Site-selective photochemistry in anthracene crystals

Volume57,numbei

CHEMICAL

2

SITE-SELECTIVE

D&l. BURLAND

PHOTOCHEMISTRY

PHYSICS

LE-ITERS

IN ANTHRACENE

1.5 J*dy 1978

CRYSTALS

and J-M. THOMAS*

IBM ResearchLaboratory. SanJose. CalifornCz 95193. USA Received

21 April

1978

Site-selectiveheterodirnerformationin anthracenecrystalscontaininga smallamount of 2hydroxyanthracene(2OEL4) hasbeen investigated.While direct irradiationat low temperaturesof the absorptionoriginsof the two 20HA sites (Oa and 09) in anthracenedid not lead to dimer formation, one of thesesites(09) is photoactivewhen irradiatedat room temperature. From an analysisof the Og and 09 fluorescenceexcitationspectrawe infer an involvementof the X-traps in heterodimerformation. Furthermoreit seemslikely that an intermediate,perhapsan exeimeris involvedin the process_

1. Introduction When anthracene crystals [I] or solutions [2] are exposed to W light, photodimerization occurs producing di-para-anthracece. In the crystal this dimeriz&ion occurs at defect sites, where the orientation of the two molecules allows the 1,4-l ,4 cycloaddition with a minimum amount of motion [3,4] _ Such sites may be associated with 2 metastable triclinic phase of anthracene that has recently been shown to coexist as microdomains inside bulk specimens of the stabIe monoclinic phase [4] _ The active sites for photodimerization could either be in the bulk of the triclinic phase or at the interface between triclinic and monoclinic phases. Recently Craig and Rajikan [S] have reported the probable formation of a mixed dimer in anthracene crystals irwolving an anthracene molecule and a 2hydroxyanthracene (20HA) molecule. 20HA is a naturally occurrmg impurity in anthracene 161. These impurity molecules occupy two sites [6] in the anthracene crystal and consequently show two fluorescence origins, OS at 24352 cm-l and 09 at 24165 cm-l _ The intriguing feature of this mixed photodimerization is that the 0s site appears to be photostable, only the 09 site being involved in dhner for* IBM VisitingScientist,Summer 1977. Presentaddress:De-

partmentof PhysicalChemistry,Dniversityof Cambridge, Ckunbridge, UK.

mation. This site selective photochemistry, observed by Craig and Rajikan, was not seen by Williams and Clarke [7], who reported both 0, and 09 sites to be photochemicahy active. In this paper we wish to report confmation of the Craig and Rajikan results. In addition we have investigated the 08 and 09 fluorescence excitation spectra and have attempted to effect heterodimer formation by selective irradiation of 0s and Og sites. We are able tp infer details of the photodimerization process from this information.

2. Experimental The crystals used in these experiments were grown from the vapor using the Radomski technique and were prepared at the University College of Wales, Aberystwyth [7,8]. The results reported here were obtained with anthracene crystals doped with lOa M/M carbazole and an unspecified but small amount of 2 OHA. The crystals were from the same batch as those used by Wtiams and Clarke [7]. The purpose of the carbazole was to enhance the concentration of X-traps which as Craig and Rajikan [S] found and as we shall see later seem to be involved in the dimerization process. X-traps are anthracene molecules that, because of their proximity to an impurity or defect, absorb and emit at an energy that differs slightly from unperturbed anthracene molecuIes. 163

VOlu.zn~ 57. nu3nber 2

The fluorescencespectra were obtained using a

1

200 W Hg-lamp as an excitation source. The exciting light was passed through 10 cm of water and a Corning 7-54 g&s falser before striking &e front szface of tie crystal. In addition the Ienses used to focus the light were made of optical crown glass so tha; little Light of wavelength shorter than 3000 A reached the crystal. The crystd was in a variable temperature cryostat and its temperatiJre cotid be varied from 2 K to room temperature. The fluorescence light from the front surface of the crystal was focused onto the slits of a high resolution monochromator and anaiyzed photoelectricalIy. The dispersion of the mo’nochromator w-ashigh enough (2.6 &mm) so that the spectra reported here are not sEt width iirnited- The excitation spectra were obtained by setting the monochromator at either the 08 or 09 fluorescence maximurn and scarming a Nz pumped puked dye laser.

