RADIATIONLESS DEACTIVATION OF TRIPLET CORONENE IN PLASTICS
4499
Radiationless Deactivation of Triplet Coronene in Plastics
by John L. Kropp and William R. Dawson Chemical Sciences Department, Physical Research Center, TR W Systems, Redondo Beach, California 90278 (Received June 22, 1967)
A method has been developed using the intensities of delayed fluorescence and of phosphorescence of coronene which permits calculation of the rate constant for crossing to S1 from isoenergetic triplet levels (ke') as well as the S1-T1 gap. For these calculations, measurements of the fluorescence yield and the temperature dependence of both the phosphorescence lifetime and the ratio of delayed fluorescence to phosphorescence were made using poly(methy1 methacrylate) specimens containing coronene-h12 and coronene-dl2. The calculated S1-T1 gap of 3930 cm-l agrees with the value obtained from the 0,O bands of the fluorescence and phosphorescence spectra. Values of ks' of coronene-d12 and coroneneh12 are equal to within the experimental error and are similar to k3, the rate constant for the reverse. intersystem crossing from S1 to TI. The delayed fluorescence accounts for only a small fraction of the total radiationless deactivation of triplet coronene. The temperature-dependent part of the rate constant for deactivation of triplet coronene is larger for ordinary coronene than for deuterated coronene.
I. Introduction The rates of intersystem crossing and internal conversion of large aromatic molecules are needed to determine the contribution of these radiationless processes to deactivation of the molecule. Rates of singlettriplet crossing have been estimated for some molecules.1r2 The back reaction from triplet to singlet is known to occur but can only be observed in solids as delayed fluorescence when the energy gap from the lowest singlet to the lowest triplet is sufficiently small to allow thermal population of the singlet from the triplet. Lewis, Lipkin, and Aiagela made observations of the delayed fluorescence of fluorescein in boric acid by measuring the lifetime change with temperature and correlated the activation energy with the &-TI energy gap. We have made similar measurements using coronene. The lowest excited triplet and singlet states of coronene, T1 and S1 (cf. Figure l), are separated by an unusually small energy gap for an aromatic hydrocarbon, 3750 cm-l, and as a result there is appreciable delayed fluorescence above 50". Initially, we did experiments similar to those of Lewis et al., and determined the lifetimes of delayed fluorescence and phosphorescence for coronene in poly(methy1 methacrylate) (PMM) as a function of temperature. The treatment of these data
did not yield an activation energy in agreement with the known spectroscopic value of the T1-Sl energy gap. The energy level diagram of coronene is given in Figure 1. Level Sl is the lowest excited singlet; T1 is the lowest triplet level. Level T, represents triplet levels isoenergetic with Sl. They are probably higher vibrational levels of T1, but one cannot exclude the possibility of another electronic triplet state lying between TI and SI. A kinetic analysis has been made which permits calculation of the activation energy for delayed fluorescence from measurements of the temperature dependence of the intensity ratio of delayed fluorescence4 to phosphorescence. The rate constant for intersystem crossing to SI from isoenergetic triplet levels may also be estimated with this analysis and compared to the rate constant for crossing from S1 to the triplet mani-
(1) c. R. Horrocks, c. Kearvell, K. Tickle, and F. Wilkinson, Trans. Faraday SOC.,62, 3393 (1966). (2) M.W. Windsor and W. R. Dawson, submitted for publication. (3) G. N. Lewis, D. Lipkin, and T. T. Magel, J . A m . Chem. Soc., 63, 3005 (1941). (4) Delayed fluorescence as used here refers only to the fluorescence due to thermal activation of SI from TI. It should not be confused with the delayed fluorescence arising from collisions of excited molecules in liquids.
Volume 71, Number IS December 1987
4500
JOHNL. KROPPAND WILLIAMR. DAWSON
Since d[C']]ldt is small compared to the other terms in eq I, we have an equilibrium between Cs and Ct such that [CS1 [c"] =
+
&TF
+
where T F = (kl kz ka)-'. The intensities of delayed fluorescence ( I F D ) and phosphorescence (Ip) are given by
IFD = k i [ C s ]
(W
k*[Ct]
(IIb)
and Ip =
Combining (IIa) and (IIb) with the relation between [es]and [C'], we have
Figure 1. Energy level diagram for coronene showing t h e rate constants for population and decay of the lowest triplet state.
fold. These studies have been made for coronene-hlz and coronene-dl2 in poly(methy1 methacrylate) (PMM).
