Some effects of molecular orientation on fluorescence emission and energy transfer in crystalline aromatic hydrocarbons

Spectrochimlca Acta, 1962, Vol. 18, pp. 439 to 448. PergamonPress Ltd. Printed in Northern Ireland

Some et~ects oi m o l e c u l a r orientation on fluorescence emi.qsion and energy transfer in crystalline aromatic hydrocarbons B. ST~,VENS '

Department of Chemistry, The University, Sheffield 10

(Received 18 October 1961) Abstracts--An examination of their fluorescence spectra in the dissolved and crystalline forms reveals a close similarity both in position and structure for long, thin, aromatic molecules where Davydov splitting of the first absorption b a n d in the crystal is small. The crystal fluorescence spectra of disk-shaped aromatic hydrocarbons on the other hand undergo a pronounced red-shift a n d loss of vibrational structure in comparison with the corresponding molecular fluorescence observed in solution; this is a t t r i b u t e d to a much closer spacing and larger overlap of molecular planes in a different type of crystal lattice, which promotes very strong molecular interaction, and it is suggested t h a t the spectrum of crystal emission is diagnostic of the type of molecular lattice adopted. The transfer of energy from solvent to solute in solid solutions of aromatic hydrocarbons appears to depend on the type of molecular lattice exhibited by both solute and solvent and m a y provide a n additional criterion of crystal structure.

INTRODUCTION A N examination of the evidence for photo-association of planar aromatic molecules has led to the conclusion that excimer formation by the diffusional process 1 (the asterisk denotes the lowest excited singlet state) is of general occurrence in fluid media [I]. In the case of dissolved pyrene [2], A * -I-A --+ A2*

(1)

excimer fluorescence (process 2) is observed as a structureless emission at A2* ~ (A2) + h~

(2)

longer wavelengths then the structured fluorescence of the monomer (process 3) A* --~ A q- h~

(3)

as shown in Fig. 1 (a and b); the absence of excimer fluorescence in concentrated solutions of other compounds is attributed to the relatively high efficiency of competing processes 4 and 5 which lead to delayed fluorescence [3] A2* ~ A * + A

(4)

A~* ~ (A~)

(5)

and self-quenching respectively and on which the evidence for excimer formation is based [1]. [1] B. STEVENS, Nature, 192, 725 (1961). [2] TH. F6RST~.X%and K. KASPE~, Z. Elebtroehem. 5 9 , 977 (1955). [3] R. WrLLIA~S, J. Chem. Phys. 28, 577 (1958). 439

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If ks is the rate constant of process n and r is the measured radiative lifetime of the monomer at infinite dilution, the quantum yield y of excimer fluorescence in the absence of quenching species is given by

The as yet unique observation of excimer fluorescence in concentrated solutions of pyrene m a y be attributed to the relatively high solubility of this low-melting compound, the comparatively long radiative lifetime of the excited monomer [4,5]

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m/~ Fig. 1. A b s o r p t i o n ( . . . . ) and fluorescence ( ) spectra of pyrene; (a) 10 -4 l~I in ethanol; (b) 10 -9 1~I in e t h a n o l ; (c) a b s o r p t i o n [22] ( . . . . ) and emission ( - - - ) spectra of crystalline pyrene.

(r ~ 10 -7 sec), the relatively fast radiative relaxation of the excimer in this case [4,5] (k9 ~ 10 6 sec -1) and the consequent low relative probabilities (k4 and ks) of competing processes at room temperature. In the case of other aromatic hydrocarbons, with fluorescence decay constants in the .characteristic region of 10 9 sec -1 [4] TH. :FS:RSTER, 5th Europ. Syrup. 3lolecular Spectrosc., Amsterdam, 1961. [5] C. A. PA.RmER and C. G. HATC~A.~D, Nature 190, 165 (1961).