3. Results In Gg. I we show the few temperatire fluorescence spectra of the anthracene crystals before and after W irradiation for one hour at room temperature. The irradiation was accornplisked by removing the 7-54 Corning glass Mter tht3sexposing the c~ysti to all radiation from the farnp to longer wavelength than 3000 a It is necessary that this irradiation be done in the absence of oxygen to prevent the build-up of an anthraqninone layer that would obscure the bdk fluorescence. Several Lteatures, previously noted by Craig and Rajikan I.51 are clear from the @ure_ First the 0, he has clearly lost-intensity with respect to the 0s line. Second one of the X-trap lines near 4000 A has also lost istmsity with respect to other lines in the pectra. These facts are summarized in table f, where the 08 , 09 and X-trap fluorescence intetities are comrjared to the line fabeled a in fig. I_ a is an intrinsic fluorescence of the anthracene crystal [9]_ Table 1 shows that the intensity changes upon W irradiation are due to decreases in the intensity of the O9 and X lines with respect to other lines in the spectra. No photochemical changes are observed if the cry.+ tai is below 77 K while being irradiated_ Nor were any observed at 2 R by directly irradiating the 0, or 0, 164

OS

Fig- 1. l%e fluOrescence Spectraat 3 K of anthracenecrystals doped with 104 M/Mcarbazoleand a small axnolrntof 20HA before and after one &our of irradiztioa The 20HA sites art?k&e&xi 08 and 09, X indicates an X-trap emission and CYrefers to au intrinsic fiuorescence Iine of anthracene,

absorption origins with a laser_ The 0s and Og Buorescence excitation spectra at 2 K are shown in fe- 2. The sharp lines in these spectra correspond to absorption lines of ZOHA. This can easily be seen if One shifts the 0, spectrnm with respect t0 the 09 spectrum in fig_ 2 so that the fluorescence origins coincide- This has been done in fig. 3, where we see that most of the sharp spectral features in the 09 spectrum lie below similar features in the 08 spectrum-Table 2 contahs measuremeatsof the positions of the lines in the 0s and 09 excitation Table 1 Relative intensities of various fines in the anthracene fiuores G?nce spew 3

Beforeirradiation After irradiation 0.94 1.22 1.55 LOO

251 1.15 0.47 0.32

d The Iabeling of t&e iiaes refers to the IaWing h fig. 1.

Volume

57. number

2

CHEMICAL PI-SYSLCSLETTERS Table 2 The fluarescencs in anthicene h
15 July 1978

excitation spectra of 2-hydroryaathracene

t (vacuum)

av’ from origin

(cm-‘)

(cm-‘)

(24352.0)

-

Og-line

Fig. 2_ The fluorescence excitation spectra of the ZOEU sites in anthracene at 2 K_ Spectrum (a) is the 09 site 2nd
0s site.

4051.8 4045.1 4040.7 4038.4 4032.7 4032.0 40255 4002.4 3995.4 3984.9

24673.4 r 3.0 24714.3 24741.2 24755.3 247835 24787.8 24834.6 249779 25021.7 25087.6

321.4 362.3 3892 403.3 4315 435.8 482.6 625.9 669.7 735.6

Og-line

spectra. Here again we see from t&e similarities in the values for E that the two spectra correspond to the abso_rptionof the same chemical species. An interesting feature in both excitation spectra is the pronounced minimum near 3985 A. This minimum coincides, within the accuracy of our measurements (-C3 cm-l), with the position in the fluorescence spectrum of the X-trap whose intensity changes upon UV irradiation_ This means that when the X-trap is excited energy transfer to both O8 and O9 20HA moiecules is impeded by some process. Since this Xtrap is proposed to be involved with the heterodimer formation, it seems Iogical to conclude that an inter-

