ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 6, с. 1003-1008
СПЕКТРОСКОПИЯ КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
YM 535.37+535.354
DEPENDENCE OF THE SPONTANEOUS EMISSION OF SINGLET OXYGEN ON THE REFRACTIVE INDEX AND MOLECULAR POLARIZABILITY OF THE SURROUNDING DIELECTRIC MEDIA
© 2014 n B. M. Dzhagarov, E. S. Jarnikova, M. V. Parkhats, and A. S. Stasheuski
B.I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk 220072, Belarus
E-mail: bmd@imaph.bas-net.by Received September 29, 2013
The rate constants (kr) for singlet oxygen O2 (a1A^) luminescence in several selected solvents and in a binary
solvent mixture (acetone—toluene) were measured. All the data have been normalized such that k^1 = 1.0 in toluene. It has been demonstrated that the changes in these rate constants were caused both by optical properties of a medium (the local field factor and density of photon states), and by an inherent property of the emitter of 1O2 (the square of the transition moment). In its turn, the value of the transition moment is directly proportional to molecular polarizability of the medium molecules. DOI: 10.7868/S0030403414060075
INTRODUCTION
For a long time, molecular singlet oxygen, O2 (a1A^), is an object of intense studies due to its unique physical and chemical properties and a prominent role in a multitude of biological and chemical reactions. Luminescence (phosphorescence) of singlet O2 which
corresponds to the alAg —»- transition was first
observed in 1976 [1]. This discovery opened the way to direct spectral control of the emergence and decay of singlet oxygen. In 1979, a laser pulse spectroscopy method has been developed for a time-resolved study of this luminescence practically in any solvent [2, 3]. The method became generally accepted in the investigation of a mechanism and dynamics of photosensitized formation of singlet oxygen and subsequent events with its participation. A description of the modern technique used in this approach can be found elsewhere [4—9]. It should be specially emphasized that singlet oxygen spontaneous emission itself is a nontrivial process in real surroundings. There are two possible excited singlet states of molecular oxygen, alAg
and b1^ . The b1^ state quickly relaxed to the lowest excited alAg state. The latter state in the free molecule (in vacuum) is extremely long-living. The elec-tric-dipole radiative transition to the ground triplet
3 _
state (X ) is strictly forbidden by symmetry, spin, and parity selection rules [10—12]. The lifetime of the a1Ag state is 72 min for the isolated molecule [12]. However in real dense media (especially in liquids), both radiative and radiationless decay are greatly enhanced by the collisions with molecules of the medi-
um. As a result of this event, the symmetry of O2 lowers
and the a1Ag —X3 transition has no more the dominant magnetic dipole character, because it borrows electric dipole character from the —► alAg transition. The latter is greatly enhanced in solvents or dense gases upon collisions, since it becomes the electric dipole—allowed transition at lower symmetry. It has been theoretically substantiated that the collision-
induced b1^
a1Ag and b1
transition
dipole moments depend on the polarizability of the solvent molecule. A more detailed discussion of this phenomenon can be found in [10—12].
The rate constant, kr, for the O2 (a1Ag) radiative transition has been determined in different solvents [12—17]. A fair correlation of kr with both the refractive index and solvent polarizability has been found. In all the above mentioned works, the experimental values of kr were analyzed only by considering the observed dependence as a result of possible changes of the intrinsic property of the O2 molecule, which is primarily the electric transition dipole moment. However, in the general case, the rate constant of the spontaneous emission depends not only on the intrinsic quantum—mechanical properties of the emitter (atom or molecule), but also on the optical characteristics of the surrounding dielectric medium [18].
Several micro- and macroscopic theoretical models have been developed and used to explain the influence of medium dielectric properties on the spontaneous emission [19—22]. For the first time such approach has been applied to explain the medium effect on the rate constant kr of singlet oxygen emission [23].
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In this work, the rate constant, kr, was determined using laser kinetic spectroscopy studies of singlet oxygen luminescence in solutions with varying refractive indexes. It was demonstrated that a strong solvent dependence resulted from two factors such as the inherent properties of the emitter and the dielectric medium characteristics, which defined the local field factor and the density of the photon states. In a theoretical paper [24], the authors investigated modification of a spontaneous emission rate constant of O2 (a1A^) near a surface of a haemoglobin-like dielectric structure. It has been shown that even for moderate dielectric contrasts one expects having significant and detectable enhancement of this rate near the surface of the biological macromolecule.
