o (1995 ΛC A PHYSICA POLONICA A o 3
I E S A ES A I E E CI O S I C EMICA Y MI E C YS A S O AC I I E WI A ACE E
O MO AWSKI A OC O OW
I s i u e o ysics o is Aca emy o Sc e ces A o ików 3 / - Wa s awa o a
(Received May 9, 1995)
e ica e o o esso s K ys o igoń ó e W o e e a isław u iewic o e occasio o ei 7 i ay?
S ec a a em o a c a ac e is ics a ei em e a u e e e e ce o e o g- i e ( os o esce ce a e aye uo esce ce emissioii o c emica y mi e c ys a s o ac i i e II ( os wi a ace e (gues we e s u ie u e co i io s o a ious s ec a eso u io s a i e e mo es o e ci a io e e e gy o - a si io s o e ci o a a o a s a es a e ee e e mi e a e a u e a e e gy sc eme o e i e s a es o c ys a s a e ee es a is e e ck e a a geme o os mo ecu es i o wo i e e ai s i e c ys a s uc u e o ac i i e II is
e ea e i e e e ge ic " ou e " s uc u e o e a s a es o o . e s a ow a e ee a s (o ac i i e a o a ace e o igi e- s ec i e y Mig a io o i e e ci a ioii e e gy i ese c emica y mi e c ys a s is co o e e omi a y y e i e - i e a i i a io o e ci o s (ei e e e oge eous o koge eous a mos i e w o e em- e a u e a ge e o ma io o i e e cime s o ac i i e was e i i e y u e ou ACS um e s 31 7 Ks 33 5 q 71 35 +-
1 I o uc io Some ime ago e wo-com o e o ga ic so i so u io s wi su s i u- io a iso e i e iso o ica y o c emica y mi e mo ecu a c ys a s seeme o e e y a ac i e as e mo e sys ems o ig y iso e e sys ems (e g mo ecu- a g asses o yme s a iqui s [1] I as u e ou owe e a i c emica y
Su mi e o i i a io o e I s i u e o ysica a eo e ica C emis y ec ica U i e si y o W ocław W ocław o a
( 9 470 O. Morawski, J. Prochorow mixed crystals, the most attractive factor, i.e. an anticipated easy-to-handle con- trol of the degree of true substitutional (translational) disorder in the host crystal lattice, is connected with very severe geometrical and symmetry requirements [2]. Furthermore for most of the candidates, for the formation of chemically mixed crystals, a solid solubility limit very seldom exceeds a fraction of mo % thus limit- ing the range and degree of the controlled substitutional disorder and by the same token the generality of conclusions. On top of that, it has turned out in the course of experiments that even in such thermodynamically favourable cases like solid solution of anthracene and acridine where the host lattice can contain as much as tens of mo % of the guest [3, 4], the analysis of experimental results may become very obscure (if not impossible at all) due to the fact that energy levels of host and guest and of different resonance aggregates are overlapping even at moder- ate guest concentrations, and both, the variety of aggregates and the overlap are strongly increasing with increasing guest concentration [5-7]. And although all these drawbacks and limitations have resulted in the shift of a centre of gravity of the research effort to isotopically mixed crystals, systematic studies of chemically mixed crystals (lightly and highly concentrated with the guest molecules) have provided some interesting results and concepts relevant to the exciton transport and trapping [8-11], formation of excited complexes and aggregation properties [12, 13] or aggregation-controlled molecular photophysics [7]. This work was aimed at identiflcation of the triplet states of a new twocompo nent solid solution of acridine (host) with anthracene (guest)which can form chem- ically mixed crystals. This system, in contrast to extensively studied "reversed" system of anthracene (host) with acridine (guest), has never been before a subject of optical studies, although chemically mixed crystals based on the acridine host lattice should be very interesting from the point of view of excitation energy trans- fer, due to the variety of crystalline modiffcations of a neat acridine crystal and the pairwise arrangement of the molecules in the lattice [14, 15] which may domi- nate in photophysics and kinetics of excited states and overwhelm the resonance (guest-guest) interactions. With this motivation we have extended our previous interest in chemically mixed crystals of anthracene (host) with acridine (guest) to the crystals under consideration. The details relating to the electronic singlet states, traps for singlet excitons as well as the verification of the formation of singlet excimers of acridine in chem- ica y mi e c ys a s o ac i i e ΙI wi a ace e a e ee ese e i ou recent paper [16] together with some preliminary results of low-resolution optical studies of long-lived emission spectra. We recall that in all studied crystals [16]: (i) the energy of the bottom of singlet excitonic band is 25450 cm -1 (it is of acri- i e S1(π π origin); (ii) the only traps for singlet excitons are of anthracene origin, with energy of ca. 24000 cm -1 but the efficiency of trapping is small even at low temperatures; (iii) singlet excitons from the singlet excistonic band are de- caying predominantly via the process of formation of excimers (either in A or B pairs — see Sec. 3 below), the energy of excimer state is ca. 21000 cm -1 and its radiative decay time is of 85 ns. These findings were in general agreement with the conclusions of earlier spectroscopic experiments performed for the neat acridine II crystals [15, 17, 18]. For the flrst time, however, we have observed the phosphores- Triplet States and Triplet Excitons in Chemically Mixed Crystals ... 471
cence emission of the crystal in acridine II crystallographic form. This observation supplied us with a tool, which together with high-resolution laser excitation, could have been used for investigations of the triplet states and triplet excitons and for verification of the concepts of a triplet excimer as well as for insight into the photophysical mechanism of deactivation of excited molecular and/or excimer sin- glet states. The results of detailed studies of the phosphorescence emission and its kinetics are summarized and discussed in this paper.