11. Kinetic Considerations A mechanism of formation of triplet coronene and the subsequent singlet formation from the triplet by thermal activation is given as
C+CS
I
(0)
cs
ki
(1)
cs+c
kz
(2)
Cs+Ct
k3
(3)
Ct +hvp
k4
(4)
ct+c
kg
(5)
Ct--fC8
ks
(6)
--f
hug
where C is coronene and the superscripts s and t indicate the lowest excited singlet and the triplet state, respectively, I is the rate of absorption of excitation light, and hvF and h v p denote fluorescence and phosphorescence. I n this mechanism all processes are assumed to be unimolecular, involving no interaction between coronene molecules, and thermal equilibrium is assumed to be achieved. Immediately after the excitation is turned off, the rate equation of singlet decay is given by
The Journal of Phyeieal Chemiatry
where T P , O = k4-1 and @F = k1rF. If the only temperature-dependent process is (6), we can rewrite kg = ks' exp(-AEIRT), where A E is the activation energy of the process. Then we have finally
IFD _ IP
@ ~ T p , o k 6 ' exp( -AE/RT)
(IV)
Thus if @F and T P , are ~ known and invariant with temperature, ke' and A E can be derived from the variation of I F D / I P with temperature.
111. Experimental Section A . Materials. Coronene-hlz was used as received from Rutgerswerke; coronene-d12 was obtained from Merck Sharpe and Dohme Ltd. of Canada. Its isotopic purity exceeded 98% ; however, it was necessary to purify coronene-dlz chromatographically on a column filled with Woelm basic alumina using benzene as the eluate, in order to eliminate an impurity absorbing in the visible r e g i ~ n . ~There is essentially no difference between the absorption spectrum of purified coronenedlz compared to coronene-h12. Specimens of coronene-hlz and coronene-dlz in poly(methyl methacrylate) (PMM) at a concentration of 1.0 X M were prepared. The coronene and polymerization initiator (0.02 wt. % of a,a-azodiisobutyronitrile) were dissolved in pure methyl meth(5) l,l2-Benzperylene is a common impurity in coronene. However, 3130-A excitation is used and absorption is almost completely by coronene. When broad-band ultraviolet excitation from flash lamps was used, the data for delayed emission from the oscillogram traces were taken 0.5 sec or longer after cutoff of excitation. The lifetime of l,l%bensperylene is only 0.3 sec, and, therefore, its contribution to the data is negligible.
RADIATIONLESS DEACTIVATION OF TRIPLET CORONENE IN PLASTICS
acrylate monomer.6 The PMM solutions were transferred to ampoules. Dissolved oxygen was removed by several cycles of freezing and thawing under vacuum. The ampoules were sealed under vacuum and the solutions were polymerized by heating for 2-day periods each a t 40, 60, and 100". A DER-332' sample with a coroM was prepared nene-hlz concentration of 1.0 x by dissolving the coronene and then 15y0 diethylenetriamine in DER-332 under vacuum and heating the sample for 1 day at 100" after the specimen solidified. The face of the PMll specimens that was used for luminescence yield measurements was ground flat, polished, and bonded to a 1.5 mm thick quartz disk using a 0.0002-in. thickness of DER-332 as adhesive. This adhesive layer absorbed less than 1% of the 3130-A excitation light,. The quartz disk prevented diffusion of oxygen into the plastic which would result in a gradual decrease in phosphorescence with time. B. Measurtments. 1. Spectra. The spectral distribution of delayed emission from coronene was obtained using a Jarrell-Ash 0.75-m spectrograph. Coronene in P3IN was excited by 3130-A light from a high-pressure Hg arc. A chopper was inserted in the system so that the spectrograph recorded only the light emitted following an excitation pulse. Spectra were traced using a Joyce-Loebel microdensitometer. The temperature was controlled to within h 10" by using a heating coil wrapped around the sealed Pyrex ampoules containing the PMJI sample. 2. The Ratio of Delayed Fluorescence to Phosphorescence. The intensity of the delayed fluorescence IFD' a t the peak of fluorescence intensity (4500 A) was compared to the maximum intensity of the phosphorescence Ip' a t 5630 A. The ratio IFD'/Ip' was determined at several temperatures between 23 and 125". For these measurements the coronene samples were in an evacuated quartz tube that was wrapped with nichrome heating wire. The temperature was measured using a thermocouple with a junction placed in a hole in the sample. This assembly was immersed in glycerol used as a heat bath in a square-section, clear dewar. The sample was excited through one face of the dewar by a l-msec pulse from a xenon flash lamp. Visible light was removed from the flash with a Corning 7-54 filter. The resulting emission from the coronene sample passed into a monochromator and was detected with a photomultiplier tube (RCA 6215). The emission intensity was recorded as a function of time on an oscilloscope trace. Oscillograms were obtained at various temperatures with the monochromator set at 4500 and 5650 A. The value of IFD'/IP' was calculated from the intensities taken at equal times after the flash. The lifetimes of phosphorescence and delayed fluores-
4501
