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On the phosphorescence rise and decay processes of phenanthrene in EPA at 77°K
JOURNAL
OF MOLECULAR
SPECTROSCOPY
27,
450-460
(1968)
On the Phosphorescence Rise and Decay Processes Phenanthrene in EPA at 77°K
of
MINORU NAKAMIZO AND TAKAHIKO MATSUEDA Government
Industrial
Research
Institute,
Tosu, Saga-Pref.,
Japan
Phosphorescence rise and decay behaviors of phenanthrene have been studied in EPA at 77°K. The rise curves follow exactly an exponential law, and the rate constants obtained from the curves are always larger than or equal to the phosphorescence decay constant, depending on excitation intensity and wavelength. Fluorescence rise curves have been also studied in the same system and found to be time-dependent at t > 0.2 set after the excitation onset. The results obtained are interpreted in terms of a kinetic model involving a process of the depletion of ground-state molecules caused by excitation to their higher electronic levels, with subsequent population in the lowest excited singlet and triplet states. INTRODUCTION
Phosphorescence of aromatic compounds has been studied spectroscopically and kinetically in various media at low temperature by many workers (I-6). It is well known that the phosphorescence intensity decreases exponentially with a certain decay constant characteristic of phosphorescent molecules after excitation cutoff. Recently, the growth characteristics of triplet state in organic molecules under optical excitation have been studied by Kinoshita et al. (7) with luminescence method and by Brinen et al. (8) with ESR method. Before their works, no systematic investigation on them had been attempted. Kinoshita et al. measured the rise and decay times of the phosphorescence and delayed fluorescence of naphthalene and phenanthrene in glassy solutions and in mixed crystals at 77X, and showed from their experimental results and kinetic considerations that the processes of triplet-triplet absorption and of depletion of molecules in the ground state caused by excitation will make the rise constant larger than the decay constant at finite intensities of exciting light. It is true that the triplet-triplet absorptions are induced under intense excitation. However, the rapid radiationless transitions between the triplet states and the rise constants of the order of 0.2 to 0.8 set-’ suggest that the difference between the rise and decay constants should directly be related to the depletion of molecules in the ground state. We studied the growth and decay characteristics of the phosphorescence of phenanthrene in EPA at 77°K and obtained results which appeared to support 450
PHOSPHORESCENCE
RISE
AND
DECAY
PROCESSES
451
this latter view from the kinetic analysis of the rise and decay curves. The fluorescence growth characteristics, which give a direct evidence for t,he depletion of molecules in the ground state just after the excitation onset, have also been discussed in this paper. EXPERIMENTAL
DETAILS
The fluorescence rise and phosphorescence rise-decay behaviors of phenant)hrene \vere studied in EPA at 77°K. Phenanthrene in EPA at 77°K was suddenly exposed to an excitation light and the phosphorescence growth characteristics can be obtained with an emission monochromator fixed at 495 rnp corresponding to the peak of the phosphorescence. The phosphorescence decay characteristics are obtained after the excitation cutoff. The fluorescence growth characteristics are also obtained in the same way as for the phosphorescence. Jleasurements of the rise and decay were made on an Aminco-Keirs spectrophosphorimeter wit,h an RCA IP21 photomultiplier and a ;\latsushita VP-541A cathode-ray oscilloscope. The excitation source was a Hanovia Xenon 150-W arc lamp. The output of t,he photSomultiplier was direct’ly put, on t,he oscillosocpe. The photomultiplier and oscilloscope combination were tested for linearity of response in the range of t,he measurement. The source int,ensity was diminished successively by means of a calibrated set of metal gauze screens for st,udying the dependence of the rise and decay behaviors on an excitation intensit’y. A shut>ter was placed and controlled manually between the excitation source and t,he excitat,ion monochromator, instead of a phosphoroscope. Rise and decay times were analyzed graphicall? from photographs of the oscilloscope traces. Phenanthrene was obtained from Tokyo Kasei Co. It was recrystallized t,wo t,imes from 95 % ethanol and then heated wit’h maleic anhydride at, about 1iiRC for 20 min. After t’he solution was cooled to 7O”C, an aqueous solution of potassium hydroxide was added to the solution and reheated at 100°C for 30 min. The precipitat,e was filtered out, washed with water, dried and recrystallized three times from 95 Yoethanol. Finally it was distilled carefully over sodium. EPA was used as a solvent and purified in the same way as that described in a paper by Kanda et al. (9). RESULTS
The phosphorescence growth and decay of phenathrene were measured at various excit’ation intensit,ies and at three different excitat#ion wavelengths of 330 rnp, 292 rnp and 255 rnp, which correspond to the first, second and third electronic absorption bands of phenathrene, respectively. Typical rise curves obtained from the phosphorescence of phenathrene in EPA at 8.5 X 10e6 M are shown in Fig. 1 with two different intensities of the exciting light at 255 rnp. The phosphorescence increases in intensity with time just after the onset of the excitation and then reaches a steady-state intensity after about, 10 set at 100 70 intensity and about IS
452
NAKAMIZO
AND
MATSUEDA
set at 5 % intensity. When the excitation intensity is decreased, the rise process is slowed down and therefore the rise time, which is the reciprocal of the rise constant, at 5 % intensity of the excitation light should be longer than that at 100 % intensity. On t’he other hand, the decay rates, as expected, are hardly affected by a change in t’he excitation intensity. Fig. 3 shows a semilogarithmic plot of the
I_’
0
I
I
4
L
I
8 Time
e
12
I
a
I
16
(set)
FIG. 1. Phosphorescence rise curves of a 8.5 X lo-” iv phenanthrene in EPA at 77”K, recorded at two different excitation intensities. At lower excit’ation intensity the amplification is increased so as to make t)he amplit,ude on the oscilloscope approximately the same. &XC = 255 mp. A-G
FIG. 2. Semilogarithmic plot of the phosphorescence rise curves (-•-•-*-a-_) and curve (---a-•-•-a--) of phenathrene at variolts excitation intensities. A: at 100y6, B: at 61%, C: at 35%, D: at 207,, E: at 127,, I?: at 7.570, G: at, 5%. Concentration: 8.5 X IO-6 n/r. &,,, = 255 mp. decay
PHOSPHORbXENCE
RISE
AND
DECAY
PROCESSES
4*5:<
