Phototransformations of Neutral Red dye in aqueous solutions in the presence of cucurbit[7]uril
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Phototransformations of Neutral Red dye in aqueous solutions in the presence of cucurbit[7]uril L. S. Atabekyana and A. K. Chibisova,b aPhotochemistry
Center, Russian Academy of Sciences, Federal Research Center "Crystallography and Photonics", Russian Academy of Sciences, Build. 1, 7a ul. Novatorov, 119421 Moscow, Russian Federation. E-mail: [email protected] bMoscow Institute of Physics and Technology, 9 Institutskiy per., 141701 Dolgoprudny, Moscow Region, Russian Federation Photophysical properties and photochemical transformations of Neutral Red (NR) dye in water in the presence of cucurbit[7]uril (CB7) were studied by absorption, fluorescence, and laser kinetic spectroscopies. Phototransformations of NR involve the triplet state of the dye and the production of NR radical cations and radical anions. The formation of inclusion complexes NR@CB7 was detected. The quantum yield of fluorescence and the efficiency of intersystem crossing to the triplet state increase in the presence of CB7. Key words: Neutral Red dye, cucurbit[7]uril, laser pulsed photolysis, fluorescence, triplet state, ascorbic acid, radical cation, radical anion, protonation, electron transfer.
Neutral Red (NR) is a phenazine-based dye used for vital staining of cell cultures in microbiology and as acidbase indicator in analytical chemistry.1 A number of studies on the molecular photonics of free NR and its complexes with cavitands (cucurbiturils and cyclodextrins) have been reported.2—11 Based on the results of fluorescence measurements, it was concluded that the ability of the protonated form of NR, NRН+, to complexation with cucurbit[7]uril (CB7) is much higher than that of the neutral form of the dye.9 However, data on short-lived intermediates of photoreactions of NR proceeding in the presence of CB7 are unavailable. In this work we present the results of our research on phototransformations of NR in aqueous solutions in the absence and in the presence of CB7.
installation12 under irradiation with a second harmonic of a laser (Solar Laser Systems) at λ = 532 nm. Dissolved oxygen was removed by bubbling argon through the solution. The quantum yields of fluorescence were determined using Rhodamine 6G dye (quantum yield of fluorescence in ethanol is 0.94).13 Cucurbit[7]uril was purchased from Aldrich. The dyes, NR and Rhodamine 6G, were used as received. Aqueous solutions were prepared using a Millipore water purification system. All measurements were carried out in conventional quartz cells (1×1 cm) at room temperature.
Results and Discussion The absorption spectra of aqueous NR solution at different pH values are presented in Fig. 1. A 1.5 3 1.0
NR
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Absorption spectra of solutions of ground-state NR were recorded on an Agilent 8453 spectrophotometer (Agilent, USA). The luminescence spectra were recorded on a Varian Eclipse spectrofluorimeter. The difference absorption spectra of shortlived products of NR photoexcitation and the kinetics of their transformations were measured on a nanosecond laser photolysis
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