Microwave induced delayed phosphorescence

Volume 4, number 6

CHEMCAL PHYSICSLETTERS

MICROWAVE

INDUCED

DELAYED

I December

1969

PHOSPHORESCENCE

J. SCHMIDT, W. S. VEEMAN and J. H. VAN DER WAALS Kamerlingh

Onnes Laboraton’um,

Received

Leiden,

9 October

The Netherlands

1969

When the molecules populating a phosphorescent triplet state decay at very low temperature, thermal equilibrium between the three spin levels is no longer maintained and they decay independently. A technique has been developed to utilize this situation for measuring decay rates of individual spin levek by observing the effect on phosphorescence decay of the sudden saturation of a microwave transition between a pair of spin levels.

1. INTRODUCTION In a previous letter we demonstrated the feasibility of observing zero-field transitions in phosphorescent triplet states by means of optical detection [l]. A further result of this study [l, fig. 4] was the observation of an effect loosely called “light echo”, but perhaps more properly referred to as microwave induced delayed phosphorescence. We shall show in the present paper how this effect can be developed into a simple method for measuring decay rates of the individual sublevels of the triplet state. By way of illustration we present experimental results on quinoline (1-azanaphthalene) and isoquinoline (2-azanaphthalene) as guests in durene crystals. Remarkable differences in lifetimes are found which together with former results on quinoxaline (l&diazanaphthalene) [2] provide information specifically related to spin-orbit coupling (SOC) mechanisms 2. PRINCIPLE

in aza-aromatics.

OF THE METHOD

AND RESULTS

The zero-field splittings of the lowest nip* triplet states of quinoline and isoquinoline are known from the ESR experiments of Vincent and Maki [3]. By analogy with quinoxaline we assume their spin components TX, TY, TZ to lie in order of increasing energy, with x assigned to the axis perpendicular to the molecular plane. (The directions of the in-plane principal axes of the fine structure tensor here no longer follow from molecular symmetry. ) In fig. 1 we have indicated these zero-field splittings together with the two kinds of rate processes that govern the decay from the triplet manifold Wfiv the rate of spin reorientation TCc+ T, ;

quinoline Y’

isoquinoline

r’ig. 1. The zero-field splittings of the lowest tripIet state of quinoline and isoquinoline. The symbol k@L) (/.L= X, g, z) stands for the total rate oE decay of compoT,J and k(F) for the radiative part (CAL arrow)_ The transitions used to induce delayed phosphores-

nent

cence signals are indicated in AIKz.

k(p) the rate of decay, radiative plus radiationless, from the component Tp to the ground state (lz, v = %Y, 2). The probabilities Wpy, like other magnetic relaxation rates, were found to decrease rapidly with temperature [ 11, whereas in general the decay rates k(p) may be expected to remain constant at low temperature [4]. Consequently, for quinoline, isoquinoline, and many other molecules by cooling to 1.3EJ°K a situation is created where << ;@), g(Y), &3 (PI v = GY,Z) * (1) wpV 341

Volume 4, number 6

CHEMICAL PHYSICSLETTERS

This implies that the sublevels are effectively isolated from each other and deca independently with decay rates k(%), k(Y), k(Z7. We first consider the specific case of quinoline for which it is found that depopulation, radiative as well as non-radiative, is fastest from the top spin component Tz, ‘ust as for quinoxaline [5]. Thus k@) >> #), k 1y). The delayed phosphorescence technique makes use of this situation in the following manner. First the sample is irradiated for some time at 1.35OK and a steady state population established. Then the exciting light is shut off at a certain moment t = 0 and the phosphorescence decay monitored with a photomultiplier COMeCkd to an oscilloscope. The signal one observes initially is almost entirely due to the fast exponential decay from the emitting top level T,, while molecules in the nonradiative levels ‘I+ and TX mainly return to the ground state without contributing to the light intensity. Then, at a time t one suddenly sweeps through the zero-field transition at 3585 MHz or

Fig. 2. Two superimposed decays of phosphorescent quinoline at 1.350K with delayed phosphorescence signals induced by sudden saturation of the 1000.3 MHz transition at two different times. The difference in height of the two signals shows the decrease of the population of the nonradiative sublevel Ts’with time. Horizontal scale 0.5 set per division.