cence are equal. Thus the value of IFD'/IP' is invariant with time after the flash. Values of IFD'/IP' are not corrected for the different instrumental responses to phosphorescence and delayed fluorescence. The absolute ratio, IFD/IP,of all of the photons emitted as delayed fluorescence to all of the photons emitted as phosphorescence was determined at 23". The luminescence was detected by using the M rhodamine B in secondary fluorescence of 5 X glycerol as a quantum counter. The response per photon of this quantum counter is constant from 3000 to 6000 A.8 The entire delayed fluorescence spectrum and over SSyo of the phosphorescence spectrum of coronene lie in this region.* As before, the ultraviolet output from a xenon flash lamp which passed through a 7-54 filter excited luminescence. The decay after the flash of the resulting total luminescence, IFD plus IP,and of the portion of the luminescence transmitted by a Corning 4-60 filter, as measured via the rhodamine B quantum counter, were displayed on separate oscilloscope traces and recorded on oscillograms. The 4-60 filter transmitted half the delayed fluorescence but none of the phosphorescence of coronene. The value of IFD/IPa t 23" is calculated from the ratio of displacements on the two oscillograms. In order to convert each relative value of IFD'IIP' to its absolute value, the following procedure was used. A least-squares fit of the values of IFD'/IP' a t various temperatures was made to eq V for coronene-hu and coronene-d12in PMM
Values of IFD'/I~'differ from values of I F D / I ~by a constant factor associated with different instrumental sensitivity to phosphorescence and delayed fluorescence which is, therefore, contained in b'. When I*D/Ip is substituted for (IFD'lIp') in eq V the slope is unchanged. The corrected value of b can be obtained by using the absolute value of IFDIIP,a t 23" together with the value of a determined from eq V. This normalized equation is
where the value of b is now free of any instrumental factor. (6) E. I. Hormats and F. C. Unterlietner, J . Phys. Chem. 6 9 , 3677 (1965) . (7) DER-332 is a water-white epoxy resin produced by Dow Chemical co. (8) W. H. Melhuish, J . Opt. Soc. Am., 54, 183 (1964).
Volume 71, Number 13 December 1967
JOHN L. KROPPAND WILLIAMR. DAWSON
4502
3 . Lifetimes of Phosphorescence. Values of phosphorescence lifetime TP were obtained from slopes of the logarithmic plots against time which were taken from the oscillograms of phosphorescence intensity used to obtain values IFD’IIP’.The measurements were extended down to -196” by cooling the sample in the dewar with liquid Ns. Measurements were made a t various times as the sample slowly warmed. 4. Fluorescence and Phosphorescence Yields. The fluorescence @F and phosphorescence @p yields of coronene in PAlM were measured by observing the luminescence intensity excited by 3130-A light from a low-pressure Hg lamp (Spectroline Hg-3) relative to the intensity obtained under similar excitation from a PMM specimen containing 1.0 X lO-3M pyrene whose fluorescence yield is known to be 0.61.8 The light from the Hg lamp was collimated, passed through an aqueous CoS04-NiS04 filter solution0 to isolate the 3130-A excitation, and directed alternately onto the quartzclad Pillill samples containing pyrene, coronene-h12, or coronene-dlSeach at 1.0 X 10-3 M . All samples had the same dimensions. Provision was made to place the front face of either the reference sample or the coronene sample in a reproducible position a t an angle of 45” to the direction of the excitation beam. This excitation is all absorbed in the surface layer of the samples. Luminescence emitted nearly normally through the the front face of PlliIhI samples was detected with an E l l 1 955SB photomultiplier tube. Two luminescence intensities It and I , from the PMM specimen were measured; 1,is the steady intensity obtained with the excitation light on and includes both fluorescence and phosphorescence, and I, is the intensity obtained immediately after the excitation light is turned off by rapidly closing a shutter. The value of I, was obtained from a photograph of the phosphorescence-decay curve on an oscilloscope. Values of @F and @p were obtained from I, and I, by comparison to the fluorescence intensity of the pyrene sample I r using @F @p
= 0.61(It - Ip)/Ir =
0.61 X 1.5(Ip/Ir)
(VW (VIII)
where 0.61 in eq VI1 and VI11 is the fluorescence yield of the pyrene specimen. The factor of 1.5 in eq VI11 is a correction for the greater sensitivity of the photomultiplier for the blue fluorescence of pyrene compared to the green phosphorescence of coronene. No phototube correct,ion is necessary for the fluorescence since the photomultiplier sensitivity was the same for the fluorescence of pyrene and coronene. The phosphorescence from pyrene in PMM was so weak that the total luminescence could be taken as I , without appreciable error. The Journal of Physical Chemistry
I
I
WAVELENGTH,
(A)
Figure 2. Delayed fluorescence (4000-4800 A) relative t o phosphorescence (5000-6300 A) of coronene& in PMM. The three delayed fluorescence curves are for different sample temperatures: (1) 67 loo, (2) 110 It loo, (3) 160 15”. The intensities of fluorescence in curve 3 should be multiplied by 2 to give the correct relative intensity. Phosphorescence intensities are all normalized to the same curve.