Some effects of molecular orientation on fluorescence emission and energy transfer

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(r ~ 10 -a see), exeimer fluorescence m i g h t be observed in fluid media at sufficiently high concentrations provided processes 4 a n d 5 do n o t compete efficiently (i.e. /ca ~-/c5 ~/c~); u n f o r t u n a t e l y such concentrations are limited b y t h e low v a p o u r pressures, a n d sparing solubilities of these large molecules a n d internal conversion (process 5) a p p e a r to be d o m i n a n t in solution. T H E STRUCTURE AND FLUORESCENCE SPECTRA OF AROMATIC CRYSTALS

I n t h e highly c o n c e n t r a t e d crystalline f o r m the diffusional process 1 necessary (a)

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Fig. 2. Molecular absorption (. . . . ) and fluorescence spectra of (a) naphthalene, (b) anthracene, (c) phenanthrene, (d) chrysene in dissolved ( ) and crystalline (- - -) forms. Ais given in m/~. for eximer f o r m a t i o n should be p r o h i b i t e d and, a p a r t f r o m effects due to reabsorpt i o n [6], a n d t h e slight ( ~ 1 0 0 cm -1) red-shift due to D a v y d o v splitting [7] in the relatively w e a k first absorption b a n d of t h e crystal, there is a close resemblance [6] J. B. BIRKS and A. J. W. CAYm~ON,Prec. Roy. Soc. (London) A249, 297 (1959). [7] D. P. CP,AIG, and P. C. HOSBI~S, J. Chem. See. 539, 2309 (1955).

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between the fluorescence spectra of certain aromatic hydrocarbons in the dissolved and crystalline forms as shown in Fig. 2 ; the approximate mirror correspondence of absorption and fluorescence moreover .establishes the identity of the emitting species as the monomer in each case. Crystalline pyrene on the other hand emits the broad structureless band shown in Fig. l(c) which is remarkably similar [8] both in position and lack of structure to that associated with excimer fluorescence in concentrated solution (Fig. lb). The

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Fig. 3. Crystal structure of pyrene [9] reproduced by kind permission of the authors and the Chemical Society.

crystal structure [9] of pyrene reproduced in Fig. 3 reveals that the molecules are already grouped in parallel sandwich-like pairs and that diffusion is not necessary for the formation of the suspected emitting species; moreover, the close resemblance between the molecular and crystal absorption spectra shown in Fig. 1 indicates that the absorbing species is monomeric in each case. The fluorescence spectra of [8] J. FEI~OUSO>T, J. Chem. Phys. 28, 765 (1958). [9] J. M. ROB~.RTSON and J. G. W~r~TE, J, Ohem. Soc. 358 (1947).

Some effects of molecular orientation on fluorescence emission and energy transfer

443

perylene and 1,12-benzperylene which crystallize with the pyrene t y p e lattice [10,11] exhibit a similar loss in structure and pronounced red-shift in passing from the dissolved to the crystalline form as shown in Fig. 4. (o)

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m/~ Fig. 4. Fluorescence spectra of (a) perylene and (b) 1,12-benzperylene; - in solution; - - - microcrystals. ROB~.RTSO~ [12] has pointed out t h a t aromatic hydrocarbons appear to crystallize with one of two distinct types of lattice according to their molecular dimensions. Those molecules like naphthalene [13], anthracene [14], phenanthrene [15] and chrysene [16], in which the aromatic plane has both long and short axes, adopt the lattice shown as t y p e A in Fig~ 5 where the periodicity of c., 6 A, and large angle of inchnation of the molecular planes to the s y m m e t r y plane of the crystal, severely limit the ~r-orbital overlap of adjacent parallel molecules. In these crystals the weak coupling between molecules in the lowest excited singlet state is small compared with the intramolecular energy and is manifest as a slight red-shift (~-~100 cm -1) of the fluorescence spectrum in which the molecular vibrational structure is preserved (Fig. 2). The disk-shaped molecules of pyrene, perylene and 1,12benzperylene on the other h a n d prefer the lattice structure shown as t y p e B 1 in [1O] D. M. DOSALDSO~r,J. M. ROBERTSONand J. G. WHITE, Proc. Roy. Soc. (London) A220, 311 (1953). [11] j. G. WHITe, J. Ghem. Soc. 1398 (1948). [12] J. M. ROS~.RTSO~,Proc. Roy. Soc. (London} A207, 101 (1951). [13] S. C. A B R a m s , J. M. ROB~.RTSONand J. G. WHITe., Acta Grys$. 2, 223 (1949). [14] A. McL. lYIATHrES0N,J. lYI.ROB~.~TSO~and V. C. Sr~cLAn~,Acta Gryst. 3, 245 (1950). [15] B. S. BASAX,Ac~a Grys$. 1, 224 (1948); Indian J. Phys. 24, 309 (1950). [16] D. M. BurNs and 5. IB~L, Proc. Roy. Soc. (London)" A257, 491 (1960).

