Aryl nitrene rearrangements: spectroscopic observation of a benzazirine and its ring expansion to a ketenimine by heavy-atom tunneling.

In the photodecompositions of 4-methoxyphenyl azide (1) and 4-methylthiophenyl azide (5) in argon matrixes at cryogenic temperatures, benzazirine intermediates were identified on the basis of IR spectra. As expected, the benzazirines photochemically rearranged to the corresponding ketenimines and triplet nitrenes. Interestingly, with the methylthio substituent, the rearrangement of benzazirine 8 to ketenimine 7 occurred at 1.49 × 10(-5) s(-1) even in the dark at 10 K, despite a computed activation barrier of 3.4 kcal mol(-1). Because this rate is 10(57) times higher than that calculated for passing over the barrier and because it shows no temperature dependence, the rearrangement mechanism is interpreted in terms of heavy-atom tunneling.

Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector.

We reevaluate the absolute fluorescence and phosphorescence quantum yields of standard solutions by using a novel instrument developed for measuring the absolute emission quantum yields of solutions. The instrument consists of an integrating sphere equipped with a monochromatized Xe arc lamp as the light source and a multichannel spectrometer. By using a back-thinned CCD (BT-CCD) as the detector, the sensitivity for spectral detection in both the short and long wavelength regions is greatly improved compared with that of an optical detection system that uses a conventional photodetector. Using this instrument, we reevaluate the absolute fluorescence quantum yields (Phi(f)) of some commonly used fluorescence standard solutions by taking into account the effect of reabsorption/reemission. The value of Phi(f) for 5 x 10(-3) M quinine bisulfate in 1 N H(2)SO(4) is measured to be 0.52, which is in good agreement with the value (0.508) obtained by Melhuish by using a modified Vavilov method. In contrast, the value of Phi(f) for 1.0 x 10(-5) M quinine bisulfate in 1 N H(2)SO(4), which is one of the most commonly used standards in quantum yield measurements based on the relative method, is measured to be 0.60. This value is significantly larger than Melhuish's value (0.546), which was estimated by extrapolating the value of Phi(f) for 5 x 10(-3) M quinine bisulfate solution to infinite dilution using the self-quenching constant. The fluorescence quantum yield of 9,10-diphenylanthracene in cyclohexane is measured to be 0.97. This system can also be used to determine the phosphorescence quantum yields (Phi(p)) of metal complexes that emit phosphorescence in the near-infrared region: the values of Phi(p) for [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) are estimated to be 0.063 in water and 0.095 in acetonitrile under deaerated conditions at 298 K, while that in aerated water, which is frequently used as a luminescent reference in biological studies, is reevaluated to be 0.040.

Photophysical properties and excitation polarization of fac/mer-ruthenium complexes with 5'-amino-2,2'-bipyridine-5-carboxylic acid derivatives.

Novel ruthenium(II) complexes, fac/mer-[Ru(MeCO-5Bpy-R)3]2+ (H-5Bpy-OH = 5'-amino-2,2'-bipyridine-5-carboxylic acid; R = -NHtBu, -NH(cHex), -N(cHex)2), have been synthesized. The fac and mer isomers have been successfully separated using HPLC techniques, and their photophysical/electrochemical properties have been investigated. In the absorption and emission spectra of fac/mer-[Ru(MeCO-5Bpy-R)3]2+ with secondary amines (R = -N(cHex)2) in acetonitrile at room temperature, the maximum wavelengths based on the MLCT are longer than those for the amide derivatives with primary amines (R = -NHtBu, -NH(cHex)). A small solvent effect on the photophysical properties between fac- and mer-[Ru(MeCO-5Bpy-NHtBu)3]2+ has been observed. The excitation polarization spectra, giving P values reflecting the relation between the absorption and the emission oscillators, for the fac- and mer-ruthenium(II) complexes (C3 and C1 symmetry, respectively) have been measured for the first time. Almost no difference in the excitation polarization spectra between the fac and mer complexes is found, and these spectra are similar to that for [Ru(bpy)3]2+ with D3 symmetry. This finding suggests that the orientations of the absorption and emission oscillators, in the case of the ruthenium(II) tris(2,2'-bipyridine) derivatives, would not be affected by the symmetries of the complexes and that the P values for any derivatives would be similar to that for [Ru(bpy)3]2+.

Molecular design of functional peptides by utilizing unnatural amino acids: toward artificial and photofunctional protein.

