Delayed Fluorescence of Pyrene in Ethanol

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Published on 01 January 1963. Downloaded by University of Aberdeen on 25/08/2013 17:46:49.

Delayed Fluorescence of Pyrene in Ethanol BY C. A. PARKERAND C. G. HATCHARD Royal Naval Scientsc Service, Admiralty Materials Laboratory, Holton Heath, Poole, Dorset Received 7th August, 1962 The spectra of normal and delayed fluorescence from solutions of pyrene in ethanol at room temperature have been investigated. Both series of spectra show bands due to singlet excited rnonomer and excited dimer. At low pyrene concentration the relative intensity of the normal fluorescence of the dimer tends to zero but the relative intensity of the delayed fluorescence of the dimer tends to a constant finite value. The intensity of delayed fluorescence of both the monomer and the dimer is proportional to the square of the rate of absorption of exciting light. The delayed fluorescence is interpreted by a mechanism in which triplet-triplet quenching results in the formation of excited dimer molecules, some of which fluoresce and some of which dissociate into excited and ground-state singlet molecules. The results are discussed in relation t o the delayed fluorescence of anthracene and phenanthrene solutions, and it is concluded that the delayed fluorescence from all three substances is produced by the same mechanism.

Pyrene solutions show concentration quenching of the normal violet fluorescence accompanied by the appearance of a blue emission band which was shown by Forster and Kasper 1 to be due to an excited dimer formed by the combination of an excited singlet molecule with a molecule in the ground state. Most of the light emission in both spectral bands has a relatively short lifetime 2 but in de-oxygenated solutions Stevens and Hutton3 observed a long-lived component of the dimer emission. More recently, we observed delayed fluorescence from solutions of anthracene and phenanthrene. The intensity was proportional to the square of the rate of absorption of exciting light and we interpreted this by a mechanism in which the collision of two triplet molecules results in the formation of one excited and one ground-state singlet molecule.4~5 It seemed possible that a similar mechanism might be responsible for the delayed emission from pyrene solutions observed by Stevens and Hutton. The square-law dependence would then explain our earlier failure to observe it with low intensities of exciting light, although it was still difficult to explain why the delayed emission observed by Stevens and Hutton should show the band of the dimer but not that of the monomer. We therefore carried out some further measurements with pyrene solutions, using higher intensities of exciting light. We then observed delayed fluorescence from both the dimer and the monomer in solutions with widely varying pyrene concentration. The results of this investigation are presented in this paper. EXPERIMENTAL M A T ER I A LS

pure " pyrene contained in a glass tube lint. diam. 9 mm) was zone refined in an atmosphere of nitrogen. After 20 passes of the molten zone, some brown impurity had separated to the bottom of the column. The top and bottom of the column were rejected and the central portion, containing about half of the original material, was transferred to a smaller tube (int. diam. 6 m m ) and zone refined again. 1 cm from the top of the column was removed and about 3 cm immediately below this (map. 155°C corr.) was used for the tests. 8 g of

"

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C . A . PARKER AND C. G . H A T C H A R D

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The ethanol was purified as follows. 3 g of potassium hydroxide pellets were rinsed with ethanol to remove surface impurities and added to 3 1. of boiling ethanol. The ethanol was then immediately fractionated and the middle 50 % collected.


