2027
J . Phys. Chem. 1991,95, 2027-2036
Phosphorescence from Phenazine in Alkane Solvents in the Glass Transition Range: Spin-Lattice, Envlronment, and Orientation Relaxation of Molecules in the Metastable Triplet State Bernhard Nickel* and A. Aadreas Ruth Max- Planck-Institut fiir Biophysikalische Chemie, Abteilung Spektroskopie, Am Fassberg, 0-3400 Gatringen, Federal Republic of Germany (Received: July 27, 1990; In Final Form: November 9, 1990) The phosphorescence from phenazine in two similar solvents was investigated in the glass transition range with the aim to find a correlation between spin-lattice relaxation (SLR) of molecules in the metastable triplet state (TI) and their orientational relaxation (OR).The solvents were 3-methylpentane (3MeP) and a 1:l mixture of 3-methylpentane and isopentane (3MeP/IP). Upon laser flash excitation, apart from the decay of the total population of TI, three relaxation processes of molecules in T I were detected: (1) the decay of the selective population of the strongly phosphorescent triplet substate TI, due to SLR, (2) a time-dependent red shift of the phosphorescencespectrum resulting from molecular environment relaxation (MER), (3) the decay of the phosphorescence polarization due to OR. All three relaxation processes are in 3MeP slower than in the less viscous solvent 3MeP/IP, but a simple relation between OR and SLR has not been found. The temperature dependence of the SLR rate constant can be represented by the sum of two terms: a or Arrhenius term dominant at low temperatures (SLRI) and an Arrhenius term dominant at high temperatures (SLRh). SLRl is assigned to SLR processes characteristic for a hard glass, and SLRh is assigned to SLR resulting from rotational diffusion. In terms of the usual distinction of aand &relaxation processes in supercooled liquids, SLRh and MER are @-processeswith roughly the same activation energy, and the slow complete OR is an a-process with roughly the same activation energy as that of the viscosity. A fast partial OR accompanying SLRh and MER is predicted.
1 . Introduction Magnetic resonance of organic molecules in the metastable triplet state has never been observed in fluid solution^.^-^ This failure results from rapid spin-lattice relaxation (SLR) due to fast orientational relaxation (OR) of molecules in fluid solution.2 The objective of this investigation has been to find out whether a quantitative connection between both relaxation processes exists in viscous solutions. The basic idea has been as follows. SLR can be easily studied in zero magnetic field with molecules exTI,from hibiting strongly selective intersystem crossing SI the lowest excited singlet state SI to the three substates TI, ( i = x , y , z ) of the metastable triplet state TI (“optical spin polari~ation”~.~). SLR in such molecules in general leads to an initial change in phosphorescenceintensity, from which an effective rate constant k, for SLR can be calculated.610 The rate constant k, for OR (due to rotational diffusion) can be obtained from the decay of phosphorescence p o l a r i ~ a t i o n . ~ We ~ - ~expected ~ that, by choosing a sufficiently viscous solvent at sufficiently low temperatures, it should be possible to measure both relaxation processes in the same temperature range. In order to find out the contribution of OR to SLR, one can use two similar glass-forming solvents that differ mainly by their
--
( I ) Weisman, S. 1. J . Chem. Phys. 1958, 29, 1189. (2) Atkins, P. W.; Dobbs, A. J.; McLauchlan, K. A. Chem. Phys. Lerr. 1974, 29,616. (3) Buckley, C.D.; McLauchlan, K. A. Mol. Phys. 1985,54, I . (4) Schwoerer, M.; Sixl, H. 2.Narurforsch. 1969, 24A,952. (5) Hauruer, K. H.; Wolf, H. C. Adv. Magn. Reson. 1976.8, 85. (6) Antheunis, D. A.; Schmidt, J.; van der Waals, J. H. Chem. Phys. Lerr. 1970,6,255. (7) Antheunis, D. A,; Schmidt, J.; van der Waals, J. H. Mol. Phys. 1974, 27, 1521. (8) Schmidt, J. Spin-Lattice and Spin-Spin Relaxation Processes in Photo-Excited Triplet States in Molecular Crystals. In Relaxution Processes in Molecular Excfred Stares; FOnfachilling, J., Ed.; Kluwer Academic Publishers: Dordrecht. 1989; p 3 ff. (9) Wins”, C. J.; Dinse, K. P.; Mlbius, K. Magnetic Resonance and Related Phenomena, Proceedings of rhe XlXrh Congress AmpPre 1976; Brunner, H., Hawser, K. H., Schweitzer, D., Eds.; Group. AmpCre: Hei. delberg, 1976; p 413, (IO) Kohmoto, T.; Fukuda, Y.; Kuroda. R.; Hashi, T. Chem. Phys. Lerr. 1985, 119, 438. (1 I ) Lombardi, J. R.; Raymonda. J. W.; Albrecht. A. C. J . Chem. Phvs. 1964, 40, 1148. (12) Miller, L. J.; North, A. M. J . Chem. Soc.. Faraday Trans. 2 1975,
.71., 12?? .-- -.