Unnatural amino acids are effective as building blocks to design functional peptides from the following two points: (1) utilization of rigid unnatural amino acids for the incorporated peptides to control the conformation to appear the function, and (2) incorporation of functional and unnatural amino acids into peptides resulting in appearance of the inherent functions. As a combined strategy, molecular design of artificial metalloproteins utilizing 5'-amino-2,2'-bipyridine-5-carboxilic acid (H-5Bpy-OH) as an unnatural amino acid is proposed. The peptide containing three residues of the unnatural amino acid would fold through coordination to a metal ion. In particular, ruthenium(II) ion would yield a ruthenium tris(bipyridine) derivative as the core complex of the artificial protein, which would appear the similar photochemical functions as that of ruthenium(II) tris(bipyridine) complex. The central complex could form two isomers, fac and mer. For selective synthesis of the mer complex, which is expected as the core complex in the artificial protein, dicyclohexylamide as a bulky group is introduced at the C-terminal of the unnatural amino acid to destabilize the fac complex due to steric hindrance. Furthermore, in order to know the photochemical properties and function of the protein mimics, ruthenium(II) tris(2,2'-bipyridine) complexes bearing amide groups at 5,5' positions have been synthesized as the model complexes. As a result, the direction of amide groups (RNHCO-or RCONH-) in ruthenium complexes is found to significantly affect the emission efficiency: the former reduces the quantum yield and the latter enhances it, respectively. The ruthenium(II) tris(5,5'-diamide-2,2'-bipyridine) complexes are also found to strongly bind with various anions [e.g., halogen ions (Cl-, Br-) and acetate anion] in acetonitrile and to detect these anions through the emission spectral changes under air. The molecular design of artificial protein is expected to develop new fields among peptide, organic, inorganic, and physical chemistry.

Design and Semisynthesis of Photoactive Myoglobin Bearing Ruthenium Tris(2,2'-bipyridine) Using Cofactor-Reconstitution.

A new strategy for semisynthesis of a photoactivatable redox protein is described. Three protohemin molecules with ruthenium tris(2,2'-bipyridine) attached by different spacers were synthesized. The Ru(bpy)(3)-protohemins were incorporated into the heme crevice of apomyoglobin (apo-Mb) to yield semisynthetic Mbs carrying Ru(bpy)(3) as a photosensitizer (Ru(bpy)(3)-Mb). The photoactivation properties and the reaction mechanisms of Ru(bpy)(3)-Mbs were investigated by steady-state photoirradiation and laser flash photolysis. The photoactivation of Ru(bpy)(3)-Mbs was spectrophotometrically demonstrated by comparison with an intermolecular control, namely an equimolar mixture of Ru(bpy)(3) and native Mb. The spacer structure considerably influenced net activation efficiency over a wide pH range as measured by steady-state visible light irradiation and quantum yield. Laser flash photolysis yielded the rate of the photoinduced electron transfer (ET) from the lifetime of the excited Ru(bpy)(3) (k(et) = 4.4 x 10(7) s(-)(1) for Mb(1b) and k(et) = 3.7 x 10(7) s(-)(1) for Mb(1c)) and the back ET rate (k(back) = (2.0-3.7) x 10(7) s(-)(1) for Mb(1b) and k(back) = (1.4-2.4) x 10(7) s(-)(1) for Mb(1c)) from the decay of the transient absorption. These data consistently explained the results of the net photoreaction as follows. (i) The intermolecular control system was less photoactivated because little ET occurred from the excited state of Ru(bpy)(3) to Mb. (ii) The short lifetime of the charge-separated state after photoinduced ET greatly decreased the photoactivation efficiency of Ru(bpy)(3)-Mb with the shortest spacer. (iii) The photochemical and photophysical data of the other two Ru(bpy)(3)-Mb derivatives (the net photoreaction, quantum yield, and ET/back ET rates) were essentially identical, indicating that flexible spacers consisting of oxyethylene units do not rigidly fix the distance between Ru(bpy)(3) and the heme center of Mb. In addition, Ru(bpy)(3)-Mbs were highly photoactivated under aerobic conditions in a manner similar to that under anaerobic conditions, although O(2) usually quenches the photoexcited state of Ru(bpy)(3). This was probably due to the accelerated intramolecular ET from Ru(bpy)(3) to heme, not to O(2) in Ru(bpy)(3)-Mbs. We therefore showed that visible light affects the content of O(2)-bound Mb even in air.