APPARATUS

The spectrophosphorimeter was similar in principle to that previously described 6.7 but the sensitivity was increased by using a Bausch and Lomb grating monochromator to analyze the luminescence spectra. A Hilger quartz prism monochromator was used, as before, to isolate either the 313 mp or the 366 m,u mercury lines used to excite the luminescence. Measurements were made at 23 f1 "C. The solutions were contained in cylindrical Pyrex or silica cells, 2.5 cm in diameter and with 2cm optical depth. The cells were fitted with a side-arm of diameter 1 cm for connection to the vacuum line. The solutions were de-oxygenated by the method previously described 6 except that an ethanol bath at - 110°C was used instead of liquid nitrogen, to avoid the risk of fracture of the cells. The air pressure remaining above the solution (isolated from the pump) before sealing offwas less than 2 x 10-5 mm Hg. Each solution was irradiated through one of the plane faces of the cell and the luminescence was viewed through the curved side of the cell. The exciting light was brought to a focus at the middle of the cell and the luminescence from the region between 0-5 and 1.5 cm from the front face was selected by means of a mask. For most solutions the optical density per cm for the exciting light was relatively small and it was assumed that the rate of light absorption was constant over the region viewed. The proportion of light absorbed in this region was calculated from the measured optical density of the solution and the incident light intensity. The incident light intensity at the focal point was measured by means of the ferrioxalate actinometer 8 and found to be approximately 2-9x 10-8 and 0 . 7 10-8einstein ~ cm-2sec-1 at 366 mp and 313 mp. The area of illumination was 0.1 sq. cm. The light intensities showed long-term fluctuations of up to 20 yo and correction for these was made using the readings of the fluorescent screen monitor.9 To measure the lifetime of the delayed fluorescence, the 800 c/sec sector discs were replaced by 100 c/sec discs and, with these rotating out-of-phase, the decay of luminescence was displayed on an oscilloscope as previously described.6 To obtain sufficient sensitivity for lifetime measurements, the band-width of the analyzing monochromator was increased to 23 mp. This was still sufficient to isolate the luminescence in one band from most of that in the other. The first-order plots of the luminescence decay were very nearly linear although in some cases there was slight evidence for the presence of a second-order component. Where the deviation from linearity was appreciable, the initial slope was used to calculate a first-order rate constant. The lifetimes found for the monomer and dimer bands in any one solution, measured with the same rate of absorption of exciting light, were in close agreement (the mean values are recorded in table 5), but the lifetimes were dependent on the rate of light absorption, and on the concentration of pyrene. The significance of this is discussed later. The lifetimes were all greater than 2msec and to convert the observed intensities of delayed fluorescence (measured with the 800 c/sec sectors) to total luminescence emitted per cycle, they had to be multiplied by a constant phosphorimeter factor 7 of 3. All results in the tables have been corrected for this factor. C O R R E C T I O N OF S P E C T R A

The analyzing monochromator was fitted with a 9558 QB photomultiplier. The spectral sensitivity curve of the combination was determined in the visible region with a calibrated tungsten lamp10 and in the ultra-violet region by means of the fluorescein monitor.11 Since the instrument recorded spectra which were linear in wavelength, the spectral sensitivity was calculated in relative units of quanta per unit wavelength interval (SA)so as to facilitate integration of the corrected luminescence bands. The spectra in fig. 1 and 2 are uncorrected. The relevant values of the correction factor ,!?A are as follows : ;ux) 425 450 475 500 550 600 350 375 mp -694 -828 -971 -757 -454 *224 -986 a953 SA *685


286

FLUORESCENCE OF PYRENE

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All spectra were measured with a monochromator bandwidth of 3-3mp. They consisted of only two spectral bands, that of the dimer with maximum (uncorrected) at 469 mp, and that of the monomer which showed several maxima, the one at 393 mp being selected for quantitative measurements because its intensity was not affected by self-absorption in concentrated solutions. Although the monomer and dimer bands overlapped, the intensity of the dimer emission at the monomer maximum, and the intensity of the monomer emission at the dimer maximum were both less than 2 % of their maximum values, so that the appropriate small corrections for overlap of the bands could be readily calculated and

wavelength, mp FIG.1 .-Normal fluorescence of pyrene in ethanol. (1) 3~ 10-3 M, (2) 10-3 M, (3) 3x 10-4 M, (4) 2x 1 0 - 6 ~ . The instrumental sensitivity settings for curves (1) and (4) were approximately 0.6 and 3.7 times that for curves (2) and (3). The short wavelength ends of the spectra in the more concentrated solutions are distorted by self-absorption.

