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Triplet States and Triplet Excitons in Chemically Mixed Crystals

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Triplet States and Triplet Excitons in Chemically Mixed Crystals o (1995 ΛCA PHYSICA POLONICA A o 3 IE SAES A IE ECIOS I CEMICAY MIE CYSAS O ACIIE WI AACEE O MOAWSKI A OCOOW Isiue o ysics ois Acaemy o Sceces A oików 3/ - Wasawa oa (Received May 9, 1995) eicae o oessos Kyso igoń óe W oee a isław uiewic o e occasio o ei 7 iay? Seca a emoa caaceisics a ei emeaue eeece o e og-ie (osoescece a eaye uoescece emissioii o cemicay mie cysas o aciie II (os wi aacee (gues wee suie ue coiios o aious seca esouios a iee moes o eciaio e eegy o - asiios o ecio a a o a saes ae ee eemie a e aue a eegy sceme o e ie saes o cysas ae ee esaise e cke aageme o os moecues io wo iee ais i e cysa sucue o aciie II is eeae i e eegeic "oue" sucue o e a saes o o . e saow a e ee as (o aciie a o aacee oigi e- seciey Migaio o ie eciaioii eegy i ese cemicay mie cysas is cooe eomiay y e ie-ie aiiaio o ecios (eie eeogeeous o kogeeous amos i e woe em- eaue age e omaio o ie ecimes o aciie was eiiey ue ou ACS umes 317Ks 335q 7135+- 1 Ioucio Some ime ago e wo-comoe ogaic soi souios wi susiu- ioa isoe ie isooicay o cemicay mie moecua cysas seeme o e ey aacie as e moe sysems o igy isoee sysems (eg moecu- a gasses oymes a iquis [1] I as ue ou owee a i cemicay Sumie o iiaio o e Isiue o ysica a eoeica Cemisy ecica Uiesiy o Wocław Wocław oa (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- icay mie cysas o aciie ΙI wi aacee ae ee esee 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- ie 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 Exprntl 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 oo-couig ecique was use o ecoig e emissio seca 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 dn 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 ui ce a e sace gou Ρ 1 /α sees as e os aice o aacee 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.
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