phosphorescence rise and decay curves for phenanthrene at different’ excitinglight intensities. For excitation \vith intensit’ies below 5 ‘G of the excitation-light int,ensity, the rise con&ants I\-ere found to be approximately equal to t’he deca?. const,ant from analyses of t’hese curves in Fig. 2. Those were 0.30 see-’ and 0.53 see -’ at 5 7 and 100 c’r int’ensities of t’he excitation light, respectively. The value of the decay constant, however, was independent’ of the excitation intensitv and found to be 0.S see-l over t’he whole int’ensity range of t’he excitation light, used. The dependence of the phosphorescence rise and decay t,imes on the excik~tion intens&y can be obtained with these rate constants for phenanthrene and is given in Fig. 3, together with that of t,he phosphorescence intensity under steady-stak conditions. A similar behavior of the phosphorescence rise and decay was also ohserved at excitation n-avelengt*hs of 292 rnp and 330 rnp. Values of the rate co11stants increase wit8h the order of the excitation wavelengths; 25.5 mp > 392 mp > 330 nip. This order is in good agreement lvith t,he order of the molar extinction coefficient of absorption band at each excitation wavelength in phenanthrene. Therefore, t,he phosphorescence rise processes depend on excitation wavelength as \vell :LSon excit#ation inten+. The fluorescence rise curves have also been studied at different) excitat’ion intensities and at different, excitaGon wavelengths I\-ith the same solution a~ that used for the phosphorescence. The curves were measured at 360 mpL?corresponding to t,he peak of the fluorescence of phenanthrene. The results obtained at, t#wo different, excitation intensities are shown in I’ig. 4, together with t,he phosphorescence rise-decay curves under the same condiGons. It should be noted thai under high intensity excitation the intensity of the fluorescence declines \vith time in a nearly exponential fashion to a ytendy-state intensity just after reaching XII initial intensity. Such a time dependency of the fluorescence rise process \\ns
0
20 40 60 80 Excitation Intensity (%).
100
FIG. 3. 1)ependence of the phosphoresence rise time (7~) and decay time (TV) of phenaut,hrene on excitat.ion intensity. Phosphorescence intensity, (I 0) p , under steady-state cow dit.iorrs are also included in the figure. Concentration: 8.5 X IO-6 M. A,,, = ZA5 mr.
454
NAKAMIZO
AND
MATSUEDA
FIU. 4. Fluorescence rise curves and phosphorescence at two different excitation intensities. Concentration: abscissae represent time in seconds.
rise-decay curves of phenanthrene 8.5 X 10S6 M. &,,, = 255 m,~. The
observed only from the dilute solutions a.t low temperature. A similar behavior of the fluorescence rise was also found in the systems of naphthalene, diphenyl and triphenylene in rigid glass solutions at 77°K. A difference between the initial and steady-state intensities decreases with decreasing intensity of excitation light as seen in Fig. 4 and depends on the excitation wavelength as in the case of the phosphorescence. During excitation at 5 % and 7.5 % intensities of the excitation light, the fluorescence intensities no longer change with time as is the case with the fluorescence of molecules at room temperature. Fig. 5 shows the effect of excitation wavelength on the fluorescence rise curves and the phosphorescence risedecay curves of phenanthrene. As is evident from the above experimental results, the fluorescence and phosphorescence rise processes depend on the intensity and the wavelength of the excitation light. These processes were also found to be dependent on the concentration. An increase in concentration gives the same effect for these curves as the excitation intensity is decreased. A semilogarithmic plot of the fluorescence rise curves gives a straight line and the rate constants obtained from a slope of this line are in general larger than the rise constants of phosphorescence under the same conditions. KINETICS
Decays of phosphorescence and delayed fluorescence have been satisfactorily interpreted by several authors (2-6) with a kinetic model. In the present paper, the kinetics of phosphorescence rise and decay processes are quantitatively treated in terms of the Jablonski model of Fig. 6, together with that of the fluorescence rise process. Here, no direct excitation of molecules to the triplet state is considered, since the Tl +-- So absorption probability of nearly all aromatic
PHOSPHORESCENCE 255
RISE
AND 292
n-p
DECAY
PROCESSES
ny
330
mJ.l
20
30
1,“1~~‘11~~~~~~~,~~‘~“‘~~~~l
0 FIG. 6. Fluorescence at different excitation time in seconds.
IO
20
30
40
0
IO
20
30
40
0
IO
40
rise curves and phosphorescence rise-decay curves of phenanthrene wavelengths. Concentration: 8..i X lOWfiM. The abscissae represent
First Excited SINGLET STATE S, ‘TATE J,
SO GROUND STATE FIG. 6. Schematic energy level diagram for the three electronic states. Constants kFQ, ky and kpg represent radiationless transition probabilities while constants kF and kp refer to the radiative kansition probabilities for fluorescence and phosphorescence, respectivel?r.
hydrocarbons is negligibly small compared with the SI +- So absorption probability. After the excitation onset the rates of changes in the concentration of molecules in the ground state, t’he lowest excited singlet state and the triplet state can be expressed in the following three differential equations with the rate const,ants defined in Fig. 6,
G”,lldt = kzbSi1-
k,[Z'I],
NAKAMIZO
456
AND MATSUEDA
where k, = RI(X)c(X),
kl = kF -I- lCFQ )
k = kp f
C-2)
kpQ,
[Sal + [&I + [Tll = A. Here SO,Sl and T1 refer to the ground state, the first excited singlet state and t,he triplet state of the molecules, respectively. Brackets represent the concentration of the molecule in each state at a time t after the excitation onset. The over-all rate associated with the production of 81 state is designated lc, , a quantity which depends on t&heexcitation-light intensity I expressed as a function of time and on t,he molar extinction coefficient Eof absorption band of the molecule at an excitation wavelength X. R is a constant appropriate to the optical system used and A t’he over-all concentration of the molecule. In order to solve these differential equations a stationary-state approximation is usually applied to IS,] under the assumption that just after the excitation onset a stationary-state is attained in a time which is very short compared with t#he phosphorescence process in question. However, this approximation cannot be applicable to the present case where the fluorescence isalso a time-dependent process at t > 0.2 set as seen in the preceding section. Substitution of [X1] from t#herelation (4) into Eq. (3) gives the equation as follows, d[!i”l]/rlt = - (kz + k,)[TJ
-
k,[&] + k,A.
