Dec 29, 1997 - micellar solutions at room temperature are ascertained. Key words: solubilization, phosphorescence at room temperature, micelle, heavy atom, ...
Journalof Applied Spectroscopy. VoL 66, No. 2, 1999
PHOSPHORESCENCE OF MOLECULES OF POLYCYCLIC AROMATIC HYDROCARBONS IN AQUEOUS MICELLAR SOLUTIONS OF SODIUM DODECYLSULFATE AT ROOM TEMPERATURE L. V. Levshin, a S. N. Shtykov, b* I. Yu. Goryacheva, b and G. V. Mei'nikov c
UDC 535.37
Deactivation of the excited states of pyrene, benzanthracene, and fluorene molecules in aqueous micellar solutions of sodium dodecylsufate is studied using steady and pulsed fluorimetry. Quenching of the singlet states of polyatomic hydrocarbons by thallium ions is considered. Effective. micellar, and biomolecular constants for the quenching rate are obtained. Phosphorescence constants for the aforementioned compounds are determined. Reasons behind the possibility of observing phosphorescence of polyaromatic compounds in micellar solutions at room temperature are ascertained.
Key words: solubilization, phosphorescence at room temperature, micelle, heavy atom, quenching, surfactants. The features of microheterogeneous self-organizing systems (direct and inverse micelles, microemulsions, liposomes, and vesicles) formed in a solution by the diphyl molecules of surfactants manifest themselves in a reactant concentration, a local variation in the properties of a medium in the microenvironment of solubilized particles, a change in their physicochemical properties, reactivity, and also in the conditions for intra- and intermolecular deactivation of the energy for electronic excitation of luminescent probes [ 1-3 ]. The latter fact is of particular interest, since it makes it possible to expose ways of transforming the excitation energy in organized molecular systems. Naturally, this calls for systematic studies to evaluate the effects exerted by the nature of surfactant molecules and luminescent probes per se, the type of organized system, and the factors controlling the excitation energy transfer on deactivation of the excitation energy. The current study aims at investigating the deactivation of the photoexcited states of luminescent probes in aqueous micellar solutions of sodium dodecylsulfate, which is an anionic surfactant. As a result of the partial dissociation of their constituent molecules, the surface of Sodium dodecylsulfate micelles had a negative charge [1 ]. As probes use was made of such molecules of polycyclic aromatic hydrocarbons as pyrene, benzanthracene, and fluorene. Their singlet excited states were quenched by adding thallium nitrate (I), which dissociated in an aqueous medium to form thallium cations (I), opposite in charge to the surfactant micelles, which were capable of stimulating spin-orbital interaction (the heavy-atom effect) [4 ]. Oxygen, which is an effective quencher of the electron-excited states of polycyclic aromatic hydrocarbons, was removed from the solution using a chemical method described in [5 ]. The sodium dodecylsulfate preparation of the Dia-M firm contained over 98% of the base material. Thallium nitrate and other employed salts were qualified as pure for analysis. Luminescence spectra for a steady excitation were obtained on SDL-1 and Hitachi MRF-1 spectrofluorimeters. Deactivation of the energy of triplet states was studied with the aid of a pulse fluorimeter [6 ]. aM. V. Lomonosov Moscow State University, Russia; bN. G. Chernyshevskii Saratov State University, 83, Astrakhanskaya Str., Saratov, 410026, Russia; CSaratov State Technical University, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 201-204, March-April, 1999. Original article submitted December 29, 1997.