(13) Rutherford, H.; Soutar, 1. J . Polym. Scl., Pblym. Phys. Ed. 1977, I S , 2213.
0022-3654/91/2095-2027%02.50/0
different viscosities q’ and q” a t low temperature^.'^ If in the glass-transition range the relation 1/q’ < l/q” holds, one may expect the relations k,‘ < k,” and k,‘ < k,”. The original main objective of this investigation was to find out whether a simple correlation exists between the differences k,” - k,’ and k,” - k,’. Two similar glass-forming solvents with the property 1/q’ > k, >> ko held and I(t) was described with sufficient accuracy by two exponentials I ( t ) = U exp(-k,t) + V exp(-k,t) (13)
if the appropriate part of the total phosphorescence decay curve was selected. With k, > k, the relaxation rate constant kj (j = s, e) was obtained as the difference k, - k, = kj. When a-OR was observed ( K # p), the relation k, >> k, k, 2 k, >> ko held. The separability of a-ORand @-ORunder these conditions will be discussed in section 5.6. Apart from the relaxation rate constants, kj (j = 0, s, e, a,8, y), the &responding relaxation times T~ = l / k j will be used. If T,, T,, and T, are considerably shorter than the triplet lifetime so, then the intensity Io* in eq 1 has a simple meaning: Io* is the intensity that is obtained by back-extrapolation of the final monoexponential phosphorescence decay to the time t = 0.
-
3. Experimental Method A schematic view of the experimental arrangement is shown in Figure 3. The different components of the experimental arrangement are described in turn. Samples. The samples were solutions of zone-refined phenazine in 3-methylpentane and in a 1:l mixture of 3-methylpentane and isopentane. The preparation of a sample included the following step: (1) A small amount of a concentrated solution of phenazine in 3MeP was filled into the degassing bulb of a fluorescence cuvette (Hellma 221 QS, cross section 10 mm X 10 mm, usable height 40 mm); the degassing bulb was cooled with ice, and the solvent was evaporated in vacuo. (2) A known amount of dry solvent was distilled into the degassing bulb cooled with liquid nitrogen. For the reproducible preparation of a 1:l mixtures (by volume) of 3-methylpentane and isopentane, each degassed component was distilled first into a calibrated tube maintained at -40 O C and then from that tube into the degassing bulb. (3) The solution of phenazine was subjected to several freeze-pumpthaw cycles, and the glass-tube connection of the cuvette to the vacuum line was sealed off. Cryostat. The sample was placed in a homebuilt nitrogen-flow cryostat that had been designed for the investigation of photochemically instable samples. Within the cryostat the sample can be moved up and down (path length 30 mm) with variable speed and rotated in steps of 90' about a vertical axis. Additionally, the whole cryostat can be moved horizontally relative to the excitation laser beam. By the combination of all three movements, a large part of the whole volume of the sample can be used. With an excitation beam diameter of 2 mm, photochemical degradation of the sample is roughly 100 times slower than that of an immobile
2030 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 sample. If the translational motion of the sample is slow enough, the distortion of the time dependence of the phosphorescence by this motion is negligible; this is in particular true for the fast relaxation processes studied in the present investigation. Temperature. The temperature of a sample was kept constant within f0.02 K (temperature controller Haake TP 24), and the absolute temperature was measured with an accuracy of i O . l K (digital PtlOO resistance thermometer Lauda R46). The thermometer was calibrated36with a vapor-pressure thermometer at the temperatures