(5)
From Eqs. (I), (3), (4) and (5)) the following second-order differential equation for [!P1]can be obtained,
where
(Yp = k,k, + kok, + k&3 + ksk, . The general solution for the homogeneous equation of Eq. (6) can be expressed in the form, [!iVJ = C1 exp (-at)
+ Cz exp (-Pi).
(SJ
With Eq. (8), one can easily obtain the following equation for [SD], KS] = (l/k*){
(CX- fc, -
ks)C1 exp (-at)
+ (P -
k, - ka)Cs exp C-P>).
(9)
A similar equation to that for [&I is also derived for [S,]. Constants Cl and C, in
PHOSPHORESCENCE
RISE
AXl)
DECAY
PROCESSES
457
volved in these equations can be determined in ordinary mathematic procedures \\ith the initial condition, [&I = A and [SJ = [Z’J = 0 at’ t = 0. Finally, one obtaiw the exact expressions for [A’,], [&I and [Y’,] as follows; [AS”]=
iz([k,(kl +
kz)/@]
+
[ko(a -
li2 -
k:O~a!ia! -
p)]
exp (-(Y,) (10)
+ [k0(P [S,] = SkO{ (k,/&)
Ii2 -
k:+)I@(@ -
CY!/LY((Y - pi] exp (-at)
+ [(k:, -
+ [C/i:, -
[Tl] =
d/i&2{
(l/C@)
-
a)] exp (--/3t)],
[l/CY(a -
/3)]
?Xtl
8)/8(8
-
cy)] exp
(--Pt)j,
(-Cd)
(1”) + [l,lB(P -
LY)]exp (--Pt)).
These expressions represent the concentration of the molecules in each state at :+ time t after the excitation onset. Con&ants 01and /3 in the above expressions can he qproximated satisfactorily n-&h the relation (7J,
provided that [&k2/(ko + /> /3, the second terms of Eqs. (lo), ( 11) and ( 12 J may be neglected without1 introducing any serious error into the results t(J be obtained. This is because of tjhe fact that exp ( -/3t) >> exp ( ---at J for times aftcl the excitation onset which arc long compared with (ko + k1 + k2)p1. Conscquently Eqs. ( lo)! (11) and i 12) can be approximated with sufficient accuraq. by the expressions, [iSo] = (A/~yflJ{&(kl
+ kq) -
k,(/3 -
[&l = ( Akoka/aB) { 1 -
[l -
[T1l =
exp ( -Pt ) ] .
( Ak,,k2/~@) ( 1 -
k, -
k,) exp (-@t)},
(,plX, J] exp ( -/It) 1,
= 1 -
{1 -
(15)
(18)
With these expressions the equations representing the rise of S1 fluorescence Tl phosphorescence wit,h t,imes are given by the follo\ving; (Z/10)F
(141
(&/x-3)} cxp i -/!3t ),
nrd
(171
:tr1d (I?‘IuJp = 1 -
respectively,
where 1” represents
exp i -$t 1,
the steady-state
intensity
(1s) of each emission
under
458
NAKAMIZO
AND MATSUEDA
excitation from the light source of constant intensity, and suffixes F and P refer to fluorescence and phosphorescence, respectively. Since the rise constant fi becomes equal to the phosphorescence decay constant k3 , as is evident from the relations (4) and (13) when the low intensity of the excitation light is used and/or the molar extinction coefficient at excitation wavelength is very small, Eqs. (17) and (18) can simply be expressed in the forms, and (I/~o)P = 1 -
exp (-k&),
(20)
respectively. This dependence of the rise constant 0 on the excitation intensity has been verified experimentally for both the fluorescence and phosphorescence of phenanthrene in dilute solution at 77°K as shown in Figs. 1, 2 and 4. DISCUSSION
Delayed fluorescence and phosphorescence decays of organic compounds have been quantitatively studied by many authors with a kinetic model, but, on the other hand, a similar treatment has never been made on the growth characteristics of Tl ---f So phosphorescence, much less those of &+ So fluorescence. We studied in detail the fluorescence rise behavior as well as the phosphorescence rise and decay behaviors of phenanthrene in EPA rigid glass at different excitation-light intensities and at different excitation wavelengths. Eqs. (17) and (18) are the general expressions for the rises of fluorescence and phosphorescence, respectively, valid when [4kokg/(ko + ICI+ kz - k3)2] << 1. The analysis of the phosphorescence rise curves at different excitation intensities leads to the conclusion that, in dilute solutions of phenanthrene, [4kokz/(ko + kl + k, - k3)‘] may be sufficiently small compared with unity even though the molecules were excited with the intense light at wavelengths of absorption bands having a large molar extinction coefficient. For example, we obtained for a 8.5 X 10e6 M solution of phenanthrene in EPA at 77”K, p -
k, = ( kokz/(ko + kl + kz -
k3) ] = 0.25 set-‘,
from the rise curve of phosphorescence under the excitation with a 100% lightintensity at 255 ml*. The rise constant /3of interest here depends directly on the over-all rate con stant k. and, therefore, on I(X) and c(X) as is evident from the relation (4). All the above formulas are derived on the basis of the depletion of molecules in the ground state caused by excitation to their higher electronic levels, with subsequent populations of the first excited singlet and triplet states. Therefore, the rise constant /Ican closely be related to the concentration of molecules in each state. Using the relation (7), CY~= (ko + kl + k2 { 123+ [ko ky’( ko + k~ + k2)] 1, one obtains the steady-state concentration of triplet-state molecules, 17’11s~= AI1 -
(&/‘P)),
(21)
PHOSPHORESCENCE and that of ground-state
RISE AND
lIECAY PROCESSES
-is9
molecules,
[S”]S, = A (h//3).