applied to the observed intensities at the maxima. To convert the corrected intensities at the maxima to relative quanta emitted by the monomer or dimer, they had to be multiplied by the effective half-bandwidths of the respective bands. These were determined as follows. For the monomer, the normal fluorescence band in 2 x 10-6 M solution (where normal fluorescence from the dimer is negligible) was corrected for the spectral sensitivity of the apparatus (SA) and the integrated area of the corrected spectrum was divided by the uncorrected maximum value. A similar procedure was applied to the dimer band, using the spectrum of delayed fluorescence observed in 3 x 10-3 M solution, where the delayed fluorescence from the monomer was small and could be allowed for. DETERMINATION OF FLUORESCENCE EFFICIENCIES

The fluorescence efficiency of the monomer in 2x 10-6 M solution at 366 mp was determined by comparison 10 with a de-oxygenated solution of 10-6 M anthracene in ethanol

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C . A . PARKER A N D C . G . HATCHARD

contained in a similar cylindrical cell. Assuming a value of 0.27 reported by Melhuish 12 for the fluorescence efficiency of anthracene, the fluorescence efficiency of 2x 10-6 M pyrene was found to be 0.65. The fluorescence efficiency & of the monomer in the more concentrated solutions was then determined by comparing the intensity of fluorescence emitted at 393 mp with that emitted by the 2 x 10-6 M solution. By dividing these relative intensities by the fractions of exciting light absorbed by the solutions, the relative values of fluorescence efficiencywere calculated, and hence also the absolute values (see table 1). Since

I

400

360

440

480

520

wavelength, mp FIG. 2.-Delayed fluorescence of pyrene in ethanol. (3) 3 x 10-4 M, (4) 2~ 10-6 M. (2) 10-3 M, (1) 3 x 10-3 M, The instrumental sensitivity settings were approximately lo00 times greater than those for the corresponding curves in fig. 1. The short wavelength ends of the spectra in the more concentrated solutions are distorted by self-absorption.

the results depended upon comparison of solutions in different cylindrical cells, which varied slightly in shape, size and wall thickness, they are not so precise as those normally obtained by comparison in rectangular cells. A further source of error in the concentrated solutions TABLE1,xONCENTRATION QUENCHING concentration of pyrene C

fraction of light absorbed

3x10-3M

-377

10-3

-293 *120

3 x 10-4 10-4

2~ 10-5 2 x 10-6

OF NORMAL FZUORESCENCE AT relative fluorescence yield intensity of monomer of monomer fluorescence 4s

65.6

0-102 0.212 0.412

106.0 84.6 38.9 0-524 -0434 -0090 9.7 0.631 ~o0090 1-00 0.65 mean value of K = 20 X lo3 1. mole-].

366 mp K=

1 . 7 9 103 ~ 1. mole-1 2-07 1.93 2.40 (1.5)


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FLUORESCENCE OF P Y R E N E

was the f i c u l t y of determining precisely the fraction of light absorbed in the volume of solution viewed. Nevertheless, the Stern-Volmer quenching constants (K in table 1) are reasonably consistent and the mean value is close to that determined indirectly from measurements of relative intensities of monomer and dimer emission (see below). With each solution the ratio of the intensities of dimer emission to monomer emission were calculated both for normal fluorescence(+D/&) and for delayed fluorescence (&/OM). These values are shown in table 2 and the significance of the derived constants, K1, K2 and K3 is discussed later. In a separate series of experiments at measured rates of light absorption, the ratios of the efficienciesof delayed fluorescence to normal fluorescence of the monomer (OM/&) were measured and used to calculate the values of 8, in table 3. The significance of the last column in table 3 will be explained later.

TABLE 2.-RATIO

OF BAND INTENSITIES OF DIMER AND MONOMER FOR NORMAL AND DELAYED FLUORESCENCE

concentration of pyrene C

ratio for normal fluorescence

K

JD=

4D

4MC

G

3 x 10-3 M

5.20

10-3 3~ 10-4 10-4 2~ 10-5 2 x 10-6

1.80 0.514 0.157 0.031

1 . 7 3 103 ~ 1. mole-1 1*80 1.71 1.57 (1.5)