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Fig. 5 where the lattice unit is a pair o f almost completely overlapping parallel molecules with an interplanar distance of c., 3.5 A. The larger molecules of coronene [17] and ovalene [18], together with those of 3,4-benzyprene and 20methylcholanthrene in the monocHn~c form [19], achieve an almost complete

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overlap at a similar interplanar distance b y stacking in columns with their planes at 45 ° to the s y m m e t r y plane in a lattice shown as t y p e B9 (Fig. 5). The fluorescence spectra of these compounds are shown in Fig. 6. The broad structureless emission bands exhibited b y crystals of lattice t y p e - B at considerably longer wavelengths than the corresponding molecular fluorescence could be attributed to the presence of impurities; however, in view of the facts t h a t [17] J. M. ROBE~TSO~and J. G. WmT~, J. Ghem. Soc. 807 (1945). [18] D. M. D o ~ D s o ~ and J. NI. ROBE~TSO~, •roc. Roy. Soy. (London) A22{}, 157 (1953). [19] J. :[B~L, Z. Kris~. 94, 7 (1936).

Some effects of molecular orientation on fluorescence emission and energy transfer

445

(a) The same technique was used to purify these compounds and those exhibiting the spectra shown in Fig. 2, (b) No trace of the molecular fluorescence spectrum is observed, (c) Similar spectra have previously been reported for some of these compounds, [6,8] it is proposed that the pronounced red-shift ( ~ 5 0 0 0 cm -1) and loss in vibrational

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F i g . 6. Fluorescence spectra of (a) coronene, (b) ovalene, (c) 3,4-benzpyrene and 20-methylcholanthrene in dissolved ( ) and microerystalline ( - - - ) forms. ( . . . ) molecular absorption spectrum o f ovalene after C ~ [34]. Fluorescence spectra o f 3,4-benzpyrene as monoclinic crystals and as sublimate are deno¢ed m

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structure of fluorescence, observed in passing from dilute solution to the crystalline state, is characteristic of molecules which crystallise with a type-B lattice. On the basis of this criterion such a lattice is adopted by disk-shaped molecules of anthanthrene according to the fluorescence spectra shown in Fig. 7, whereas the blue-fluorescent modification of 3,4-benzpyrene (Fig. 6) obtained as a sublimate or by rapid evaporation from certain solvents [20], has a type-A lattice. Crystalline naphthacene and [20] F. W~.mEI¢~ and ft. C. Mo~'RxM, Nature 145, 895 (1940).

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Fig. 7. The fluorescence spectra of anthanthrene in solution ( and as microcrystals (- - -) after NORTHI~OPand SIMPSolv[28]. pentacene are virtually non-fluorescent at room temperature [21] and, to this author's knowledge, a detailed X - r a y analysis of their crystal structures has n o t been reported; however preliminary measurements [22] indicate t h a t naphthacene prefers a type-A lattice which might be expected for both compounds on the basis of their molecular dimensions. The large red-shift and loss in vibrational structure of the crystal fluorescence spectrum undoubtedly originates in a strong electronic interaction [23] between the closely packed molecules in a type-B lattice. SI~PsoN and PETERSON [24] conclude that the crystal absorption spectrum reflects either the behaviour of independent molecules or of the crystal as a whole depending on the relative magnitudes of this interaction and the spacing of molecular vibrational levels; it is possible that these two extreme cases are illustrated in emission b y the molecular lattice types A and B respectively, although the absence of corresponding absorption bands in those cases investigated [8,25] indicates an even smaller interplanar separation of adjacent overlapping molecules in the excited configuration than that of ~ 3 . 5 A in the unexcited crystal. The effect observed in crystalline pyrene, perylene and 1,12benzperylene could be attributed to a large splitting of the two-fold degenerate levels of the B z lattice unit upon which a much weaker coupling between these units is superimposed; whilst if the emission exhibited b y a B 2 lattice originates from an "impurity" centre of a molecular sandwich-pair (as in the B 1 lattice) arising from a lattice imperfection at the surface or inside the crystal, there is a formal analogy between the behaviour of these molecules in the fluid and crystalline states. Measurements of the decay times for t y p e - B lattice emission however provide [21] [22] [23] [24] [25]