(2”)
Eq. (21) is the same equation as that derived by Alfimov et aZ. (IO j, and permits t,he determination of [TI]s~ when t’he rise and decay constants of phosphorescence are known. For example, n-e obtained from the solution containing S.5 X lo-” .I/ phenant,hrene in EPA at, 77”1\, [T&, = 4.0 X lo-” 31 and [&I,, = 4.5 X lo@ .I/ when the excitation was made 1vit.h a 100 7: light,-intensity at 25.5 mp. Under such conditions, the order of magnitude of [T 1]6T is comparabIe nit.h that of [S,I],VT. A:: is evident from Eqs. (14) and (161, t,he relat,ion, [So] + [TJ :x -4, alw:~~~ holds. This means that [&‘I]is negligibly small compared wit)h [So] and [T,] during the excitation. A decrease in the fluorescence intensity observed in the rise curve at t > 0.2 see gives only one evidence for the depletion of molecules in t)he ground stnt.cl. since the ratio of t’he steady&ate to t’he appareut initial iutensities of fluoresccwcr oht,ained with the exctation light of moderate iutensity is nearly equal t,o the ratio of the calculated [S&, to [So],=” in the ground state. Recently, Kinoshita et al. (7’) shot\-ed in t,heir &dies ou t’he phosphoresccnc*e gro\vt,h and decay of phenanthreue molecules that the diffcrencc ,9 - hi, depends on T, + T, absorption besides excit.atiou wavelength and int,ensity, solvents medium, solute concent,rat,iou, aud tempernt.ure, and that. t.he depeudcucc of 0 - /ca on the excitation wavelength contains information ou higher euerg! arc induced under hightriplet, states, Ti . It is true that the T, + T1 absorptions intensity excitation. But it does not seem that the rapid radiationless tJransitions between t,he lowest t,riplet’ state and higher-lying triplets are rat,e determining fat the phosphorescence rise processes having the rate constants of the order of 0.2 to 0.8 set-I. 111 the cases where there is an overlap between the phosphoresceurtt emission and the triplet-triplet’ absorption, however, t,he observed rise const,:mts should become larger than t’he true ones. Brinen et al. ( II ) report)ed the effecat of reabsorption of the phosphorescence by triplet’ molecules on kinetic processes involving population and depopulation of t,he lol\,est. triplet. state. They fourld that t.he phosphorescence rise curves of phenatlthrcne-tl10 m-e t~onexponertti:tl \vhen followed over several lifet,imes at wavelengths \\-here t,here ~vas an overlap between the phosphoresceuce emissiotl and ?‘i +- TI absorption, :uid that th(% effect, of reabsorption is to make the apparent phosphorescence rise time Tq shorter than the t’rue 74 . Deviation from the exponentiality was also fouud for the phosphorescence decay curves measured at wavelengths where the phosphorescence and the T, +- Tl absorption overlap, and the apparent, decay times were longer than the true decay t,ime 7P . On the ot)her hand, the agreement bytn-een ESR and phosphorescence determinations of T.~ and 7P occurs only where there is no appreciable Ti - TI absorption (11). Even for the phosphoresceuccb rise and decay constants measured at wavelengths where there is uo T< c T1 :tbsorption, however, ,f3is larger than ktr . This suggest.s t.hnt the phosphoresccwcr
460
NAKAMIZO
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MATSUEI)A
rise processes are to be independent of the Ti +- T1 transit8ions, and that’ the difference /3- lc:i may be attributed to other fact)ors t#hanthe transitions between triplets. The constant /3obtained from the fluorescence rise curves were generally larger than t’hose from the phosphorescence rise curves. The processes responsible for this discrepancy have not yet been identified, but it may be ascribed to the omission of the terms involving exp (--at) in Eys. (lo), (11) and (la), or there may be other factors affecting the fluorescence rise processes. A further study on this aspect, is presently in progress. In conclusion, the phosphorescence rise curves obtained here exactly obey an exponential law and can well be explained in t’erms of a kinetic model involving the process of the depletion of ground-state molecules caused by excitation, with subsequent populations of the first excited singlet and kiplet states. The rate constant ,8 obtained from the phosphorescence rise curves is always larger than or equal to the phosphorescence decay constant k3 , depending on the intensity and navelength of excitation light. High-intensity excitation at wavelengths of absorption bands having a large molar extirkon coefficient make a steady-state concentration of t’riplet molecules comparable wit’h t’hat of ground-state moleules. The relation, [TI] + [&I 2 A, always holds during excitation. Time-dependent behaviors were observed in the fluorescence rise curves at t > 0.2 set after excitation onset. This gives orrly one evidence for the deplet,ion of molecules in the ground state caused by excitation.
The authors wish to t,hauk Professor Yoshiya Kauda of Kyushu University for many helpful suggest.ious aud discussions during this work and I)r. H. Kakiyama of this Iustitttte for his interest aud eucouragemeut. Valuable commeuts ou this work by Professor Il. Sumi of Kurume Techuical College are gratefully acknowledged.
REcErvEd:
:1Iarch ‘3, 198s REFERENCES
1. 8. K. Low~za xm ?\l. A. EL-SAYI’;U, Chenl. Kev., 66, 199 (1966), aiid references therein: G. N. Lswrs _~NDM. K.~sH~, J. Am. Chem. Sot., 66, 2100 (1944). 1). 8. MCCLURE, J. Chem. Phys., 17, 905 (1949). E. H. GILMORX AND E. C. LIM, J. Phys. Ch.em., 63, 15 (1959). 9. H. STERNLIGHT, G. C. NIX~Z.~N,AND G. W. ROBINSON, J. Chem. Phys., 38, 1326 (1963). S. II. OLNESS :\ND H. SPONDJR,J. Chenz. Phys., 38, 1779 (1963). 4. T. AZUMI hlr;~ S. P. M&LYNN, J. Chern. Phys., 39, 1186 (1963). 5. 1). Y.0r.4~0~0, Nippon Kagaku Zasshi, 73, 739 (1952); 74, 8, 173 (1933). 6. S. K.vro END M. KOIZUMI, Bull. Chem. Sot. Japan, 27, 189 (1953); 30, 27 (1957). 7. NI. KINOSHITA, T. MISR.~, END 6. P. MCGLYNN, J. Mol. Speclry., 21, 333 (1966). 8. J. S. BRINEN, W. G. HODGSON A\~~1\1.K. ORI,OFF, J. Mol. Spectly., 23, 112 (1967). 9. Y. K.~Nu.I, AND R. SHIM.IDS, Spectrochim. dcta, 16, 211 (1959). 10. ;\I. V. ALFIMOV, N. Y.4. BUB~N, A. I. PRISTUP~, AND V. N. SHEMSHDX,Otp. Spectry., 20, 232 (1966). 11. J. S. BRINXN .\NDW. (;. HODGSOK, J. Chew. Phys., 47, 2946 (1967).