2 Exp r nt l Anthracene and acridine were purified by vacuum sublimation and zone-refin- ing. Crystals of 0.02, 0.1 and 1.0% M/M anthracene in acridine host were grown by sublimation [4]. With the use of the standard powder diffractometry technique a crystal stucture of the host lattice in mixed crystals was found to coincide with the crystal structure of acridine II Α o o -cou i g ec ique was use o eco i g e emissio s ec a Several modes of excitations have been employed, i.e. 365 and 405 nm lines isolated from a mercury HBO 200 lamp; for selective excitation within the 395-415 nm range — a dye laser (Lambda Physik FL3001) pumped by YeCl excimer laser (Lambda Physik LPX105); for excitation spectra — the above laser setup equipped with KDP frequency doubler (FL 30 SHG crystal) tuned within 605-660 nm range with a linewidth of 0.4 cm -1 or 0.08 cm -1 (with FL 82 Fabry-Perot etalon set). A details of the experimental setup, including optical helium cryostat and the electronics as well as the techniques of decays measurements, have been pub- lished previously [19].
lt nd d n At least five different crystalline modiflcations of acridine have been reported up to now (see [15] and the references therein). In the present study of chemically mixed crystals, a crystalline form of acridine II (monoclinic, with 8 molecules in e u i ce a e s ace g ou Ρ 1 /α se es as e os a ice o a ace e molecules. The crystallographic stucture of acridine II, with two molecules per asymmetric unit, has a complex arrangement of molecules. The packing units, each containing two molecules, are not all equivalent as the molecules are arranged in two types of antiparallel pairs about the centre of symmetry (designated as A and B pairs, respectively). The separation distance between molecules in orientation A is 0.349 nm and in normal projection they are not exactly overlapped but are staggered. In B orientation, the Separation distance is 0.361 nm and in normal projection there is only an overlap of the terminal aromatic rings of the two acridine molecules [14]. In general, such a crystallographic stucture of the host crystal should pro- vide more sites for substitution and larger variety of local environments for guest molecule than a "reversed" mixed crystal of anthracene (host) with acridine (guest). However, it has been shown long ago that in the acridine II structure anthracene is not able to pack as closely as acridine molecules [3] and this fact strongly limits an accessible concentration range of guest molecules in these chemically mixed 472 0. Morawski, J. Prochorow crystals. Hence, our investigations have been limited to three mixed crystals of acridine with anthracene concentrations 0.02, 0.1 and 1.0% M/M. In all these crystals phosphorescence emission was observed and its detailed features and the temperature dependence are discussed in the following.
