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.
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...
Synthesis of some new monohydroxy-substituted diarylporphyrins with different positions of the reactionary center in the macrocycle was performed. An ability of their zinc complexes to bind methyl esters of ortho-, meta-and paraaminobenzoic acids in toluene was studied by spectrophotometric titration and1 H NMR spectroscopy.
It was determined that the diprotonated form of 2,8,12,18-tetramethyl-3,7,13,17-tetrabutyl- H 21, H 23-porphine ( H 4III2+) acts as a bromide selective receptor. It forms stable complexes H 4III2+( CH 3 CN )( Br −) ( log K d1 = 5.50 ± 0.03), H 4III2+( Br −)2 ( log K d2 = 3.25 ± 0.03) in the system H 2III-( Et )4 NBr - HClO 4-acetonitrile at 25°C. Spectrophotometric method allows to determine the bromide ions' concentration 3 × 10−8 M−1.
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