OF MOLECULAR
SPECTROSCOPY
27,
450-460
(1968)
On the Phosphorescence Rise and Decay Processes Phenanthrene in EPA at 77°K
of
MINORU NAKAMIZO AND TAKAHIKO MATSUEDA Government
Industrial
Research
Institute,
Tosu, Saga-Pref.,
Japan
Phosphorescence rise and decay behaviors of phenanthrene have been studied in EPA at 77°K. The rise curves follow exactly an exponential law, and the rate constants obtained from the curves are always larger than or equal to the phosphorescence decay constant, depending on excitation intensity and wavelength. Fluorescence rise curves have been also studied in the same system and found to be time-dependent at t > 0.2 set after the excitation onset. The results obtained are interpreted in terms of a kinetic model involving a process of the depletion of ground-state molecules caused by excitation to their higher electronic levels, with subsequent population in the lowest excited singlet and triplet states. INTRODUCTION
Phosphorescence of aromatic compounds has been studied spectroscopically and kinetically in various media at low temperature by many workers (I-6). It is well known that the phosphorescence intensity decreases exponentially with a certain decay constant characteristic of phosphorescent molecules after excitation cutoff. Recently, the growth characteristics of triplet state in organic molecules under optical excitation have been studied by Kinoshita et al. (7) with luminescence method and by Brinen et al. (8) with ESR method. Before their works, no systematic investigation on them had been attempted. Kinoshita et al. measured the rise and decay times of the phosphorescence and delayed fluorescence of naphthalene and phenanthrene in glassy solutions and in mixed crystals at 77X, and showed from their experimental results and kinetic considerations that the processes of triplet-triplet absorption and of depletion of molecules in the ground state caused by excitation will make the rise constant larger than the decay constant at finite intensities of exciting light. It is true that the triplet-triplet absorptions are induced under intense excitation. However, the rapid radiationless transitions between the triplet states and the rise constants of the order of 0.2 to 0.8 set-’ suggest that the difference between the rise and decay constants should directly be related to the depletion of molecules in the ground state. We studied the growth and decay characteristics of the phosphorescence of phenanthrene in EPA at 77°K and obtained results which appeared to support 450
PHOSPHORESCENCE
RISE
AND
DECAY
PROCESSES
451
this latter view from the kinetic analysis of the rise and decay curves. The fluorescence growth characteristics, which give a direct evidence for t,he depletion of molecules in the ground state just after the excitation onset, have also been discussed in this paper. EXPERIMENTAL
DETAILS
The fluorescence rise and phosphorescence rise-decay behaviors of phenant)hrene \vere studied in EPA at 77°K. Phenanthrene in EPA at 77°K was suddenly exposed to an excitation light and the phosphorescence growth characteristics can be obtained with an emission monochromator fixed at 495 rnp corresponding to the peak of the phosphorescence. The phosphorescence decay characteristics are obtained after the excitation cutoff. The fluorescence growth characteristics are also obtained in the same way as for the phosphorescence. Jleasurements of the rise and decay were made on an Aminco-Keirs spectrophosphorimeter wit,h an RCA IP21 photomultiplier and a ;\latsushita VP-541A cathode-ray oscilloscope. The excitation source was a Hanovia Xenon 150-W arc lamp. The output of t,he photSomultiplier was direct’ly put, on t,he oscillosocpe. The photomultiplier and oscilloscope combination were tested for linearity of response in the range of t,he measurement. The source int,ensity was diminished successively by means of a calibrated set of metal gauze screens for st,udying the dependence of the rise and decay behaviors on an excitation intensit’y. A shut>ter was placed and controlled manually between the excitation source and t,he excitat,ion monochromator, instead of a phosphoroscope. Rise and decay times were analyzed graphicall? from photographs of the oscilloscope traces. Phenanthrene was obtained from Tokyo Kasei Co. It was recrystallized t,wo t,imes from 95 % ethanol and then heated wit’h maleic anhydride at, about 1iiRC for 20 min. After t’he solution was cooled to 7O”C, an aqueous solution of potassium hydroxide was added to the solution and reheated at 100°C for 30 min. The precipitat,e was filtered out, washed with water, dried and recrystallized three times from 95 Yoethanol. Finally it was distilled carefully over sodium. EPA was used as a solvent and purified in the same way as that described in a paper by Kanda et al. (9). RESULTS
The phosphorescence growth and decay of phenathrene were measured at various excit’ation intensit,ies and at three different excitat#ion wavelengths of 330 rnp, 292 rnp and 255 rnp, which correspond to the first, second and third electronic absorption bands of phenathrene, respectively. Typical rise curves obtained from the phosphorescence of phenathrene in EPA at 8.5 X 10e6 M are shown in Fig. 1 with two different intensities of the exciting light at 255 rnp. The phosphorescence increases in intensity with time just after the onset of the excitation and then reaches a steady-state intensity after about, 10 set at 100 70 intensity and about IS
452
NAKAMIZO
AND
MATSUEDA
set at 5 % intensity. When the excitation intensity is decreased, the rise process is slowed down and therefore the rise time, which is the reciprocal of the rise constant, at 5 % intensity of the excitation light should be longer than that at 100 % intensity. On t’he other hand, the decay rates, as expected, are hardly affected by a change in t’he excitation intensity. Fig. 3 shows a semilogarithmic plot of the
I_’
0
I
I
4
L
I
8 Time
e
12
I
a
I
16
(set)
FIG. 1. Phosphorescence rise curves of a 8.5 X lo-” iv phenanthrene in EPA at 77”K, recorded at two different excitation intensities. At lower excit’ation intensity the amplification is increased so as to make t)he amplit,ude on the oscilloscope approximately the same. &XC = 255 mp. A-G
FIG. 2. Semilogarithmic plot of the phosphorescence rise curves (-•-•-*-a-_) and curve (---a-•-•-a--) of phenathrene at variolts excitation intensities. A: at 100y6, B: at 61%, C: at 35%, D: at 207,, E: at 127,, I?: at 7.570, G: at, 5%. Concentration: 8.5 X IO-6 n/r. &,,, = 255 mp. decay
PHOSPHORbXENCE
RISE
AND
DECAY
PROCESSES
4*5:<