3.1. Phosphorescence a{ low temperatures Figure 1 illustrates the long-lived emission spectra of chemically mixed crys- tals of acridine and anthracene, at 1.7 K, for different concentrations of anthracene in the acridine II host lattice. This emission is composed of a broad, and structure- less delayed fluorescence band, spread ou: in 21000-17000 cm -1 spectral range, which is identical in its shape and spectral position with the spectrum of prompt fluorescence of the crystals under consideration [16]. On the low-energy side of delayed fluorescence band (below 16000 cm -1 ), a phosphorescence emission is ob- served. The phosphorescence spectum is composed of very sharp and fairly well resolved bands. The energy of maximum of the highest-energy band in phospho- rescence spectrum (a presumable 0-0 transition of phosphorescence emission) is ca. 15691 cm -1 . At 1.7 K, the decay time of phosphorescence monitored at this band is 33 ms. Both, the energy of this phosphorescence band and its decay time can be compared to the energy of 0-0 transition (15813 cm -1 ) and the decay time of phosphorescence (34 ms) observed for acridine isolated (at low concentrations) in the crystalline host lattice of 2,3-dimethylnaphthalene [20]. And such a com- parison would suggest a rather safe assignment of the phosphorescence spectum observed in these mixed crystals as the phosphorescence of acridine. However some other characteristics of phosphorescence emission indicate that such an assignment would be an oversimplified one. First of all, it is seen in Fig. 1 that intensity distribution among different subbands of the phospho- rescence spectrum is different for different concentrations of anthracene in the acridine host lattice. In mixed crystals with 0.02 and 0.1% of anthracene the 0-0 band at 15691 cm -1 is the most intense one (Fig. 1a and 1b), but in the crystal with the highest concentration (1.0%) of anthracene its intensity is strongly di- minished and the band at 14829 cm -1 is the most intense one (Fig. c). Another, very distinct observation related to the changes of the intensity distribution within the subbands of phosphorescence spectrum is given in Fig. 2, which shows that even for a fixed concentration of anthracene, the relative intensities are dependent on the wavelength of excitation. If excitation is carried out via singlet excitonic band (27472 cm -1 excitation) the 0-0 band of phosphorescence of acridine, at 15690 cm -1 , is the most intense one. When the excitation is tuned below the singlet excitonic band, directly to the singlet traps (24619 cm -1 excitation) the intensity of the 0-0 band (and of some other subbands) is decreasing, while the intensity of the band located at 14829 cm -1 is strongly enhanced. A more detailed analysis of the vibrational pattern of the phosphorescence spectum enables the identification (with accuracy of ±5 cm -1 ) of such vibra- tional frequencies as 225, 400, 620, 1276, 1410 and 1569 cm -1 which are char- acteristic of acridine (and have been all identified in phosphorescence spectum of acridine embedded as a dopant into the crystal of fluorene [21]). Thus we no- tice that a ca. 860 cm -1 shift between the most intense bands under considera- Triplet States and Triplet Excions in Chemically Mixed Crystals 73
io (15 91-1 9 cm -1 is o co ec e wi a y i a io a mo e o ac i i e mo ecu e a e ce e a a 1 9 cm -1 mus e e a e o e emi i g s ecies (si e i e e om ac i i e mo ecu e is co c usio is u e su o e y e ac a e ecay ime o os o esce ce mo i o e a is a is ms a 1 7 K w ic is i e e om 3 ms o se e o 15 91 cm -1 a I iew o ese o se a io s e 1 9 cm -1 a ca e i e i ie as a o a ace e o igi o com a iso e - a o a ace e os o esce ce i e os a ice o i e y c ys a is oca e a 1 73 cm -1 [ ] a i s ecay ime is S [ 3] (i e a i e c ys a i s osi io is 1 cm -1 [ ] e ce we a i e a e co c usio a e os o esce ce o mi e c ys a s o ac i i e wi a ace e co sis s o wo o e a i g s ec a e ig -e e gy os o esce ce s ec um wi e - a a 15 91 cm -1 (a e ecay ime o 3 ms w ic is o ac i i e o igi a e ow-e e gy s ec um wi e - a a 1 9 cm-1 (a e ecay ime o s w ic is o a ace e o igi Eac o ese wo s ec a ca omi a e i e o a os o esce ce o mi e c ys a s e e i g o ei e co ce a io o a ace e (c ig 1 a e mo e o e ci a io (c ig o e em e a u e a ge (see e iscussio i Sec 3 owe e e ge e a a e o e os o esce ce s ec um i e mi e c ys a s u e s u y c ea y s ows a e os o esce ce o igi a es om 7 O. Morawski, J. Prochorow