-

-

ratio for delayed fluorescence OD -

4=

(E-

Kz)/c

OM

9.35 4-18 1.69 0.970 0.777 0.706

2 . 8 8 ~103 1. mole-1 3.47 3.28 2.64 3.55

-

(= K21

mean values : K1 = 1.70 x 103 1. mole-1, K3 = 3 . 1 6 ~103 1. mole-1. TABLE3.-EFFICIENCY concentration of pyrene C

3x10-3 10-3 3 x 10-4 10-4 2~ 10-5 (no filter) 2~ 10-5 (with filter) 2 x 10-6

rate of light absorption,* einstein 1-1 sec-1

OF DELAYED FLUORESCENCE

efficiency of delayed fluorescence of monomer

lifetime of triplet,? SeC

OM -

4; Iur;

In

OM

=t

1-08x 10-5 0.837 0.342

5.o~ 10-3

0.125

1.49 x 10-4 4-45 9.88 17.0

0.212

51.9

9.4

1.1

0.0231

10.4

10-8

1.5

5.8 6.8 10.4

( 5 . 2 ~108) 1.7 0.9 0.9

1.3 10.6 mean = 1 . 2 108 ~ * Wavelength was 366 mp for the first 4 results and 313 mp for the last 3. t Lifetime of triplet assumed to be equal to twice the lifetime of delayed luminescence measured at the same rate of light absorption. 0-0329

13.4

V A R I A T I O N O F R A T E O F LIGHT A B S O R P T I O N

The intensity of delayed fluorescence in both monomer and dimer bands was measured as a function of the rate of absorption of the exciting light. To allow measurements to be made over a sufficiently wide range of intensities, the band width of the analyzing monochromator was increased to 1Omp. With each of the four solutions investigated the rate of light absorption was varied by inserting gauze screens of known transmission in front of the lens which focused the beam of exciting light on the samples. The ratios

C . A . P A R K E R A N D C. G . HATCHARD


of the intensities of the delayed fluorescence to the squares of the intensities of the exciting light, expressed in terms of the value obtained when no filter was interposed, are shown in table 4. TABLE 4.-RELATIONSHIP Published on 01 January 1963. Downloaded by University of Aberdeen on 25/08/2013 17:46:49.

BETWEEN INTENSITY OF DELAYED FLUORESCENCE AND RATE OF LIGHT ABSORPTION

filter transmission

(no filter)

relative value of

(Id2

3 x 10-JM

relative value of (delayed fluorescence)/(l$ 10-JM 2 x 1O-sM

-278

1-OOO -0773

-109

-0119

-082

*0067

1-00 0.89 0.93 0.87 0.96 0.91

-047

-0022

0-88 0.90

0.98

0.84

0.93

1-00 0.96

0.98 1a07 1.10 1*02 1-03

1a00 1-49 1*52 1.82 1.82 1.85 1.76 1.85 1.80

2 x 10-6M

1-00 0.94

0.97 1-04 0.91 0-97 1.05 1005 1.05

The fist of each pair of results refers to the relative intensity of delayed fluorescence from the monomer, the second of each pair refers to the dimer. The rates of light absorption without filter were approximately 1.0 x 10-5 and 0.8 x 10-5 einstein 1.-1 sec-1 at 366 mp for 3 x 10-3 M and 10-3 M, and 0-2 and 0-03 einstein 1.-1 sec-1 at 313 mp for 2 x l w M and 2 x 10-6 M. S P E C T R A OF N O R M A L A N D D E L A Y E D FLUORESCENCE