phosphorescence rise and decay curves for phenanthrene at different’ excitinglight intensities. For excitation \vith intensit’ies below 5 ‘G of the excitation-light int,ensity, the rise con&ants I\-ere found to be approximately equal to t’he deca?. const,ant from analyses of t’hese curves in Fig. 2. Those were 0.30 see-’ and 0.53 see -’ at 5 7 and 100 c’r int’ensities of t’he excitation light, respectively. The value of the decay constant, however, was independent’ of the excitation intensitv and found to be 0.S see-l over t’he whole int’ensity range of t’he excitation light, used. The dependence of the phosphorescence rise and decay t,imes on the excik~tion intens&y can be obtained with these rate constants for phenanthrene and is given in Fig. 3, together with that of t,he phosphorescence intensity under steady-stak conditions. A similar behavior of the phosphorescence rise and decay was also ohserved at excitation n-avelengt*hs of 292 rnp and 330 rnp. Values of the rate co11stants increase wit8h the order of the excitation wavelengths; 25.5 mp > 392 mp > 330 nip. This order is in good agreement lvith t,he order of the molar extinction coefficient of absorption band at each excitation wavelength in phenanthrene. Therefore, t,he phosphorescence rise processes depend on excitation wavelength as \vell :LSon excit#ation inten+. The fluorescence rise curves have also been studied at different) excitat’ion intensities and at different, excitaGon wavelengths I\-ith the same solution a~ that used for the phosphorescence. The curves were measured at 360 mpL?corresponding to t,he peak of the fluorescence of phenanthrene. The results obtained at, t#wo different, excitation intensities are shown in I’ig. 4, together with t,he phosphorescence rise-decay curves under the same condiGons. It should be noted thai under high intensity excitation the intensity of the fluorescence declines \vith time in a nearly exponential fashion to a ytendy-state intensity just after reaching XII initial intensity. Such a time dependency of the fluorescence rise process \\ns
0
20 40 60 80 Excitation Intensity (%).
100
FIG. 3. 1)ependence of the phosphoresence rise time (7~) and decay time (TV) of phenaut,hrene on excitat.ion intensity. Phosphorescence intensity, (I 0) p , under steady-state cow dit.iorrs are also included in the figure. Concentration: 8.5 X IO-6 M. A,,, = ZA5 mr.
454
NAKAMIZO
AND
MATSUEDA
FIU. 4. Fluorescence rise curves and phosphorescence at two different excitation intensities. Concentration: abscissae represent time in seconds.
rise-decay curves of phenanthrene 8.5 X 10S6 M. &,,, = 255 m,~. The
observed only from the dilute solutions a.t low temperature. A similar behavior of the fluorescence rise was also found in the systems of naphthalene, diphenyl and triphenylene in rigid glass solutions at 77°K. A difference between the initial and steady-state intensities decreases with decreasing intensity of excitation light as seen in Fig. 4 and depends on the excitation wavelength as in the case of the phosphorescence. During excitation at 5 % and 7.5 % intensities of the excitation light, the fluorescence intensities no longer change with time as is the case with the fluorescence of molecules at room temperature. Fig. 5 shows the effect of excitation wavelength on the fluorescence rise curves and the phosphorescence risedecay curves of phenanthrene. As is evident from the above experimental results, the fluorescence and phosphorescence rise processes depend on the intensity and the wavelength of the excitation light. These processes were also found to be dependent on the concentration. An increase in concentration gives the same effect for these curves as the excitation intensity is decreased. A semilogarithmic plot of the fluorescence rise curves gives a straight line and the rate constants obtained from a slope of this line are in general larger than the rise constants of phosphorescence under the same conditions. KINETICS
Decays of phosphorescence and delayed fluorescence have been satisfactorily interpreted by several authors (2-6) with a kinetic model. In the present paper, the kinetics of phosphorescence rise and decay processes are quantitatively treated in terms of the Jablonski model of Fig. 6, together with that of the fluorescence rise process. Here, no direct excitation of molecules to the triplet state is considered, since the Tl +-- So absorption probability of nearly all aromatic
PHOSPHORESCENCE 255
RISE
AND 292
n-p
DECAY
PROCESSES
ny
330
mJ.l
20
30
1,“1~~‘11~~~~~~~,~~‘~“‘~~~~l
0 FIG. 6. Fluorescence at different excitation time in seconds.
IO
20
30
40
0
IO
20
30
40
0
IO
40
rise curves and phosphorescence rise-decay curves of phenanthrene wavelengths. Concentration: 8..i X lOWfiM. The abscissae represent
First Excited SINGLET STATE S, ‘TATE J,
SO GROUND STATE FIG. 6. Schematic energy level diagram for the three electronic states. Constants kFQ, ky and kpg represent radiationless transition probabilities while constants kF and kp refer to the radiative kansition probabilities for fluorescence and phosphorescence, respectivel?r.
hydrocarbons is negligibly small compared with the SI +- So absorption probability. After the excitation onset the rates of changes in the concentration of molecules in the ground state, t’he lowest excited singlet state and the triplet state can be expressed in the following three differential equations with the rate const,ants defined in Fig. 6,
G”,lldt = kzbSi1-
k,[Z'I],
NAKAMIZO
456
AND MATSUEDA
where k, = RI(X)c(X),
kl = kF -I- lCFQ )
k = kp f
C-2)
kpQ,
[Sal + [&I + [Tll = A. Here SO,Sl and T1 refer to the ground state, the first excited singlet state and t,he triplet state of the molecules, respectively. Brackets represent the concentration of the molecule in each state at a time t after the excitation onset. The over-all rate associated with the production of 81 state is designated lc, , a quantity which depends on t&heexcitation-light intensity I expressed as a function of time and on t,he molar extinction coefficient Eof absorption band of the molecule at an excitation wavelength X. R is a constant appropriate to the optical system used and A t’he over-all concentration of the molecule. In order to solve these differential equations a stationary-state approximation is usually applied to IS,] under the assumption that just after the excitation onset a stationary-state is attained in a time which is very short compared with t#he phosphorescence process in question. However, this approximation cannot be applicable to the present case where the fluorescence isalso a time-dependent process at t > 0.2 set as seen in the preceding section. Substitution of [X1] from t#herelation (4) into Eq. (3) gives the equation as follows, d[!i”l]/rlt = - (kz + k,)[TJ
-
k,[&] + k,A.