1 December 1969

1000.5 MHz of fig, 1. In this way molecules are taken from the slowly decaying non-active levels TX or TV to repopulate the T, level and a sharp increase in light intensity is observed (fig. 2). It is easy to show that the height of the leading edge of the delayed signal hCc’z caused by microwave saturation at time t obeys the proportionality J#-+%)

= C(Nlr(t)-N,(t)}(k~)-K~~)~,

(2)

N&) is the number of molecules populating Tp at time t, kip) is the probability of radiative decay from Tp, I.L= x,y. Provided the delay is long enough, e.g. t a 5[k@)lm1, we may put N-(t) = 0 and find that to a good approximation hCr z(t) is proportional to Np( t)_ E@ repeating the experiment with different values of t one obtains a measure for the number of molecules in the nonactive levels TX and Tu as a function of time (fig. 2) and so finds their total decay rates k(x) and k(Y), see table 1. Let us now turn to isoquinoline. Here the situation is slightly different because it is found that appreciable radiative depopulation not only occurs from the top component TZ but also from Ty. Hence, immediately after shutting off the exciting light one observes a signal which is the sum of the decay of these two levels. Delayed phosphorescence signals can be generated by saturating either the 3335 MHz or the 2655 MHz transition (fig. l), which repopulate T, or Ty from the slowly decaying non-active level TX. Saturation of the Ty-TZ transition, however, does not lead to a measurable si al, because of the near-equality of k(z) and kb 8”. This situation allowed us to make the following measurements for isoquinoline (see fig. 3). 1. Decay rate kb) of the notbradiative level TX The heights of the signals generated by the delayed microwave pulses at 3335 MHz and 2655 MHz are given by

Table 1 of individual spin components of the triplet state. The values for the aza-aromatics were derived from the present type of experiment at 1.350K and have an estimated limit of error of *lo%, The values in the last column for nnphthalene were obtained from ESR experiments by Schwoerer and Six1 [17] at 4.20K.

Decay_ rates (in set-I)

Perdeuteroquinoxaline in durene [2]

k(z)

342

6.7

k(fl

=s 0.28

k!x)

G 0.24

Quinoxalire in perdeuteronaphthalene [2] 11.1 1.0 0.34

Quinoline in durene

Isoquinoline in durene

Naphthalene

r171

3.1

3.0

0.65

0.37 0.20

2.9

0.40

0.38

0.15

VoIume 4, number 6

CHEMICAL PHYSICS LETTERS

1 December I969

. hX-z(t)

= C{NJt) - ivp)}Ik~)

- k(,x$,

(3)

If again the delay time is sufficiently long we may neglect Nz(t) and NY(t) in eqs. (3) and (4). In each case the signal then is proportional to N%(t) and the decay rate k(x) of TX follows in the same way as for quinoline. Both transitions were used for the measurement and it was gratifying to find that they led to the same result for &) to within 5%. 2. Ratio of the radiative

If kb)

rates kp) and kg). kb) >>kb) r then eqs. (3) and (4) for a

r ’ r sufficiently long delay time simplify to hx-z(t)

z CiV,,(t)k(rZ) ,

(5)

kry(t)

= C N’(t) key) _

(6)