The spectra observed with four of the six solutions investigated are shown in fig. 1 and 2. The intensities of the monomer and dimer bands in all six solutions, measured under various conditions, are summarized in tables 1-4. The spectra of normal fluorescence are similar to those previously reported by Forster and Kasper.1 The relative intensity of the dimer band increases as the concentration of pyrene increases and the simultaneous reduction in fluorescence efficiency & of the monomer follows the Stern-Volmer quenching ~ 1. mole-1 (table 1). law with a mean quenching constant of 2 . 0 103 The relative intensity of the dimer band in the delayed fluorescence spectrum (fig. 2) is in every solution greater than that in the normal fluorescence spectrum (fig. 1). The most striking difference is shown in the 2 x 10-6 M solution where the efficiency of normal fluorescence of the dimer ($D) is negligible, but the efficiency of delayed fluorescence of the dimer (6,) amounts to 0-7 of the value for the monomer (OM) as shown in column 4 of table 2 (see also fig. 2, curve 4). It is clear from these results that the delayed fluorescence of the dimer cannot be accounted for by any mechanism involving direct quenching of excited singlet molecules since the encounter rate in 2 x 10-6 M solution is too low, as indicated by the absence of normal dimer fluorescence. Since the intensities of delayed fluorescence of both monomer and dimer bands were found to be proportional to the square of the rate of absorption of exciting light under nearly all conditions (see table 4 and below), the delayed fluorescence is assumed to be produced by a triplet-triplet quenching mechanism similar to that proposed for anthracene and phenanthrene.4~5 Thus, the sequence of reactions giving rise to singlet excited monomer and excited dimer as observed by normaZ fluorescence are as follows : hv

+S

-+

-+

s s* s; *. When the exciting light is shut off both excited states decay rapidly to much lower concentrations which are maintained by triplet-triplet quenching :

T+T-+SZ*+S*+S. Thus, in normal fluorescence, it is the singlet excited monomer which is first formed, while in delayed fluorescence it is the excited dimer which is first formed. Clearly, if S* and Sz*


290

F L U O R E S C E N C E OF P Y R E N E

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fluoresce before equilibrium between them is established the proportion of dimer observed in delayed fluorescence will always be greater than that observed in normal fluorescence, and as the concentration of pyrene is decreased the relative intensity of the delayed fluorescence of the dimer will decrease to a constant, finite value. This outline scheme thus accounts qualitatively for all the features shown by the spectra in fig. 1 and 2. The detailed reaction scheme shown in fig. 3 was therefore set up, to take into account all likely reactions of S* and S,**. In this scheme, larepresents the rate of absorption of exciting light, kfand k; the rate constants for the radiative transitions of monomer and dimer, and ko and kh their respective rate constants for radiationless conversion. kq is the bimolecular constant for triplet-triplet quenching, kt is the rate constant for triplet formation and kh the rate constant for its radiationless conversion (the constant for radiative transition of the triplet state has been omitted as it is negligibly small in solution at room temperature compared with kh). ku is the bimolecular rate constant for association of excited singlet monomer with ground state monomer and kd is the first-order rate constant for dissociation of excited dimer. Some quantitative tests of the validity of this reaction scheme are described in the following sections. R A T I O OF I N T E N S I T I E S OF MONOMER A N D D I M E R B A N D S

Under all conditions investigated the intensity of delayed fluorescence was small compared with that of normal fluorescence. Thus, when the exciting light is switched on we can ignore triplet-triplet quenching and consider the population of excited states (S*, S$*) to be maintained only by direct excitation from S to S*. The rate of formation of S j * is then equal to its rate of disappearance, i.e., Hence, the ratio of fluorescence efficiencies of dimer and monomer is given by

where c is the concentration of pyrene. When the exciting light is cut off, i.e., when the delayed fluorescence is measured, the population of excited states (S*, S!*) is maintained only by triplet-triplet quenching. Considering then the formation and lsappearance of s*, Hence the ratio of efficiencies of delayed fluorescence of dimer and monomer is given by

i.e., This expresses the fact that at low pyrene concentrationsOD/&, tends to the constant value K2, which from measurements with 2 x 10-6 M solution was found to be 0-706. Values of Kl and K3 for the other solutions were calculated (table 2) and found to be reasonably constant. A check on the consistency of the constants K1, K2 and K3 can be obtained from the measurements of the normal fluorescence quenching constant for the monomer (table 1). By considering the stationary concentration of S* for normal fluorescence, and applying the relationship between Sl* and S* given by eqn. (l), it can be shown that

The value of K calculated from the mean values of K1, K2 and K3 (table 2) was 2.1 x 103 1. mole+, i.e., in almost exact agreement with the mean observed value of 2Ox 1031. mole-1 (table 1).