wo ki s o i e a s o i e e e a o igi e e a e s a ow a s o ac i i e o igi a muc ee e a s ( y ca. 7 cm -1 o a ace e o igi w ic mig a e ee e ec e i iew o e e e gy i e e ce e wee e i e s a es o o mo ecu es As we a e s ow ea ie [1 ] si g e a s i e mi e c ys a s o ac i i e wi a ace e a e o me e c usi e y o a ace e mo ecu es (a ose ca e ace i o e uo esce ce a a so io s ec a o e c ys a a is co c usio is i ge e a ag eeme wi e o se e e a ceme o a ace e os o esce ce u o e ci a io u e o si g e a s e ow e si g e e ci o ic a (c ig e s a ow ac i i e a s a e o u a e mos y ia e ci a io o e si g e e ci o ic a w ic as we a e emo s a e ea ie se es as a ecu so o e o ma io o ac i i e e cime i e c ys a [1 ] As e o se e ac i i e os o esce ce s ec um as a e y s a a c ea -cu mo ecu a - ike c a ac e i e c ys a a as e e a ow em e a u es i s i e si y is a mos ee o e s o mag i u e sma e a e i e si y o e cime - ike uo esce ce i seems ig y im o a e a ac i i e a s ca e o u a e wi e a ici a io o e cime s a es us - we mus co c u e a e mos o a e mec a ism o o u a io o ac i i e i e a s is a co ec e wi i ec i e sys em c ossi g om e si g e e ci o ic a o e i e e ci o ic a o e os c ys a om w e e s a ow ac i i e a s a e ei g o u a e We a so s ess a co a y o co i me o ma io o si g e ac i i e e cime s e os u a e o ma io o i e e cime s [15 1 19] i e ac i i e II c ys a ca e i i e y e u e ou A i s ec io o e e e a i e y ow- eso u io s ec a gi e i ig is e ea i g a e - a s o o e ac i i e a a ace e os o es- ce ce s ec a (as we as some o e su a s i o s ec a e i i a com- osi e s uc u e e ig - eso u io os o esce ce s ec a c ea y s ow ei " ou e " s uc u e as i us a e i ßig 3 o e ac i i e a o e os o- esce ce s ec um e com o e s o e " ou e " a e e e gies o 15 91 a 15 9 cm-1 es ec i e y e cm -1 se a a io e wee e " ou e " com- o e s is c a ac e is ic o a e o e a s i e s ec um (a e same se a a io c a ac e i es e " ou e " s uc u e i e a ace e os o es- ce ce s ec um o w ic e " ou e " com o e s o e - a a e oca e a 1 9 a 1 7 7 cm -1 es ec i e y Suc a " ou e " c a ac e o e os- o esce ce om o ki s o a s (o ac i i e a o a ace e o igi ca o y e accou e o i o e assumes a i o cases e e a e wo i e e si es o a i g mo ecu es Si ce e se a a io e wee e " ou e " com o e s is ac ica y e same o e se s o s a ow (ac i i e a s a ee (a ace e a s e a i g si es mus e co ec e wi a s eci c s uc u e o e ac i- i e II c ys a a ice a mus e ec e i e e ces i e " oca " si ua io o e i e s a es o a i g mo ecu es wi i e A a ai s (see egi i g o Sec 3 I e e o -o e a o ima io e e e gy o e i e s a e o ei e e os mo ecu e (ac i i e o su s i u io a y e aci g i a ace e mo ecu e wou e co o e y e is a ce e wee e com o e s o e ai w ic is e same o ac i i e a a ace e mo ecu e i a y gi e ai ( o i e i ese c emica y mi e c ys a s a ue su s i u io a e aceme akes ace u is i e e i A a i ai s Ou ese esu s o o a ow o e Triplet States and Triplet Excions in Chemically Mixed Crystals ... 475
precise determination of the local stucture of the shallow and deep triplet traps and/or trapping sites and for instance "an anti-parallel arrangement" of acridine molecules within the A and B pairs as a possible triplet trap cannot be excluded. Nevertheless, the identification of two different sets of triplet traps, each of which is divided into two different subsets (sites) of traps is beyond any doubt. In order to determine the energy of the triplet excitonic band of the acridine II host crystal we have measured phosphorescence excitation spectra with the use of a narrow-band tunable dye laser. The excitation spectrum of acridine phosphorescence (in the 0-0 region) is presented in Fig. 4. The two most intense lines at 15821.0 cm -1 and 15752.9 cm -1 can be ascribed to the two Davydov components of the host triplet excitonic band. Other lines of much smaller intensity form two progressions with identical shifts (255, 400, 440 cm -1 , etc.) from both Davydov components. At still higher resolution the shape of both Davydov components (cf. Fig. 5) seems to suggest a presence of more "finer splitting" which could not be further resolved under present experimental conditions. This should be investigated under appro- priate resolving power, especially as it should be born in mind that although the crystal stucture of acridine II is arranged pairwise (which probably dominates all the existing couplings and interactions), still, however, there are 8 molecules in the elementary unit cell of the crystal, what may result in the complicated pattern of the Davydov splitting [25]. The above-presented results and discussion can be summarized in the en- ergy scheme of the triplet states in the chemically mixed crystal of acridine with anthracene given in Fig. 6. From this we can determinate the depths of triplet traps observed in these crystals. These depths (relative to the lower Davydov component) are equal to 62 and 104 cm -1 for the set of acridine traps and 924 • and 966 cm -1 for the set of deep anthracene traps, respectively. The reliability of these values and validity of some conclusions formulated in the preceding can be verified by investigations of the kinetics of triplet excitons. 