Hence, the ratio of hX2(t) and F-J’(t) at the same t gives us the ratio of k$?) and k$‘)_ Experimentally we found @) _ Kr$) = 1 : 2.9. 3. Total decay rates k(z) a~2 k(y) . The delayed microwave pulses repopuIate selectively either TZ or TY from TX, depending on the microwave frequency used. Hence the tail of a delayed signal represents the decay of one sublevel only and directly yields its decay rate. This value is difficult to obtain from the signal immediately after shutting off the exciting light, because it is composed of the combined decay of TZ and TY. To demonstrate the essence of things we have talked in terms of limiting situations where molecules are transferred from a non- (or weakly) emitting occupied level to a strongly emitting empty level. In actual practice one has to keep in mind that the decay is governed by a set of coupled differential equations in which all six constants WcLVand k(p) occur. Thus, in order to

obtain reasonable precision a more sophisticated analysis is needed in many cases. Moreover, the initial steady state situation is often affected by preferential population on intersystem crossing (spin alignment) [5.6]. So, on shutting off the light it may happen that, say IV,(O) << &(a) and thus the condition given for the lower Limit of the delay time, t 2 5[k(Z)]-1 in our first esample may be insufficient. In the case of isoquinoltie, for instance, the data recorded in table 1 were obtaind from experin;?nts with delay times ex-

Fig.

3. Decay

of phosphorescent

isoquinoline

at 1.35°K;

horizontal scale 0.5 set per division. a, Delayed signal

induced by saturating the 3335 MHz TX-Tz transition. b. Delayed signal induced by saturating the 2655 MHz TX-TV transition. The delay time is the same for a and b, and the difference in height of the two signals thus reflects the difference in the radiative rates k(“) and kq).

ceeding 5 sec. Actually, the photographs fig. 2 and fig. 3 served as an illustration to explain the method, and in our quantitative experiment we used a slightly more refined technique, because of the complications just mentioned_ &I example is shown in fig. 4, where the traces A-C represent the decay of isoquinoline at 1.35oK under different experimental conditions_ All traces start 4.8 set after shutting off the exciting Eight and the time scale is 0.1 set per division. Trace A re_uresents the “normal” decay in the absence of microwaves; trace B is the delayed signaL induced by a sudden sweep through the T,u T, transition at 2655 MHz and C the decay observed when the T,v -T

y

transition

is corrtkzcorcsl_v

sat-

urated_ In the latter case the slowly decaying level TX is kept in contact with TV and the two levels decay together; at the moment trace C starts (t = 4.8 set) we may then assume TX to be 343

Volume 4, number 6

CHEMICAL

PHYSICS

LETTERS

(approximately)

1. December planar

one of these

axes,

1969

in our

convention the xaxis, must be perpendicular to the molecular plane. For the in-plane y and z spin axes the situation differs for the four molecules: in naphthalene and quinoxaline they are by symmetry constrained to lie along, and perpendicular to the central bond; for the two others this is no longer true. Yet, in quinoline the y and z spin axes probably still are quite close to the y ’ and z’ axes drawn in fig. 1, but in isoquinoline this is at best only approximately so [3,7]. From the theory of spin-orbit coupling [8-lO]t it follows that for TZ* triplet states of (hetero) aromatic systems Fig. 4. The decay of phosphorescent isoquinoline at 1.35OKunder different experimental conditions. All traces start 4.S set after shutting off the exciting light and the time scale is 0.1 set per division. A: normal decay in the absence of microwaves. B: delayed signal induced by a sudden sweep through the TX-T,, transition at 2655 MHz, C: decay observed when TX- T,, is continuously saturated, D: zero signal. We believe the difference between C and D to be due to s!ow deactivation of phosphorescent impuriiies in the durene crystal. empty. Hence the difference between A and C must be caused by molecules in TX, which in case A contributed to the light intensity with their weak radiative decay h(F or via k({) and k(g) with the help of FvXyand IV,,. Now, at the moment we sweep through the transition in B this contribution to the light from molecules left in TX is suddenly halved as the populations of T, and TJI are equalized by the microwaves. Accordingly, we assumed the base line of the delayed signals to be halfway between A and B. One of the advantages of this procedure is that eqs. (5) and (6) then hold, even if k($) is not quite negligible. 3. ZRELIMlNARY