of liquid oxygen ( ~ 9 K) 0 and of solid carbon dioxide (=I95 K). The temperatures refer to the Practical International Temperature Scale of 1968. Excitation. The sample was excited with a nitrogen laser (337 nm, Lambda Physik M 1000); see Figure 3. The fluence of the excitation light in the sample was adjusted by shifting the lens L2 along the light path, and the diameter of the excitation light beam in the sample was adjusted to -2 mm with the diaphragm D. The polarization of the excitation light was defined by the Glan-Taylor prism PI. Since the photodegradation of the samples due to T, T, excitation of phenazine was not negligible, the fluence of the excitation pulses was kept as low as possible. Luminescence Light Path. The horizontal luminescent zone of the sample is imaged onto a pair of vertical slits SI1of a mechanical chopper Ch by two achromatic lenses AI and A2 and two 90° deflection mirrors M I and M2 (see Figure 3). With respect to the propagation direction of the luminescence light, the image of the luminescent zone and the luminescence polarization are rotated by 90' through the two reflections. Thus, horizontal orientation of the polarizer P2 corresponds to the measurement of the intensity I,,and vertical orientation to that of the intensity I,. The chopper slit SIIis imaged onto the entrance slit of a grating monochromator GM. The polarization-dependent transmittance of the monochromator is eliminated by a Hanle depolarizer HD. The monochromator can be used either as a single monochromator (as shown in Figure 3) or as a double monochromator. In the present investigation the single-monochromator mode has been used since stray luminescent light has been no problem, and a high spectral resolution has not been necessary. The use of the single-monochromator mode has the advantage that, for a given signal-tenoise ratio, a lower excitation intensity is needed, and hence the photochemical degradation of the sample is slower. The luminescence was detected with a red-sensitive photomultiplier tube (EM1 9558 QA, S20 photocathode, sensitive area reduced to 12." diameter with a defocusing magnet CI 22/ 12). For the measurement of phosphorescence decay curves, instead of the monochromator often interference filters were used (Oriel, Type XFS10-50 with the center wavelength X in nanometers and a half-width of =lO nm; we shall use the abbreviation IFX). For this purpose the 90° deflection mirror M3 (see Figure 3) was placed between A3 and Ad. By imaging the chopper slit SI1to the slit SI2,conceivable disturbing effects of luminescence from an interference filter were reduced to a negligible level (of the total filter luminescence only a fraction of the order of can pass SI2). Elimination of Polarization Bias. Whdn the polarization of the luminescence was of no interest, the polarization bias was eliminated by one of two ways (cf. configurations 1, and 2,* in Table 1 of ref 32): (1) The sample was excited with vertically polarized light (polarizer PI, 4, = OO), and the luminescence was detected with the polarizer P2 at the magic angle 42 z 54.7O relative to parallel polarization. (2) The sample was excited with light of the polarization angle = 90° - 54.7O = 35.3O, and the luminescence was detected without the polarizer P2. (In this case the 90° geometry of luminescence detection is essential.) Chopper. In the present type of experiment, the reliable measurement of the phosphorescenceat times t 3: 1 ps after pulsed excitation is of interest. Although phenazine is only weakly fluorescent, the intensity of the total luminescence in the first microsecond is higher than the phosphorescence in the second
-
~
~~
~~
~~~
(36) Lesche, H.; Klemp, D.; Nickel, 8 . Z . Phys. Chsm. (Munich) 1986, 141, 239.