476 O. Morawski' J. Prochorow
3.2. Temperature dependence of phosphorescence and the kinetics of triplet excions The quoted earlier Fig. 3 illustrates also the temperature dependence of the "doublet" components of phosphorescence spectum originating from acridine traps. With an increase in temperature the high-energy component is loosing its Triplet States and Triplet Excions in Chemically Mixed Crystals ... 477
i e si y muc as e a e ow-e e gy com o e a a o e 1 K o y is a e o e ca e s i o se e I s i e si y is quick y ec easi g wi em e a- u e a a o e 15 K e os o esce ce o ac i i e is o o ge e is i g i e os o esce ce s ec um o e mi e c ys a W a is e is e a ace e a o os o esce ce s ec um o e c ys a w ic (a e some i i ia i c ease i i e si y i e em e a u e a ge o 15- K is a so oosi g i s i e si y wi i c easi g em e a u e a e e ua y ceases o e is i e em e a u e a ge o -1 K e em e a u e e e e ce o e w o e os o esce ce s ec um oge e wi e em e a u e c a ges o e aye uo esce ce s ec um a e s ow i ig 7 o ow- eso u io s ec a e em e a u e e e e ce o e i eg a i e si y o e aye uo esce ce a oug somew a mo e com ica e a a o se e o os o esce ce e ea s ou c a ac e is ic egio s o em e a u e o w ic i s e a iou is c ea y is aye I s a s o g ow a ow em e a u es (a o e K eac es e ma imum (a 15 K a a e a s ig o emai s co s a i e a ge o me ium em e a u es (u o i K a e s a s o ce agai o i c ease s a y a eac es i s seco ma imum (a i 1 K A o e 1 K e i e si y o e aye uo esce ce is s ow y u co i uous y e- c easi g wi i c easi g em e a u e A ese em e a u e c a ges o i e si y o e aye uo esce ce a a os o esce ce o ac i i e a a ace e a e accom a ie y em e a u e c a ges o ei ecay imes (c ig 478 O. Morawski, J. Prochorow
These observations of temperature changes of delayed fluorescence are typ- ical for delayed fluorescence of triplet-triplet annihilation origin (P-type delayed fluorescence) in the almost whole range of investigated temperatures. At deep tem- peratures (1.7-2.0 K) most of the excitons are trapped, which results in intense phosphorescence (mostly from shallow acridine traps) and relatively low-intensity delayed fluorescence which must be of heterogeneous triplet-triplet annihilation between mobile triplet excitons in the excitonic band and trapped excitons. This is a so co i me y e ac a e ecay ime o e aye uo esce ce a 1 7 Κ is 15 ms (cf. Fig. 8) which is almost twice as short as the decay time of acridine phosphorescence (34 ms) — a feature typical for the emission originating from the annihilative events. When the temperature is increased, excitons from shallow acridine traps are thermally activated to the triplet excitonic band, the intensity of phosphorescence drops (c.f. Figs. 3 and 7) and its decay time is getting shorter. A e same ime (i -15 Κ a ge e i e si y o e aye uo esce ce is quick y increasing (see above). In this temperature range the delayed fluorescence is due to the homogeneous annihilation of mobile excitons (although annihilation of mobile excitons and excitons trapped on deep anthracene traps can also contribute, espe- cially in the mixed crystals with higher concentration of anthracene). Some fraction of all excitons which are thermally activated from acridine traps is also being •re- pumped to deep anthracene traps, as can be inferred from the above-mentioned i c ease i i e si y o a ace e os o esce ce a 15- Κ a ge We o ice a a Κ a u o 5 K e ecay ime o e aye uo esce ce (c ig is 1 ms (a oug i as a e ow o 1 ms a 15 Κ a is a mos wice as short as the decay time of anthracene phosphorescence (40 ms). This indicates that after depopulation of shallow acridine traps, the heterogeneous annihilation of mobile excitons and excitons trapped on deep anthracene traps becomes im- portant. When the temperature is increased above ti 40-50 K, the intensity of anthracene phosphorescence and its decay time start to decrease quickly and this Triples States and Triplet Excitons in Chemically Mixed Crystals ... 