In present work we have measured relative rate constants for O2 (a1A^) luminescence in several solvents and in an acetone-toluene binary solvent mixture. All data have been normalized such that k^1 = 1.0 in toluene. The key idea of this work is to separate the influence of intrinsic quantum—mechanical properties of the 1O2 molecule on the rate constant of the
a1Ag —► X3 E radiative transition from that of external properties of a dielectric environment.
EXPERIMENT
The 1O2 molecule was directly detected by measuring its luminescence at 1270 nm. Singlet oxygen was generated by known standards namely the tosylate salt of 5,10,15,20-tetrakis (4-N-methylpyridyl) porphyrin (TMPyP) and mesoporphyrin IX dimethyl ester (MP).
MP was used in toluene, acetone, acetonitrile, chloroform, and in acetone-toluene mixtures with volume ratios of 80 : 20, 60 : 40, 40 : 60, and 20 : 80. TMPyP was used in acetonitrile, methanol, ethanol, H2O, and D2O. All the solvents were dried and distilled before use. TMPyP has a quantum yield (yA) of 0.77 ± ± 0.04 for photosensitized production of singlet oxygen in water and heavy water [25]. We used the same values of yA to study 1O2 in ethanol, methanol, and acetonitrile. For MP, ya = 0.75 ± 0.05 was taken. This value has been assumed based on analysis of literature data for yA [26, 27] and yT (quantum yield of triplet state formation) [28, 29]. The yA walue in acetonitrile was additionally determined by comparison the intensity of singlet oxygen luminescence sensitized by mesoporphyrin and Ga-tetraphynylporphin (yA = 0.98). The data for both photosensitizers (MP and TMPyP) in acetonitrile closely corresponded to each other.
All experiments were carried out at 20 ± 1°C in 10 x 10 mm quarts cuvettes in air-saturated solutions. To minimize sample photodegradation, continuous magnetic stirring was used.
The refractive indices, n, of the solvents were measured on an Abbe IRF-22 refractometer at 20°C using
the yellow sodium line at XD = 589.3 nm. The absolute error was ±0.0002. The refractometer was calibrated using distilled water (n = 1.3329 for H2O). A small additional error that we did not take into account was due to the fact that singlet oxygen emission took place at 1270 nm, where the n value was slightly less. Control absorption spectra were recorded on a Cary-500 Scan spectrophotometer (Varian).
The luminescence of singlet oxygen was measured using a laser near-infrared luminescence lifetime spectrometer (detection range of 950—1400 nm) developed at the Institute of Physics, National Academy of Sciences of Belarus. The sensitizers were excited by laser pulses at a wavelength of 531 nm (STA-01SH Nd : LSB laser, STANDA Ltd., Lithuania) with a maximum energy of 4 J a pulse duration of 0.7 ns, and a repetition rate of 1 kHz. Luminescence radiation is collected with a high-throughput optical system and directed via an MS2004i monochromator (SOLAR TII Ltd., Belarus) to an H10330-45 photo-multiplier (Hamamatsu Photonics K.K., Japan), operated in the photon-counting mode. The signal was processed by a P7888-2 multiscaler (FAST ComTec GmbH, Germany). The channel width of the multiscaler was set to 32 ns. The detailed presentation of this spectrometer was given elsewhere [9].
Time-resolved 1O2 luminescence traces were approximated by an expression (1):
I(t) = Wexp(—t/xA) - exp(-t/xT)]/(xA - xr), (1) where
Io = bkryAEL( 1 -10 ^) /n
(2)
and b is a factor, which remains constant in all experiments and depends on the parameters of the detector/amplifier system and geometry of optical elements in the experimental setup; yA — the singlet oxygen formation quantum yield; EL — the excitation photon flux; A — optical density at the excitation wavelength; n — refractive index. ta and tt—lifetimes of singlet oxygen and the photosensitizer triplet state, respectively.
After approximation of the experimental traces by equation (1), we can use the values of /0n2/(1—10—A) to calculate kr by a relative method, because the remaining parameters are ignored when yA and conditions for the luminescence excitation and recording do not change on going from one solvent to another.
In our experiments, the dependence of /0n2/(1— 10—A) on El was linear with a slope a1 due to the low excitation energy and relatively small optical density at the wavelength of excitation (A < 0.4). To determine kr by the relative method, we should compare the signals for each solvent with that for the standard:
, rel , ,, st , st
kr = kr/kr = a1/a1.
Relative rate constants (k^1) and lifetimes (ta) for O2 (a1Ag) luminescence in different solvents, the measured refractive indices (n), and the photosensitizer triplet state lifetimes tt
, rel kr a x 10 24, cm3 n 22 Í ",+ 2ln í 3 2 I2 3n I n \ 2n + y xA, Tt, ^s
D2O 0.11 1.26 1.3
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