DISCUSSION OF RESULTS

The purpose of the present paper is to present a new method, rather than an interpretation of the data obtained Yet, a few features stand out for brief comment. When thinking about our results for quinoline and isoquinoline it is helpful to make a comparison with naphthalene and quinoxaline. The four molecules have very similar no* lowest triplet states, as borne out by their phosphorescence, zero-field splitting and spin density distribution

PI* As before,

X, y, z label the principal axes of the zero-field splitting tensor, further referred to as “spin axesR. Since all four molecules are 344

I the TX components couple with ZZ* singlet states; II the Ty and (or) Tz components couple with (a) irdc and OZ* singlet states, and if the molecule is a heterocyclic one, with (b) n7;* singlet states. Here o(8) are bonding (antibonding) core orbitals and n is the special case of a “lone pair” orbital on e.g. nitrogen. Coupling Ha is known to be stronger than I by an order of magnitude, and whenever it arises, Il’b is much more effective again - as demonstrated by the remarkable effect of nitrogen substitution on intersystem crossing and phosphorescence lifetime [13]. Further, in scheme II the amount of singlet character acquired by Ty and Tz individually depends on the direction relative to the y, z spin axes of the u, ti or n orbital(s) involved [lo]. In discussing decay rates we have to take account of two factors. Firstly, the probability for radiative as well as radiationless decay from a given component TP depends on the amount of singlet character T, acquires through spin-orbit coupling. (The admixture of TP-character into the ground state also is effective, but in the present symmetry-oriented discussion there is no need to consider it explicitly.) Secondly, for radiative decay to occur to an appreciable extent from TP t its total spin @ orbit symmetry has to comply with the selection rules for electric dipole radiation from Tp to the ground state. (We tacitly ignore any selection rules for the radiationless decay, as very little is known about them.) Let us now turn to the data of table 1. The first thing to note is that k(x) is the smallest in every column; presumably because T, is least contaminated with singlet character owing to the $ For references to subsequent work on spin-orbit coupling see e.g. Hameka [ll] and van der Waals and de Groot [ 121.

Volume 4, number 6

CHEMICAL PHYSICS LETTERS

ineffectiveness of scheme L Also, since the nitrogen lone pair orbital(s) do not enter into this scheme, it is gratifying to find that I&) is only slightly affected by nitrogen substitution_ The rate constants K(Y) and k(z) offer a different picture. In naphthalene and quinoxaline the total symmetry of the components TZ and TY is such that “allowed” radiative decay can only occur from TZ - in agreement with experiment. From the radiative lifetime given by Bennett et al. [14] one estimates k(:) = 0.10 set-1 for naphthalene. A corresponding value for quinoxaline cannot be given, but it should be close to the 6.7 SeC-1 total rate COILStaIIt measured for C6Np6; an enhancement which reflects the strong spinorbit coupling provided by Ilb. As to the radiationless parts of &‘) and @), it appears from the data that these primarily depend on the relative amount of singlet character mixed into Ty and Tz_ In naphthalene spin-orbit coupling (by Ha) is between the r or ii* orbitals and the C-C and C-H sigma orbitals [9, lo]; of the latter there are so many geometrically inequivalent ones that Tz and TY must acquire comparable amounts of singlet character, as witnessed by the near-equality of the radiationless rates k(Y) and k@) - k(:) for naphthalene in table 1. With the insertion of one or more N atoms into the ring the possibility of the strong coupling ILb is introduced; which of the spin components is (are) involved, depends on the symmetry of the n orbital concerned. In fact, to a good approximation [lo]: if (Yis the angle between the direction of the n orbital and the y spin axis, then TZ and TY share in the enhanced coupling provided by this orbital in the ratio cos CL:sin (Y. In quinoxaline, and very nearly in quinoline, the lone pair orbital(s) point along the y spin axis (a = 0). Hence for these molecules it is specifically the decay from Tz that is strongly enhanced by the nitrogen substitution, resulting in increased values of k(z) and its radiative part k(g). The case of isoquinoline, finally, is an interesting one. The fact that radiative decay now OCcurs from both TZ and TV must be an illustration of the skewness of the n orbital relative to the y and z spin axes in this molecule. The position of these axes is not accurately known; if y were pa,rallel to y’ in fig. 1 (as it nearly seems to be [3,?]) then a ratio kp) : kp) = cos2600 : sin2600 = 1 : 3 would be expected, .just as observed. boquinoline differs from quinoline in that a B atead of an (Y CH group in the naphthalene system is replaced by N. Since the spin density in the triplet state is appreciably lower at the B