212
0021-9037 / 99/6602-0212522.00 9 1999 Kluwer Academic/Plenum Publish ers
Fig. 1. Luminescence spectra for pyrene (a) (C .~ 5-10 -5 mole/liter) and benzanthracene (b) (C ~ 5.10 - s mole/liter) in a sodium dodecylsulfate solution for various TINO3 concentrations: 0 (1), 0.0005 (2), 0.001 (3), and 0.03 mole/liter (4). In aqueous solutions of sodium dodecylsulfate with a concentration of surfactant molecules exceeding the critical concentration of micelle formation by no more than an order of magnitude (_< 10 -1 M), spherical micelles form that are capable of solubilizing low-polarity molecules of polycyclic aromatic hydrocarbons. The efficiency of the conversion of polycyclic aromatic h y d r o c a r b o n molecules from an aqueous m a c r o p h a s e to a ~micellar micropseudophase is characterized by the magnitude of the distribution constant, which for example, for pyrene is 1.7.106 [7 ]. Such a large magnitude of the distribution constant indicates that over 99% of the molecules of this probe are bound to the micelle, and the observed processes of transfer and deactivation of the electronic excitation energy belong to a micellar pseudophase. It is well-known that the solubilized molecules of polycyclic aromatic hydrocarbons are localized in this case in the region of the surface layer of micelles, i.e., near the charged Stern layer [7 ]. Figure 1 presents the luminescence spectra for aqueous micellar solutions of pyrene and benzanthracene molecules. Clearly, in the absence of thallium ions (curves 1) there are intense fluorescence bands with ~lmax = 394 nm for pyrene and 395 nm for benzanthracene. Adding TINO3 even in small quantities (curves 2) decreases the fluorescence intensity for all polycyclic aromatic hydrocarbons considered. This phenomenon can be attributed to the fact the introduction of thallium nitrate into the micellar solution of sodium dodecylsulfate causes the replacement of Na + ions in the surface layer of micelles by TI + ions. This leads to a localization of and an increase in the effective concentration of heavy TI + ions in the Stern micellar layer, i.e., near the solubilized molecules of pobycyclic aromatic hydrocarbons. The latter factor, according to [8 ], causes an enhancement of their intercombination conversion from the singlet to the triplet state. Naturally, in this case the process is accompanied by a decrease in the fluorescence intensity for the molecules of polycyclic aromatic hydrocarbons considered. We determined the effective, micellar, and biomolecular constants of fluorescence quenching for the polycyclic aromatic hydrocarbons considered [9, 10 ]. The S t e r n - F o l m e r effective quenching constant was obtained from the familiar equation
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TABLE 1. Spectral Photophysical Characteristics of Pyrene, Benzanthracene, and Fluorene Molecules *) in Aqueous Solution of Sodium Dodecylsulfate **)
Compound
An, nm
Aph, n m
Pyrene Benzanthracene
394 395
595 604
Fluorene
318
456
k eft St-F" 10-3, liter/mole
km
kQfl" 10 -8, St-F, ' liter/mole liter/mole.see
4.9
78
3.5
55
2.0
k0h 910 -2, sec -1
~'ph, /~sec
0.8 2.5 2.7
12.5 4
3.7
Note./ln and 2ph are the wavelengths corresponding to the maximum intensities of fluorescence and phosphorescence of the molecules of polycyclic aromatic hydrocarbons considered. *)C = 5.10 -5 mole/liter; **)C = 0.05 mole/liter.
