude owing to the Q transition, which acts as a near-resonance intermediate state, and also owing to the presence of gerade energy levels, which we identify in this spectral region. In tetraazaporphyrins symmetrically substituted with strong electron acceptors, we find further enhancement (up to 2 ϳ 1600 GM). As a possible application, we demonstrate for the first time to our knowledge the photosensitization of singlet-oxygen luminescence by TPA in porphyrin.

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Abstract-We demonstrate efficient generation of singlet oxygen upon two-photon excitation with 150-fs 780-nm laser pulses of a new porphyrin photosensitizer molecule whose two-photon absorption cross section has been considerably enhanced by chemical design.

The individual absorption spectra of the two NH tautomers of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole are assigned on the basis of the Gouterman four-orbital model and a quantum chemical TD-DFT study. The assignment indicates that the red-shifted T1 tautomer is the one with protonated pyrrole nitrogen atoms N(21), N(22) and N(23), whereas the blue-shifted T2 tautomer has pyrrole nitrogen atoms N(21), N(22) and N(24) protonated. A wave-like nonplanar distortion of the macrocycle in the ground state is found for both NH tautomers, with the wave axis going through the pyrroles containing N(22) and N(24). The 7C plane determined by the least-squares distances to the carbon atoms C1, C4, C5, C6, C9, C16, and C19 is suggested as a mean corrole macrocycle plane for the analysis of out-of-plane distortions. The magnitude of these distortions is distinctly different for the two NH tautomers, leading to substantial perturbations of their acid-base properties, which are rationalized by the interplay of the degree of out-of-plane distortion of the macrocycle as a whole and the tendency of the pyrrole nitrogen atoms toward pyramidalization, with the former leading to a basicity increase whereas the latter enhances the acidity.

Protonation of a free-base meso-pyrimidinyl-substituted AB 2 -corrole (H 3 AB 2 ) in ethanol solution by stepwise addition of sulfuric acid has been studied in the temperature range from 293 to 333 K. The formation rate of protonated species was found to depend profoundly on the temperature at which the titration was undertaken. Two steps in the titration curve were identified at temperatures around 293−298 K, whereas one-step formation of protonated species was found to occur at temperatures above 308 K. The protonation product was the same in both cases, i.e., H 4 AB 2 + corrole, protonated at the macrocycle core nitrogen atoms. The two steps in the protonation kinetics at lower temperatures were attributed to protonation of individual tautomers of the free-base H 3 AB 2 corrole. To the best of our knowledge, this is the first well-illustrated (spectrophotometric) observation of individual properties of corrole NH tautomers in fluid solution. Concomitant increase in the NH tautomerization rate with increasing temperature is proposed to account for the one-step protonation. Evidences for the role of individual corrole NH tautomers in the protonation process as well as their optical features are discussed based on spectroscopic results and simulation data. ■ INTRODUCTIONCorroles, contracted tetrapyrrolic macrocycles lacking one meso-carbon atom, have emerged as attractive porphyrinoid materials, notably during the past decade, in which novel synthetic pathways toward meso-triaryl-substituted corroles and numerous postmacrocyclization functionalization protocols have considerably enlarged the set of available corrole structures and their potential applications.1,2 Corrole-based materials are nowadays extensively studied as novel catalysts, antitumor treatment, and imaging agents as well as active materials in the design of new sensors and optoelectronic devices. 3,4 Two major structural peculiarities of corroles relative to porphyrins are the presence of three rather than two NH protons in the coordination core and their lower symmetry. The phenomenon of NH tautomerization has been known for a long time for free-base (Fb) porphyrins and has been studied in detail by NMR and optical spectroscopy methods in both the liquid and solid states. 5 The presence of three NH protons in the corrole macrocyclic core and the inherent asymmetry impose many questions concerning the specific details of the corrole tautomerization mechanism. Quantum chemical calculations predict a substantially lower barrier for corrole tautomerization (2.45 kcal mol −1 )6 as compared to that of porphyrins (17 kcal mol −1 ), 7 which leads to a much faster rate of proton migration in the tetrapyrrolic core. The lower symmetry of the Fb corrole macrocycle (C s ) compared to that of a Fb porphyrin (D 2h ) implies that the two tautomeric forms are distinct and structurally quite different (see Scheme 1), which opens the possibility to detect the two NH tautomers for any corrole derivative, independently of the peripheral substitution pa...

ABSTRACT:The fluorescence spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in the temperature range from 4.2 to 332 K. For the first time, the individual fluorescence profiles of the two corrole NH tautomers have been assigned over the whole temperature range. The pronounced temperature dependence of the fluorescence spectra of the meso-pyrimidinylcorrole under study was found to originate from switching between the fluorescence emissions of the two tautomers due to a reduced NH tautomerization rate with decreasing temperature. As a result, the long wavelength tautomer dominates the total emission spectrum at room temperature, whereas at low temperatures, the majority of the emission comes from the short wavelength tautomer. Energy level diagrams (involving the two NH tautomers) explaining the excitation energy deactivation channels in the meso-pyrimidinylcorrole at room temperature and below are presented. A significant H/D isotope effect on the NH tautomerization rate has been observed, resulting in an enhanced contribution of the short wavelength tautomer to the total fluorescence spectrum at the expense of that of the long wavelength tautomer. Substantially different fluorescence quantum yields have been determined for the individual NH tautomers, leading to a pronounced temperature dependence of the overall fluorescence quantum yield. The obtained results allow the unambiguous statement that the two NH tautomers of corroles coexist in fluid and solid solutions in a wide range of temperatures, with the proportion depending on the corrole substitution pattern. Moreover, this study shows that the (future) interpretation of the fluorescence properties of mesopyrimidinylcorroles and all other corrole materials should be done (more) carefully, taking into account the coexistence of NH tautomers with individual spectral signatures.

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