C . A. P A R K E R A N D C . G. HATCHARD

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29 1

D E P E N D E N C E OF I N T E N S I T Y OF D E L A Y E D FLUORESCENCE O N T H E R A T E O F LIGHT ABSORPTION


In general, the rate of disappearance of triplet is equal to kh[T]+kq[T]2. At low rates of light absorption, the second term is small and the standing concentration of triplet during illumination is given by

kdTI=

(3)

Ia4t9

where 4t is the quantum efficiency of triplet formation and is given by

Using eqn. (3), and setting up the steady state equations for [S,**] in delayed fluorescence and [S*] in normal fluorescence it can be shown that at the low rates of light absorption to which eqn. (3) applies, the ratio of delayed to normal fluorescence of the monomer is given by

eM

Since &, is independent of the rate of light absorption la, should be proportional to I,, and the intensity of delayed fluorescence of the monomer should be proportional to 1;. Further, since OD/& is constant for any given solution (see eqn. (2)) the intensity of delayed fluorescence of the dimer should also be proportional to I:. Tests of the predicted square-law dependence are shown in table 4. For three of the solutions the law was obeyed, within experimental error, over the whole range of light intensities investigated, which varied by a factor of 20. In 2 x 10-5 M solution there was some falling off in intensity of delayed fluorescence at the highest rates of light absorption. The lifetime of the delayed fluorescence was also intensity-dependent (table 5 ) and it is possible that laand #t in this solution were both sufficientlyhigh to make the rate of triplettriplet quenching appreciable compared with its rate of radiationless conversion, and eqn. (3) would then no longer be valid. Eqn. (4)can also be used for a further test of the proposed mechanism, and to derive an approximate value for 4:, the triplet formation efficiency at infinite dilution. If we substitute

6t = +Mkt/kf, and

l/kh = r,,

where Z~is the lifetime of the triplet (which will be equal to twice the lifetime of delayed fluorescence 5 ), then eqn. (4)reduces to

The left-hand side of eqn. ( 5 ) should thus be constant for all solutions. The results in column 5 of table 3 show that, with the exception of the strongest solution for which the calculation of la was difficult, the values were constant to within a factor of 2. This is considered to be sufficient confirmation of the proposed mechanism in view of the errors involved in the various measurements and the fact that & appears in the expression as the cube. If it is assumed that triplet-triplet quenching is diffusion controlled, i.e., kq = 0-6x 1010 1. mole-1 sec-1, the mean value of OM/@&$ from table 3 can be substituted in eqn. ( 5 ) to calculate a value for &', which was found to be 0.13. LIFETIME OF D E L A Y E D FLUORESCENCE

At low light intensities, triplet-triplet quenching does not contribute appreciably to the rate of disappearance of triplet molecules and hence the triplet concentration during the dark periods should be proportional to exp (- kht). Since the rate of production of delayed


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fluorescence is proportional to [T]z, its intensity during the dark periods will be proportional to exp (-%hi). The delayed fluorescence should thus decay exponentially with a lifetime equal to one-half of that of the triplet molecules. The observed rates of decay were nearly exponential although there was slight evidence for the presence of a second-order component in some cases. The lifetimes varied with concentration of pyrene and with rate of absorption of exciting light. To facilitate comparison, the results are arranged in four groups (table 5 ) according to the approximate standing concentration of triplet molecules (as measured by I,+,, see eqn. (3)) at which they were observed, The lifetimes decreased with increasing concentration of triplet and this suggests that, at the higher values of I&,, the second order triplet-triplet quenching term was appreciable so that eqn. (3) was not strictly valid. For the 2 x 10-5 M solution, direct calculation seems to confirm this. Thus, the longest lifetime observed was 5.4 msec indicating a triplet lifetime of at least 11 msec for pyrene in ethanol at 23°C. With 4t = 0.13 and Ia = 0 . 2 10-5 ~ einstein 1.-1 sec-1, the calculated initial rate of triplet self-quenching is 0-5x 10-7 mole sec-1 compared with 2.6~ 10-7 mole sec-1 for the initial rate of fist-order decay. The fact that the 2 x 10-5 M solution did not obey the square law at high intensities (table 4) could thus be explained. It is, however, difficult to explain why the 3 x 10-3 M and 10-3 M solutions did obey the square law at high intensities, yet showed an appreciable decrease in lifetime. It would be desirable to repeat the lifetime measurements, using a more precise method which allowed the luminescence decay to be followed over several half-lives so that the question of a second-order component could be definitely settled.