(5)
From Eqs. (I), (3), (4) and (5)) the following second-order differential equation for [!P1]can be obtained,
where
(Yp = k,k, + kok, + k&3 + ksk, . The general solution for the homogeneous equation of Eq. (6) can be expressed in the form, [!iVJ = C1 exp (-at)
+ Cz exp (-Pi).
(SJ
With Eq. (8), one can easily obtain the following equation for [SD], KS] = (l/k*){
(CX- fc, -
ks)C1 exp (-at)
+ (P -
k, - ka)Cs exp C-P>).
(9)
A similar equation to that for [&I is also derived for [S,]. Constants Cl and C, in
PHOSPHORESCENCE
RISE
AXl)
DECAY
PROCESSES
457
volved in these equations can be determined in ordinary mathematic procedures \\ith the initial condition, [&I = A and [SJ = [Z’J = 0 at’ t = 0. Finally, one obtaiw the exact expressions for [A’,], [&I and [Y’,] as follows; [AS”]=
iz([k,(kl +
kz)/@]
+
[ko(a -
li2 -
k:O~a!ia! -
p)]
exp (-(Y,) (10)
+ [k0(P [S,] = SkO{ (k,/&)
Ii2 -
k:+)I@(@ -
CY!/LY((Y - pi] exp (-at)
+ [(k:, -
+ [C/i:, -
[Tl] =
d/i&2{
(l/C@)
-
a)] exp (--/3t)],
[l/CY(a -
/3)]
?Xtl
8)/8(8
-
cy)] exp
(--Pt)j,
(-Cd)
(1”) + [l,lB(P -
LY)]exp (--Pt)).
These expressions represent the concentration of the molecules in each state at :+ time t after the excitation onset. Con&ants 01and /3 in the above expressions can he qproximated satisfactorily n-&h the relation (7J,
provided that [&k2/(ko + /> /3, the second terms of Eqs. (lo), ( 11) and ( 12 J may be neglected without1 introducing any serious error into the results t(J be obtained. This is because of tjhe fact that exp ( -/3t) >> exp ( ---at J for times aftcl the excitation onset which arc long compared with (ko + k1 + k2)p1. Conscquently Eqs. ( lo)! (11) and i 12) can be approximated with sufficient accuraq. by the expressions, [iSo] = (A/~yflJ{&(kl
+ kq) -
k,(/3 -
[&l = ( Akoka/aB) { 1 -
[l -
[T1l =
exp ( -Pt ) ] .
( Ak,,k2/~@) ( 1 -
k, -
k,) exp (-@t)},
(,plX, J] exp ( -/It) 1,
= 1 -
{1 -
(15)
(18)
With these expressions the equations representing the rise of S1 fluorescence Tl phosphorescence wit,h t,imes are given by the follo\ving; (Z/10)F
(141
(&/x-3)} cxp i -/!3t ),
nrd
(171
:tr1d (I?‘IuJp = 1 -
respectively,
where 1” represents
exp i -$t 1,
the steady-state
intensity
(1s) of each emission
under
458
NAKAMIZO
AND MATSUEDA
excitation from the light source of constant intensity, and suffixes F and P refer to fluorescence and phosphorescence, respectively. Since the rise constant fi becomes equal to the phosphorescence decay constant k3 , as is evident from the relations (4) and (13) when the low intensity of the excitation light is used and/or the molar extinction coefficient at excitation wavelength is very small, Eqs. (17) and (18) can simply be expressed in the forms, and (I/~o)P = 1 -
exp (-k&),
(20)
respectively. This dependence of the rise constant 0 on the excitation intensity has been verified experimentally for both the fluorescence and phosphorescence of phenanthrene in dilute solution at 77°K as shown in Figs. 1, 2 and 4. DISCUSSION
Delayed fluorescence and phosphorescence decays of organic compounds have been quantitatively studied by many authors with a kinetic model, but, on the other hand, a similar treatment has never been made on the growth characteristics of Tl ---f So phosphorescence, much less those of &+ So fluorescence. We studied in detail the fluorescence rise behavior as well as the phosphorescence rise and decay behaviors of phenanthrene in EPA rigid glass at different excitation-light intensities and at different excitation wavelengths. Eqs. (17) and (18) are the general expressions for the rises of fluorescence and phosphorescence, respectively, valid when [4kokg/(ko + ICI+ kz - k3)2] << 1. The analysis of the phosphorescence rise curves at different excitation intensities leads to the conclusion that, in dilute solutions of phenanthrene, [4kokz/(ko + kl + k, - k3)‘] may be sufficiently small compared with unity even though the molecules were excited with the intense light at wavelengths of absorption bands having a large molar extinction coefficient. For example, we obtained for a 8.5 X 10e6 M solution of phenanthrene in EPA at 77”K, p -
k, = ( kokz/(ko + kl + kz -
k3) ] = 0.25 set-‘,
from the rise curve of phosphorescence under the excitation with a 100% lightintensity at 255 ml*. The rise constant /3of interest here depends directly on the over-all rate con stant k. and, therefore, on I(X) and c(X) as is evident from the relation (4). All the above formulas are derived on the basis of the depletion of molecules in the ground state caused by excitation to their higher electronic levels, with subsequent populations of the first excited singlet and triplet states. Therefore, the rise constant /Ican closely be related to the concentration of molecules in each state. Using the relation (7), CY~= (ko + kl + k2 { 123+ [ko ky’( ko + k~ + k2)] 1, one obtains the steady-state concentration of triplet-state molecules, 17’11s~= AI1 -
(&/‘P)),
(21)
PHOSPHORESCENCE and that of ground-state
RISE AND
lIECAY PROCESSES
-is9
molecules,
[S”]S, = A (h//3).