E. J. B o w ~ , N~ure 142, 10.81 (1938). E. HmmT~T.and H. W. BERGK,Z. Physik. Chem. (J~e~z~J) B38, 319 (1936). L. E. LYONS,J. Chem. Hoe. 1347 (1958). W. T. SrEPso~ and D. L. P~.T~.Rso~, J. Chem. Phys. 26, 588 (1957). R. M. HOC~ST~ASS~.~,Can. J. Chem. 39, 451 (1961).

Some effects of molecular orientation on fluorescence emission and energy transfer

447

values in the region of 10 -~ to 10-9 sec and the small slow component observed in a number of cases is non-exponential [26].

E N E R G Y TRANSFER IN AROIkiATIC CRYSTALS The extremely efficient sensitization of impurity emission by the host lattice exhibited by solid solutions of aromatic hydrocarbons is attributed to the extensive migration [27] of excitons which are trapped at the impurity sites; a necessary condition for this phenonemon is that the emitting level of the impurity has a lower energy than the exciton band of the crystal. Since both anthracene and pyrene are quenched by dissolved perylene, coronene and ovalene [28], exciton migration is c o m m o n to both type-A (anthracene) and type-B (pyrene) lattices; however, whereas crystalline anthracene is quenched in addition by dissolved naphthacene and pentacene, no transfer is observed from crystalline pyrene to these solutes, both of which satisfy the energy requirement but which are believed to crystallizewith a type-A lattice. This is consistent with the observation [29] that solid 3,4-benzpyrene does not sensitize the emission of dissolved naphthacene unless this solvent has the blue-fluorescent modification for which a type-A lattice is proposed on the basis of its fluorescence spectrum. Despite the small number of systems investigated--the much lower energy of the exciton band in a type-B lattice severely limits the number of type-A solutes which satisfy the energy requirement--the sharp distinction between a relatively efficienttransfer from pyrene to type-B solutes and its complete absence in the case of dissolved naphthacene and pentacene is undoubtedly related to the configuration of solute and solvent molecules [28], and in the absence of other satisfactory criteria it is suggested t h a t provided the energy condition is fulfilled a type B host lattice is quenched only by those impurities which themselves have the same lattice structure. On the basis of this postulate the transfer from pyrene to dissolved anthanthrene, violanthrene and isoviolanthrene [28] confirms the lattice assignm e n t based on the emission spectra of anthanthrene, and indicates t h a t the latter compounds adopt a t y p e - B lattice in accordance with their molecular dimensions. These assignments are summarized in Table 1. At low concentrations the correspondence between the emission spectrum of the solute and its molecular absorption spectrum [28] signifies a small energy of interaction between the host lattice and the molecularly dispersed solute; however at higher concentrations the molecular fluorescence spectrum of pyrene dissolved in solid naphthalene or fluorene, is replaced by the excimer band [30] (as in liquid solutions) illustrating the tendency of this solute to dissolve as associated pairs e v e n in an alien lattice. A similar effect has been observed from solid solutions ofperylene in naphthalene [31] and is less clearly demonstrated by anthanthrene in anthracene owing possibly to the very low fluorescence yield of molecular pairs in this case [28]. [26] [27] [28] [29] [30] [31]

E. HUTTONPh.D. Thesis, Sheffield (1961). O. Srmeso~, Proc. Roy. Soc. (London) A238, 402 (1956). D. C. NORTHROPand O. SI~PSO~, Proc. Roy. Soc. (.London) A234, 136 (1956). F. W~.m~.RT,Trans. Faraday Soc. 86, 1033 (1940). A. SCHm~L~,~,Z. 17aturforsch 169, 5 (1961). B. STEVENSand T. DICKINSOn.Unpublished data.