TABLE ~ . - L ~ T I MOF E

DELAYED FLUORESCENCE

The values of l&t were within the following ranges : (a) 0.45-0.75, (b) 0.15-0.25, (c) 0.050.08, and (d) 0.02-0.03 x 10-6 mole 1.-1 sec-1. The maximum rates of light absorption were approximately twice as great as the maximum rates used for the results in table 4. w ncentrat ion of pyrent

3 x 10-3 M 10-3

3~ 10-4

lifetime of delayed fluorescence, msec (b)

2.1 2.1 2.2 2-3 2-7 2.8

2.4 2.6 2.9

3.1

3.4

3.7

3.8 3.9 4.7

5.2

10-4 2~ 10-5

(4

(a)

2.9 3.3

2 x 10-6

(4

3.0

4.6

5-4 4.2 4.4

5.3

For a given value of I,$,, the lifetimes show an appreciable decrease with increasing pyrene concentration. This may be due to quenching of the triplet by the pyrene itself, or by traces of impurity present in the pyrene. The lifetimes are, of course, also critically dependent on the efficiency of de-oxygenation of the solutions and slight variations from one cell filling to the next may thus account for part of the variation shown in table 5. Such variations will not, however, affect the validity of the calculations presented previously. S U M M A R Y OF CONSTANTS

In a previous paper 2 we measured the oxygen quenching of the normal fluorescence of the monomer and dimer in cyclohexane, from which limiting values for the lifetimes of the monomer fluorescence (T& and the dimer fluorescence (T,,) were calculated. If we assume these values to represent the lifetimes in ethanol, they can be combined with the

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293 data presented in the present paper to determine approximate values for all the constants shown in fig.3. Thus,


C. A. P A R K E R A N D C . G . HATCHARD

Hence,

To determine the corresponding constants for the h e r , we first calculate the values of (&+&) from the values of & in table 1, and the values of in table 2. It is found

FIG. 3.-Proposed mechanism for normal and delayed fluorescence.

that the values of (#,+&) are, within experimental error, independent of concentration of pyrene. This implies that KlK2 = K3-K1, and substituting the values from eqn. (1) and (3, k;/(k;+ kb) = 4; = 0.65.

But

and

+kh + kd) sec, Kl/K3 = kd/(k;+ kb +kd)= 0.54. TI) = l / ( k ;

N

Hence,

The association constant for the excited singlet monomer with a ground-state molecule can be determined from

+

K 3 / K 2= k a / ( k f +ko k,) = 4-5 x lo3 1. mole", whence

ka = 0.45 x lolo 1. mole-' sec", which is close to the diffusion-controlled rate ( 0 . 6 1010 ~ 1. mole-1 sec-I), a value which we have also assumed for kq.