(2”)
Eq. (21) is the same equation as that derived by Alfimov et aZ. (IO j, and permits t,he determination of [TI]s~ when t’he rise and decay constants of phosphorescence are known. For example, n-e obtained from the solution containing S.5 X lo-” .I/ phenant,hrene in EPA at, 77”1\, [T&, = 4.0 X lo-” 31 and [&I,, = 4.5 X lo@ .I/ when the excitation was made 1vit.h a 100 7: light,-intensity at 25.5 mp. Under such conditions, the order of magnitude of [T 1]6T is comparabIe nit.h that of [S,I],VT. A:: is evident from Eqs. (14) and (161, t,he relat,ion, [So] + [TJ :x -4, alw:~~~ holds. This means that [&‘I]is negligibly small compared wit)h [So] and [T,] during the excitation. A decrease in the fluorescence intensity observed in the rise curve at t > 0.2 see gives only one evidence for the depletion of molecules in t)he ground stnt.cl. since the ratio of t’he steady&ate to t’he appareut initial iutensities of fluoresccwcr oht,ained with the exctation light of moderate iutensity is nearly equal t,o the ratio of the calculated [S&, to [So],=” in the ground state. Recently, Kinoshita et al. (7’) shot\-ed in t,heir &dies ou t’he phosphoresccnc*e gro\vt,h and decay of phenanthreue molecules that the diffcrencc ,9 - hi, depends on T, + T, absorption besides excit.atiou wavelength and int,ensity, solvents medium, solute concent,rat,iou, aud tempernt.ure, and that. t.he depeudcucc of 0 - /ca on the excitation wavelength contains information ou higher euerg! arc induced under hightriplet, states, Ti . It is true that the T, + T1 absorptions intensity excitation. But it does not seem that the rapid radiationless tJransitions between t,he lowest t,riplet’ state and higher-lying triplets are rat,e determining fat the phosphorescence rise processes having the rate constants of the order of 0.2 to 0.8 set-I. 111 the cases where there is an overlap between the phosphoresceurtt emission and the triplet-triplet’ absorption, however, t,he observed rise const,:mts should become larger than t’he true ones. Brinen et al. ( II ) report)ed the effecat of reabsorption of the phosphorescence by triplet’ molecules on kinetic processes involving population and depopulation of t,he lol\,est. triplet. state. They fourld that t.he phosphorescence rise curves of phenatlthrcne-tl10 m-e t~onexponertti:tl \vhen followed over several lifet,imes at wavelengths \\-here t,here ~vas an overlap between the phosphoresceuce emissiotl and ?‘i +- TI absorption, :uid that th(% effect, of reabsorption is to make the apparent phosphorescence rise time Tq shorter than the t’rue 74 . Deviation from the exponentiality was also fouud for the phosphorescence decay curves measured at wavelengths where the phosphorescence and the T, +- Tl absorption overlap, and the apparent, decay times were longer than the true decay t,ime 7P . On the ot)her hand, the agreement bytn-een ESR and phosphorescence determinations of T.~ and 7P occurs only where there is no appreciable Ti - TI absorption (11). Even for the phosphoresceuccb rise and decay constants measured at wavelengths where there is uo T< c T1 :tbsorption, however, ,f3is larger than ktr . This suggest.s t.hnt the phosphoresccwcr
460
NAKAMIZO
AND
MATSUEI)A
rise processes are to be independent of the Ti +- T1 transit8ions, and that’ the difference /3- lc:i may be attributed to other fact)ors t#hanthe transitions between triplets. The constant /3obtained from the fluorescence rise curves were generally larger than t’hose from the phosphorescence rise curves. The processes responsible for this discrepancy have not yet been identified, but it may be ascribed to the omission of the terms involving exp (--at) in Eys. (lo), (11) and (la), or there may be other factors affecting the fluorescence rise processes. A further study on this aspect, is presently in progress. In conclusion, the phosphorescence rise curves obtained here exactly obey an exponential law and can well be explained in t’erms of a kinetic model involving the process of the depletion of ground-state molecules caused by excitation, with subsequent populations of the first excited singlet and kiplet states. The rate constant ,8 obtained from the phosphorescence rise curves is always larger than or equal to the phosphorescence decay constant k3 , depending on the intensity and navelength of excitation light. High-intensity excitation at wavelengths of absorption bands having a large molar extirkon coefficient make a steady-state concentration of t’riplet molecules comparable wit’h t’hat of ground-state moleules. The relation, [TI] + [&I 2 A, always holds during excitation. Time-dependent behaviors were observed in the fluorescence rise curves at t > 0.2 set after excitation onset. This gives orrly one evidence for the deplet,ion of molecules in the ground state caused by excitation.
The authors wish to t,hauk Professor Yoshiya Kauda of Kyushu University for many helpful suggest.ious aud discussions during this work and I)r. H. Kakiyama of this Iustitttte for his interest aud eucouragemeut. Valuable commeuts ou this work by Professor Il. Sumi of Kurume Techuical College are gratefully acknowledged.
REcErvEd:
:1Iarch ‘3, 198s REFERENCES
1. 8. K. Low~za xm ?\l. A. EL-SAYI’;U, Chenl. Kev., 66, 199 (1966), aiid references therein: G. N. Lswrs _~NDM. K.~sH~, J. Am. Chem. Sot., 66, 2100 (1944). 1). 8. MCCLURE, J. Chem. Phys., 17, 905 (1949). E. H. GILMORX AND E. C. LIM, J. Phys. Ch.em., 63, 15 (1959). 9. H. STERNLIGHT, G. C. NIX~Z.~N,AND G. W. ROBINSON, J. Chem. Phys., 38, 1326 (1963). S. II. OLNESS :\ND H. SPONDJR,J. Chenz. Phys., 38, 1779 (1963). 4. T. AZUMI hlr;~ S. P. M&LYNN, J. Chern. Phys., 39, 1186 (1963). 5. 1). Y.0r.4~0~0, Nippon Kagaku Zasshi, 73, 739 (1952); 74, 8, 173 (1933). 6. S. K.vro END M. KOIZUMI, Bull. Chem. Sot. Japan, 27, 189 (1953); 30, 27 (1957). 7. NI. KINOSHITA, T. MISR.~, END 6. P. MCGLYNN, J. Mol. Speclry., 21, 333 (1966). 8. J. S. BRINEN, W. G. HODGSON A\~~1\1.K. ORI,OFF, J. Mol. Spectly., 23, 112 (1967). 9. Y. K.~Nu.I, AND R. SHIM.IDS, Spectrochim. dcta, 16, 211 (1959). 10. ;\I. V. ALFIMOV, N. Y.4. BUB~N, A. I. PRISTUP~, AND V. N. SHEMSHDX,Otp. Spectry., 20, 232 (1966). 11. J. S. BRINXN .\NDW. (;. HODGSOK, J. Chew. Phys., 47, 2946 (1967).