79 mea s a e e ma ac i a io o ee i e a s ecomes mo e a mo e e ec i e A o e - K ee i e a s a e o a ac ica u oses em y ( os o esce ce ca o o ge e o se e a e e aye uo esce ce w ic is s i o se e is ue o e omoge eous i e - i e a i i a io o mo i e e ci- o s A co i uous ec ease i i s i e si y a i ecay ime o se e o ig e em e a u es is ue o sca e i g o mo i e e ci o s a e i g oug e c ys a a ice as e sca e i g i c eases wi i c easi g em e a u e o e c ys a I comes om e above-outlined, qua i a i e esc i io a e e a e wo em e a u e a ges w e e e i e si y o e aye uo esce ce (I is go e e a co o e y e e ec i e e ma ac i a io o a e i e e ci o s o e i e e ci o ic a i e
w e e is e o u a io o e a ΔE is e ac i a io e e gy o e e- o u a io ocess k — o ma co s a Si ce e os o esce ce i e si y (I is o o io a o e o u a io o a a A e ius y e o o o (I /Ι e sus eci oca em e a u e (1/ s ou i g a ou a ac i a io e e gy ΔΕ o e ocess o e ma e o u a io o gi e a s I ig 9 suc a A e ius o w ic e a es o e ma e o u a io o ee a ace e a s (5 -9 K em e a u e a ge is gi e e ac i a io e e gy o is ocess o - ai e om e o i ig 9 is equa o 9 ± cm -1 I we com a e is a ue o e a e age e o a ace e a s w ic we e o ai e om s ec osco ic a a (see Sec 3 1 a w ic a e 9 a 9 1 cm -1 we see a ese a ues a e i e ce e ag eeme A a a ogous oce u e a ie o s a ow ac i i e a s ( o -1 K em e a u e a ge yie s e ac i a io e e gy o ± cm -1 w ic is a so i goo ag eeme wi cm -1 e e mi e om s ec osco ic a a (see Sec as e e o ig e -e e gy su se o ac i i e i e a s ese ac i a io e e gies s ow a e o ose o oki e ic sc eme o o y we ac- 480 O. Morawski, J. Prochorow counts for the observed behaviour of the long-lived emission of these chemically mixed crystals (although this scheme would necessitate some minor corrections for some of the temperature ranges in the case of crystals which contain higher concentrations of anthracene (0.1 and especially 1.0%)), but also is self-consistent from the quantitative point of view.
3.3. Concluding remarks Determination of the nature and the energy scheme of the triplet states in the chemically mixed crystals of acridine II with anthracene has revealed that the triplet traps of both types (of acridine and anthracene origin) are within each type very distinctly segregated into two groups (of energy). We believe that this is a clear demonstration of a very selective role of the stucture of the host crystal lattice, which in the case of acridine II with two molecules per asymmetric unit offers two distinctly different local environments (sites) for anthracene guest molecule. It is also a proof that with this moderate concentration of guest molecule a true substitutional replacement of host by guest takes place. A complete lack of any traces of the formation of triplet excimers of acridine may be considered as a piece of evidence which would ule out the nonradiative sinnglet-triplet intersystem crossing (from excimer singlet state) as a process con- trolling the decay of acridine singlet excimers, thus leaving a thermally activated internal conversion as the main nonradiative mode of excimer's decay. Still, how- ever, population of the triplet states (traps), though of very low efficiency, takes place in these crystals and an understanding of how the triplet states (traps) are populated is probably of great importance for photophysics of acridine (not only in the crystalline phase). In order to solve this problem one has to probe the dynamics of the formation and the decay of singlet excimer of acridine with the picosecond time-resolved emission studies and this is now in the course of investigations for acridine II and its chemically mixed crystals in this laboratory.
Ack ow e gme s We are deeply grateful to Dr. R. Radomski (Technical University of Wroclaw) for supplying us with many samples of chemically mixed crystals, among them, with those particular which were used in this work. We also wish to thank him for stimulating inspiration and many valuable suggestions on various aspects of chem- ically mixed molecular crystals, from which we have benefited over the years. We also acknowledge the flnancial support of this research from the State Committee for Scientific Research (Republic of Poland) under the Project nr 2 2333 92 3
e e e ces
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