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The longest lifetime of delayed fluorescence observed was 54msec. Our proposed mechanism implies that this is equal to one-half of the triplet lifetime. Hence This value will include a contribution due to bimolecular quenching by any impurities present in the solution. The absolute values of the constants can be regarded as only approximate because of the assumptions made about the values of zh(and q,,and the errors involved in determining the other data. They are considered, however, to be of at least the correct order of magnitude. It would be desirable to determine zEAand zD by direct measurement so that more precise values of the constants could be calculated. DISCUSSION The results obtained in this investigation provide strong evidence that the mechanism proposed in fig. 3 to account for the delayed fluorescence of pyrene is essentially correct. It is of interest to compare this mechanism with that proposed to account for the delayed fluorescence from anthracene and phenanthrene solutions in which only monomer emission was observed. We there assumed 41 5 that triplet-triplet quenching produced a species X which carried the energy from two triplet molecules and that this species then gave rise to an excited singlet molecule. It now seems likely that the delayed fluorescence from all three hydrocarbons is produced by the same mechanism. We should then identify the species X as an excited dimer which, for anthracene and phenanthrene, dissociates so rapidly into excited and ground-state singlet molecules that emission from the dimer itself is very small. (It is noted that the delayed fluorescence spectrum of anthracene previously reported 5 shows a slightly greater intensity in the long wavelength tail compared with the spectrum of normal fluorescence. This may be due to the presence of traces of impurity but it might also represent a very weak emission from an anthracene excited dimer and it is worth further investigation.) With regard to the mechanism whereby two triplet molecules can combine to form a relatively short-lived (and presumably singlet) dimer, it is of interest that Colpa,l4 on theoretical grounds, has already proposed the reuerse of this process to account for concentration quenching of fluorescence. He suggested that an excited singlet dimer, formed by combination of an excited singlet molecule with a normal molecule, could be converted to a second singlet excited dimer which would then dissociate to triplet molecules. We propose that delayed fluorescence is produced by the reuerse of this process, viz.,

s*+s. We think that delayed fluorescence of this kind may well be quite common among aromatic hydrocarbons and possibly amongst other fluorescent substances. It may also account for delayed fluorescence observed in the vapour state.15 The main pre-requisites are a relatively high fluorescence efficiency,a relatively high triplet formation efficiency, a relatively stable triplet state in fluid medium, and absence of oxygen or other quenching impurities. The delayed fluorescence produced by triplet-triplet quenching is to be sharply differentiated from that observed with compounds such as eosin6 or proflavine.16 The latter type of delayed fluorescence has the same lifetime as the triplet and its intensity is proportional to the first power of the rate of light absorption. It is produced by thermal activation of molecules from the triplet level to the excited singlet level and will occur with any substance for which the separation between these levels is sufficiently small. With anthracene, phenanthrene or pyrene, however, the separation is too great for this type of delayed fluorescence to be appreciable. To facilitate distinction between the two types we suggest that the first be called eosin-tjpe (or E-type) delayed fluorescence, and the second, pyrenetype (or P-type) delayed fluorescence. In an earlier paper6 it was shown how the observation of triplet-singlet phosphorescence could be used to investigate the lifetime and reactions of certain triplet molecules


C. A . PARKER AND C . G . H A T C H A R D


in solution. It now seems that the measurement of P-type delayed fluorescence can provide similar information and can be used for substances such as anthracene or pyrene with which the observation of triplet-singlet emission is difficult or impossible. Although neither method can be so universally applicable as flash absorption spectroscopy, they both have the advantage over the latter that much lower light intensities can be used and the d f i culty of isolating the first-order and second-order triplet decay processes avoided. This paper is published with the permission of the Superintendent, Admiralty Materials Laboratory. Forster and Kasper, 2. Elektrochem., 1955, 59, 976. Parker and Hatchard, Nature, 1961, 190, 165. 3 Stevens and Hutton, Nature, 1960, 186, 1045. 4 Parker and Hatchard, Proc. Chem. SOC.,1962, 147. 5 Parker and Hatchard, Proc. Roy. SOC. A , 1962, 269, 574. 6 Parker and Hatchard, Trans. Faraday SOC.,1961, 57, 1894. 7 Parker and Hatchard, Analyst, 1962, 87, 664. Hatchard and Parker, Proc. Roy. SOC. A , 1956, 235, 518. 9 Parker, Nature, 1958, 182, 1002. 10 Parker and Rees, Analyst, 1960,85, 587. 11 Parker, Anal. Chern., 1962, 34, 502. 12 Melhuish, J. Physic. Chem., 1961, 65, 229. 13 Jackson, Livingston and Pugh, Trans. Faraday Soc., 1960, 56, 1635. 14 Colpa, paper presented at the Fifth European Congress on Molecular Spectroscopy (Amsterdam, 1961). 15 Stevens, Nature, 1961, 192, 725. 16 Parker and Hatchard, J. Physic. Chem., in press. 1 2

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