B. ST~.VENS

448

Table 1. Crystal lattice assignments according to

naphthalene anthracene phenanthrene chrysene naphthacene pentacene pyrene 3,4-benzpyrene monoclinic sublimate perylene 1-12,benzperylene 20-me~hylcholanthrene anthanthrene coroncne ovalene violanthrene isoviolanthrene

Molecular shape

X-ray analysis

Fluorescence spectrum

A A ,4 .4 ,4 ,4 B A,B

A A A A ,4 ?

A A .4 .4

B1

B

B2

B .4 B B B B B B

B B ,4,B B B B B B

B1 B1 B2 B~ B2

Exciton capture

A A

A B B B B B B

T h e p r e f e r e n c e for m o l e c u l a r a s s o c i a t i o n e x h i b i t e d b y " d i s k - s h a p e d " a r o m a t i c molecules b o t h in t h e p u r e l a t t i c e ( t y p e B1) a n d in solid s o l u t i o n could a c c o u n t for t h e a p p e a r a n c e of a n e w emission b a n d [32] a t longer w a v e l e n g t h s , as t h e c o n c e n t r a t i o n of c h l o r o p h y l l - a is a n e t h a n o l glass is increased. Unless o t h e r w i s e s t a t e d t h e fluorescence s p e c t r a p r e s e n t e d i~ this p r e l i m i n a r y r e p o r t were r e c o r d e d p h o t o e l e c t r i c a l l y in reflexion o n e q u i p m e n t f r e e l y m a d e a v a i l a b l e b y Dr. GR~,GORIO WV,B~,R; t h e r e c o r d i n g s h a v e b e e n c o n v e r t e d t o r e l a t i v e intensities b y c o r r e c t i n g for p h o t o m u l t i p l i e r r e s p o n s e a n d t r a n s m i s s i o n o f t h e s c a n n i n g m o n o c h r o m a t o r . S a m p l e s of a n t h a n t h r e n e a n d o v a l e n e g e n e r o u s l y d o n a t e d b y Dr. D. C. NORTHROP w e r e u s e d w i t h o u t p u r i f i c a t i o n ; o t h e r c o m p o u n d s o f t h e h i g h e s t p u r i t y o b t a i n a b l e f r o m c o m m e r c i a l sources w e r e s u b j e c t e d to f r a c t i o n a l m i c r o s u b l i m a t i o n [33] a n d t h e s a m e s p e c i m e n s u s e d t o o b t a i n fluorescence s p e c t r a in t h e c r y s t a l l i n e a n d dissolved forms. Acknowledgements--The author would like to express his thanks to Dr. WEBER and to M~. JA~r~s LO~GWORTHfor his assistance with the recordings; to Drs. D. C. I~ORTm~OP,J. B. BIRKS and J. C. S P E ~ for illtuninating and orienting discussions, and to the Royal Society and ~he Chemical Society for grants-in-aid. Note added in proof--Since this paper was written the crystal structures of naphthacene [35] and pentaccne [36] have been published and confirm the assignments given in Table 1. The author is grateful to Dr. TRoTTv.R for drawing his attention to these publications. [32] [33] [34] [35] [36]

S. S. BRODY, Science 128, 838 (1958). W. H. I~ELlfUISH,Nature 183, 1933 (1959). E. CT.AI~,A~omatische Kohlenwasserstoffe. J. M. ROBERTSO~, V. C. Sn~cv.Ara and J. TROTTER, Acta Gryst, 14, 697 (1961). R. B. CAMPBELLJ. M. ROB~.RTSO~ and 5. TROTTER, Acta Cryst. 14, 705 (1961).