1 December 1969

than the (Ypositions, one wculd expect this difference to be reflected in the relative values for the two molecules of (a) the strength of the spin-orbit coupling, (b) the oscillator strength of the ii* - n transition from which the phosphorescence derives its intensity. In agreement with this qualitative picture the phosphorescence of isoquinoline is indeed found to be appreciably weaker than that of quinoline. On the other hand, one would also expect the radiationless decay of isoquinoline to occur at a markedly lower rate than for quinoline, and again preferentially from Ty . This, however, is not found experimentally, when taking into account that the decay rates of table 1 for isoquinoline (just as for naphthalene) must be largely those of the radiationless processes. Further experiments on perdeutero compounds are in progress and the assembly of data should then provide a challenge for recent, more sophisticated theories of radiationless decay [15,16i.

ACKNOWLEDGEMENT The present work owes much to the support given by Dr. T. J. Schaafsma and J. van Egmond who prepared the crystalline samples for us.

REFERENCES [l] J. Schmidt and J. H. van der Waals. Chem. Phg. Letters 2 (1968) 640. (21 J. Schmidt, Chan Iu Yam and J. IL van der Waals, Proc. Conf. Transitions non rzdiatives dans Les mol&ules. Paris (1969), to be published in J. Chim. Phys. [3] J. S. Vincent and A. H. Maki, J. Chem. Phys. 39 (1963) 3088; 42 (1965) 865. 141 _ . G. W. Robinson and R. P. Frosch. J. Chem. Phys. 37 (1962) 1962: 38 (1963) 1187. 151 M. S. de Groot, I. A. M Hesselman and J. H. van der * ’ Wanls, Mol. Phys. 12 (1967) 259; &I.S.de Groot, I.A. M. Hesselman. J.Schmidt and J. H. van der Waals, hfol. Phys. I5 (t968) 17. [6] M. Schwoerer and H. C. WoIf. in: Proc. of the XIVth Colloque AmpEre 1966. ed. R. Blinc (KorthHolland, Amsterdam, 1967) p. 87. [7] Y. Gondo and A. H. hlaki. J. Chem. Phys. 30 (1969) 3270. [8] D. S. McClure,

J. Chem. Phys. 17 (1949) 665; 20 (1952) 682. [S] B. R. Henry and W.Siebrand. J. Chem. Phys., to be published. [lo] W.S.Veeman and J.H.van der Waals, Mol. Phys., to be published. [II] H. F. Hameka, in: The triplet state (Cambridge University Press, London, 1967).

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Volume 4, number 6 [12] J_ H. van der Waala and M. S. de Groot,

CHEMICAL

in: The triplet state (Cambridge University Press, London, 1967). [13] M. Kasha, Radiation Res. Suppl. 2 (1960) 243. [14] R. G. Bennett et al., J. Chem. Phys. 41 11964) 3042. [15] M. Bixon and J. Jortner. J. Chem. Phys. 48 (1968) 715;

PHYSICS LETTERS

1 December

1969

J. Jortner and R. S. Berry. J. Chem. Phys. 48 (l968) 2757. (161 W. Siebrand. J. Chem. Phys. 44 (l966) 4055; W. Siebrand and D. F. Williams, J. Chem. Phys. 49 (1968) 1860. [17] M. Schwoerer and H. Sixl, Chem. Phyg. Letters 2 4968) 14.