Nickel and Ruth
Figure 4. Corrected phosphorescence spectrum and phosphorescence polarization spectrum of phenazine in 3-methylpentane at 84.8 K.
microsecond. Therefore, due to signal-induced background, without suppression of the strong prompt signal it would be impossible to measure reliably the initial phosphorescence decay due to SLR. The most reliable method of suppressing the prompt signal is the use of a fast mechanical chopper. Our home-built chopper consists of a single disk3' with two segments running between two pairs of slits in an evacuated housing. The first slit pair (SIIin Figure 3) is in the luminescence light path; the second slit pair is combined with an infrared light gate by which the laser and the detection system are synchronized with the chopper. The dead time between the laser pulse ( t = 0) and the full opening Of the Slit pair is tdad = (ddcad+ dslit)/(2rrf,01), where ddcad is the distance of the disk segment edge from the slit edge at t = 0,0.5 mm I dslitI2.0 mm is the slit width, r = 90 mm is the effective radius of the chopper disk, and 20 Hz I fm, I lo00 Hz is the rotational frequency. The lower limit of tdad was achieved withj;,, = 1000 Hz, ddad = 0.2 mm, and dslil= 0.5 mm: tdad = 1.2 fis. Most experiments were performed withf,,, = 800 Hz and dset = 1.0 mm. Spectra and Decay Curves. The photon-counting technique was used. Spectra were measured with a four-channel counter (Ortec 974). The counter was synchronized with the monochromator, and data were transferred to a personal computer. Time-resolved spectra were measured with an electronic gate with variable delay relative to the laser pulse and with variable width. Phosphorescence decay curves were measured with a signal averager (Nicolet 370) with a shortest channel dwell time of 1 NS; the excitation pulse repetition frequency was typically between 5 and 10 Hz. Measured count rates were corrected for pulse overlap and dark counts. For the evaluation of phosphorescence decay curves in general the time t = 0 of sample excitation had to be known. For this purpose the excitation light having passed the sample was strongly attenuated, and a very small fraction of the attenuated excitation light passed through an interference filter (IF333) and a silica fiber light guide to the photomultiplier housing. This light marked the time of excitation ( t = 0). Viscosity. Viscosities were measured with an improved lowtemperature capillary viscosimeter similar to that previously described.36 The temperature interval was l .O K. Intermediate viscosity values were obtained by linear interpolation of log (q/P), The absolute accuracy of the viscosity values is estimated to &S%. 4. Results 4.1. Confirmation of Known Phosphorescence Data. The
phosphorescence spectrum and the phosphorescencepolarization spectrum of phenazine are shown in Figure 4. Both spectra are practically identical with the corresponding spectra published by Pavlopoul~s.~*The longtime decay of the phosphorescence from phenazine in 3MeP at 85 K is monoexponential (see Figure 5) and yields so = 12.51 f 0.03 ms, in excellent agreement with the triplet lifetime 12.6 0.1 ms reported for phenazine in 3MeP at
*
(37) The chopper disk wa8 designed and produced by Arthur Pfeiffer Vakuumtechnik Wetzlar GmbH, D-6334 Abslar, Germany. (38) Pavlopoulos, T.0 . J . Chcm. Phys. 1969, SI, 2936.
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 2031
Phosphorescence from Phenazine in Alkane Solvents 1
0
F
4
’I
0
10
20
t /ms
~
~
I- 12000 1 -
2ooot; 2 0 2
30
Figure 5. Phosphorescence decay curve of phenazine in 3MeP at 84.8 K (measured with monochromator in 0,O transition at 638 nm). The interruptions of the decay curve in this figure and in Figures 9 and IO
result from the mechanical chopper. The lower part of the figure represents the weighted residuals.
I
0
I
I
300
400
I
100 200
tips
1-4
500
Figure 7. Description of the effect of SLR of phenazine in 3MeP at 76.8 K by eq 13. The relative intensity is given in counts per channel; the
channel dwell time was 1 ps. The lower part of the figure represents the weighted residuals.
, II nm
’
0
I
0100Mo3004olco5o3 tils Figure 6. Effect of SLR on the initial phosphorescence decay of phe-
nazine in 3MeP at different temperatures.