( I 0 / 1 ) - 1 = k err
St-F
[Q],
where I o is the fluorescence intensity for the molecules of polycyclic aromatic hydrocarbons in the absence of a ~.eff quencher; I is the fluorescence intensity with TINO3 added to the solution; ~St-F is the S t e r n - F o l m e r effective constant, and [Q ] is the quencher concentration. It was assumed here that virtually all molecules of polycyclic aromatic hydrocarbons were solubilized in sodium dodecylsulfate micelles. With allowance for the microheterogeneity of the medium, the constant for fluorescence quenching in micellar systems (kst_F), m in conformity with [9 ], was predicted by the equation m
kst-F
= keff S~-F VC ,
where V is the molar volume of surfactants, which is equal to 0.32 liter/mole for sodium dodecylsulfate [10 ] and C is the surfactant concentration. Using the micellar constant it is possible to calculate the biomolecular quenchingrate constant kQfl: 111
kQfI = kst_F/Z where 9 is the mean lifetime of the molecules of polycyclic aromatic hydrocarbons in the excited singlet state [11 ]. The obtained constants for benzanthracene, fluorene, and pyrene are listed in Table 1. Starling with a TINO3 concentration of 0.0002 mole/liter, the above-stated decrease in the fluorescence intensity for deoxygenated aqueous micellar solutions of polycyclic aromatic hydrocarbons was followed by an afterglow in the region of 570-720 nm for pyrene and benzanthracene and 450-500 nm for fluorene, the maxima of whose spectra are presented in Table 1. The triplet nature of this afterglow is corroborated by its sensitivity to the presence of oxygen in the solution (the afterglow disappears in the presence of oxygen). The new long-wave persistent glow in the selected polycyclic aromatic hydrcr at room temperature is related to the phosphorescence stimulated by heavy ions: thallium ions. The latter fact is not trivial, since phosphorescence was generally observed either when solutions froze or when a solid substrate was used [12, 13 ]. The appearance of phosphorescence in some polycyclic aromatic hydrocarbons in aqueous micellar solutions was observed also by the authors of [14, 15 ]. The quantum yield of fluorescence in these conditions was 0.76 for pyrene and 0.64 for bensanthracene [14 ]. However, these studies did not include a detailed analysis of the luminescence spectra for the molecules of polycyclic aromatic hydrocarbons at different concentrations of quenchers in the solution. T h e observed s h a r p decrease in the fluorescence intensity for the molecules of polycyclic aromatic hydrocarbons and the simultaneous significant increase in the phosphorescence intensity (curves 3 a n d 4 in Fig. 1) indicate that in this case TI+ ions lead to an increase in the rate of intercombination conversion Sl ~> TI and in 214
Fig. 2. Constant of phosphorescence quenching rate blph = 604 rim) for benzantharacene (C = 5" 10 -5 mole/liter) in a sodium dodecylsulfate solution (C = 0.05 M) vs. TINO3 concentration. the population of tl'~e triplet state T 1. The two aforementionet~ factors and the incr-ase in the rate of radiative deactivation T 1 ~ ,So of the triplet state Tl give rise to phosphorescence and enhance its intensity. In this connection, we studied the kinetics of the phosphorescence quenching rate for the molecules of polycyclic aromatic h y d r o c a r b o n s . With a rise in the thallium nitrate concentration the constants of t h e phosphorescence deactivation rate/r increased (Fig. 2). Based on the data obtained we determined the constants for the rate of natural phosphorescence deactivation in the absence of thallium ions. The values obtained for the rate constants k~ and-mean lifetimes of the excited triplet states Tph are supplied in Table 1. The mean lifetime of the triplet states is found to dramatically decrease when moving from pyrene to fluorene. Previous studies [16 ] established an increase in the lifetime of the triplet states of anthracene molecules (rt = 6/zsec) in a n aqueous micellar solution of sodium dodecylsulfate in comparison with ethanol solutions (zt = 4 /~sec). This is associated with the creation, by surfactant molecules, of a protective enclosure around the luminescent probes, which reduces the effect of quenchers (mainly of solubilized oxygen), increases the medium viscosity, and, therefore, reduces the translational mobility of the molecules of polycyclic aromatic hydrocarbons. All these processes reduce the portion of nonradiative losses in energy transformation in the excited state. The significant lifetime of triplet molecules indicates that the constant of the nonradiative deactivation rate increases only insignificantly with an increase in the concentration of heavy atoms in the solution. Thus, the onset of phosphorescence for the molecules of polycyclic aroni~tic hydrocarbons at room temperature is fostered by the following processes in aqueous micellar solutions: a convergence of molecules; an increase in their effective concentration and in the time of contact between the molecules of polycyclic aromatic hydrocarbons and heavy ions; a rise in the rate of intercombination conversion induced by heavy ions; a decrease in the mobility of a molecule of polycyclic aromatic hydrocarbon per se as a result of solubilization in miceUes, which reduces its nonradiative deactivation; and a shielding of the molecules of polycyclic aromatic hydrocarbons in micelles from extraneous quenchers. The work was conducted with the support of the Russian Foundation for Fundamental Research, Grant No. 97-03-33393a. REFERENCES .
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