*
IV. Results and Discussion A. Rate Constants and Activation Energy. The delayed emission spectra of coronene at three temperatures in PMM are given in Figure 2. The phosphorescence spectrum is normalized to the same intensity at each temperature and the relative amount of delayed fluorescence compared to the phosphorescence is shown as the temperature is varied. At room temperature the delayed luminescence appears green, and only a small amount of light is emitted as delayed fluorescence. However, as the temperature is raised, the delayed emission becomes more blue, indicating that more light is emitted as delayed fluorescence. Above 100” the intensity of delayed fluorescence exceeds that of phosphorescence. Lifetime measurements show that the lifetimes of delayed fluorescence and phosphorescence are the same within 5% at all temperatures studied. Most values of IFD’/IP‘ were determined from the intensity as a function of temperature using phototube detection. The absolute values of IFD/IP deter= 0.025 for both coronene-hlz mined at 23” are IFD/IP and coronene-dlz. The plot of log (IFD/IP)us. 1/T for both coronene-$2 and coronene-dlz are given in Figure 3. The lines drawn correspond to least-squares fits of the is 0.025 at 23“ data that are adjusted so that IFD/IP (cf. eq VI1 and VIII). Some values of IFD’/IP’ for (9) E. J. Bowen, “The Chemical Aspects of Light,” Oxford University Press, London, 1946, p 279.
RADIATIGNLESS DEACTIVATION OF TRIPLET CORONENE IN PLASTICS
0.01
I2.5
1
1
I
I
I
2.7
2.9
3. I
3.3
3.5
T-’
lo3
(OK
Figure 3. Ratio of delayed fluorescence to phosphorescence IFD/ZPfor mronene in PMM vs. reciprocal temperature: A, coronenedln; A, coronene-h12; X , coronenedl, (data obtained from areas under spectra similar to those given in Figure 2 ) . Lines are least-squares fits to experimental points.
coronene-d12 in PMM determined by integrating the area under the curves of total emission spectra are included in Figure 3 and agree quite well with the data taken using photomultiplier detection at one wavelength. The value of (PFTP,Ok6 in eq IV is determined as the antilog of the intercept of the lines in Figure 3 at 1/T = 0 and is 2.8 X lo6 for coronene-dl2 and 7.9 X lo6 for coronene-h12. The value of AE can be determined from the slope and is 3840 and 4032 cm-’ for the deuterated and protonated samples, respectively. The intercepts in Figure 3 are i(D) = 6.45 and i(H) = 6.90. The difference is less than 10% and is within the experimental error. All of the data points in Figure 3 for both coronene-dl2 and coronene-h12 can be fitted to a common straight line of slope corresponding to 3930 cm-l and an intercept of 4.5 X lo6. I n either case the activation energy agrees reasonably well with the known energy gap of 3750 cm-l for both coronene-d12 and coronene-h12 determined from fluorescence and phosphorescence emission spectra.10 Values of &(H) and h ( D ) are needed in order to determine the values of k 6 ( H ) and k6’(D) from eq IV. The values of %(H), (PF(D), (Pp(H), and @P(D) are given in Table I. Values of (PF are accurate to lo%,
*
4503
and those of (PP to *20%. The ratios should be more accurate. The phosphorescence yield is enhanced three times upon deuteration. This increase in phosphorescence yield is expected since the lifetime of phosphorescence of coronene increases from 6 to 23 sec at room temperature upon deuteration. The different percentage increases in (PP and T P upon deuteration may well be due to experimental errors. The values of (PF(D) (PF(H)are the same within experimental errors, which agree with previous lifetime data for other aromatics gathered by Laposa, Lim, and I