77 K.39 To is also close to (k,)-’= 13.5 ms for phenazine in biphenyl’ at 1.2 K (see values of k, in Figure 2). 4.2. Spin-Lattice Relaxation. In Figure 6 the initial part of the phosphorescence decay curve of phenazine in 3MeP is shown for different temperatures. The decay curves in Figure 6 can be described by eq 13; a representative example is shown in Figure 7. The values of the time constant r,‘ = (k,- kJ’ of the fast initial relaxation process are listed in Table I. Qualitatively analogous results were obtained with phenazine in 3MeP/IP, and the corresponding values of r/ are also given in Table I. We assign the fast initial phosphorescence decay to SLR. Our arguments for this assignment are as follows: 1. If the initial fast decay of the phosphorescence is completely due to SLR,then the relation between s in eq 4 and the preexponential factors U and V in eq 13 is s = V/U.From ref 7 (see parameter values in Figure 2) follows s = 1.78 for phenazine in a biphenyl host crystal at 1.2 K. Since the value of s is not likely to depend sensitively on matrix and temperature (as long as the ordering of excited singlet and triplet states is not changed by the change of the matrix), one should expect a similar value of s in the present case.4o The values of V / U in Table I are indeed close to the value of s from ref 7. (39) Kallir, A. J.; Suter. G. W.; Wild, U.P.J . Phys. Chem. 1985.89, 1996. (40)In principle, a syatcmaticerror results from the finite time resolution.
One can show that this systematic error leads to an increase of the value of V/Uby a factor/,, = 1 + (tdw$l!/T,)’/12, where tdynll is the effective channel dwell time. With fdwrll = 0.8 ps and T , 2, 2.6 ps we obtain a maximum syltematic error of the value of V/Uof +0.8%. The largest error results from the uncertainty of h0.3 p i of the time of laser excitation; the resulting uncertainty of V / U is *IO% for the shortest SLR time.
15.4
15.6
15.8
lO”P I cm-’
1E.O
Figure 8. Time-resolved spectrum (---)
and spectrum without time resolution (-) of the origin band of the phosphorescence from phenazine in 3MeP at 84.8 K (spectral bandwidth -5 nm). The time-resolved spcctrum was measured in the first 10 ps after pulsed excitation. TABLE I: Measured SLR Times T~ and Amplitude Ratios V / U of Phenazine in 3MeP (’) and in 3MeP/IP (”) at Different Temperaturesa
75.8 76.8 78.8 79.8 79.8 79.8 79.8 80.8 82.8 83.8 84.8 86.8 88.8 90.8
30.6 27.7 23.6 20S6 20.gC 20.6d 20.S 18.9 14.6 13.0b 11.1 8.2 5.3 3.3
30.86 28.02 23.04 20.83 20.83 20.83 20.83 18.75 14.87 13.04 11.29 8.06 5.36 3.34
30.80 27.99 23.00 20.78 20.78 20.78 20.78 18.69 14.82 13.00 11.26 8.05 5.34 3.30
1.70 1.71 1.63 1.68 1.68 1.70 1.69 1.60 1.53 1.60 1.47 1.40 1.60 1.60
17.17 16.76 13.9 14.25 14.34 9.9 9.68 10.12 7.93 8.33 7.93 8.33 7.93 8.33 7.93 8,33 6.5 6.48 6.77 4.2 4.30 4.32 3.6 3.50 3.40 2.6 2.85 2.66 1.90 1.60 1.27 0.96 0.86 0.57
1.58 1.56
1.55 1.62 1.64 1.85
‘The labeled values of T,’ refer to phosphorescence decay curvca measured with interference filters: ( b ) lF636, (c) 1F640, (d) IF660, (e) IF702. All other phosphorescence decay curves were measured at 638 nm with a spectral bandwidth of -5 nm. The error limits of the T , values are estimated to range from f 2 % for the largest value of T,’ to *IO% for the smallest value of T,”. For the error limits of V / U cf, footnote 40. The SLR relaxation times T,, and Tlb Were c a k u h d with eqs 15a and 15b and the parameter values in Table IV (cf. section 5.2). 2. If the phosphorescence spectrum of phenazine had a background of a rapidly decaying impurity luminescence, then,
2032 The Journal of Physical Chemistry, Vol. 95, No. 5, 1991
ou
0
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Nickel and Ruth 0.1 r
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t/ps t/ps Figure 9. Effect of MER on the time dependence I ( f ) of the phosphorescence from phenazine in 3MeP I ( t ) was measured with the interference filters IF636 (left side) and IF 640 (right side).
-r
+2
0
-2
TABLE 11: MER Times T: and Approximate Triplet Lifetimes T,' of Phenazine in 3 M e P T,K 7=', ps 7,,,', ms V,'/U,' T ~ < .ps 7,