40825 4076.6 4075.7 40695 4063.2 40625 4056.0 40533 4049.6 4030.4 40179 3997.1 3985.2

(24165.0) 244879 24523.3 24528.7 24566.1 24604.2 24608.4 24647.9 24663.1 24686.8 24804.4 24881.6 25011.1 25085.7

3225 358.3 363.7 401.1 4396 443.4 4829 498.1 521.8 639.4 716.6 846.1 920.7

involved in dimer formation is inbibiting energy transfer (see later).

mediate

1 Jd Fii. 3. The fluorescenceexcitation spectxaof the (a) 08 and (b) 09 &es % h f%. 2. Here the 08 Iipem ed so that 08 .md 09 origks correspond.

has be% shift-

4. Discussion Craig and Rajikan [S] interpreted their experimentaI results as being due to the “probable” formation of a mixed dimer. It would perhaps be well to emphasize, however, that while their results and the results reported here strongly support this interpretation, the interpretation has not been proven. With this cautionary note we will assume that the photochemistry we are observing involves the formation of a mixed antbracene-20HA dimer. Much is known about dimer formation in puR an165

Vollllne 57, number 2

cx3EhfICAL PHYSICS

thracene crystals. Lattice defects and other faults, including metastable phases, appear to play an important role in the process [3,4,10,1 l] _ In a perfect crystal of anthracene neighboring molecules do not have the correct orientation for stable dimer formation_ However, at some of the faults that have now been identified the orientation may allow two neighboring antbracene molecules to form stable dimers. Some of these faults arise as a result of stress; some, it now seems clear [l l] may be a consequence of the disruption of the lartice by impurities. The actual mechanism of photodimerization is not known in detail. It obviously involves the initiai excitation of one of the dimerizing molecules. It has also been suggested ]12,13] that an excimer state is involved as an intermediate. Chandross et al. 1131 have identified two types of anthracene excimer, only one of which is an intermediate in the formation of a stable dimer. It is clear from Ferguson’s recent work on the anthracenophanes 1141 that a wide range of smoothly varying conformations and orientations of two interacting antbracene molecules may all display excimer behavior. Another observation that will be important for our discussion of heterodimer formation is the importance of molecular motion to-the dimer formation. Jones and Nicol 115J have noted that, when antbracene crystals are subjected to-hydrostatic pressure, photodimerization does not occur even though extimer ffuorescence is observed_ One interpretation of these results is that stable dimer formation requires moIecuIar motion_ Since this motion is restricted in antbracene crystals under pressure, dimer formation itself is reduced in these systems. In the light of this previous work on anthracene photodimerization, we can now outline a plausible explanation of our results on anthracene-20HA heterodimer formation. The relevant experimental observations to be explained are: (i) the relationship between the intensity decrease upon n-radiation of the 09 fluorescence line and the parallel decrease of the X-trap line; (ii) the restriction of photochemistry to high temperatures; (iii) the decrease m 0, and Og fluorescence intensity upon direct excitation of the X-trap. The Grst point has been considered by Craig and %jikau [S] _ From it they inferred that mixed dimer formation could occur only at 09 sites perhaps be166.

LETTERS

15 July 1978

cause of the relative orientation of the 2OH.A molecule with respect to neighboring anthracene molecules at this site. The X-trap is also associated with this reactive site, Either these X-traps themselves or anthracene molecules in their vicinity are involved in heterodimer formation_ Excitation of one of the pair of dimerizing molecules seems to be necessary but not sufficient for stable dimer formation_ Observation (ii) above implies that a considerable amount of thermal motion is necessary for dimerization. The remarkable feature about observation (iii) is that both 0, and Og sites seem to be affected in a similar way by X-trap excitation (see fg. 2). In both cases energy, that might be expected to be partially transformed into 20HA fluorescence, is drained off by some other process. One possrble explanation of these results is that heterodimer formation involves the initial formation of an intermediate such as an excimer. At low temperatures thermal motion is not sufficient to permit stable dimer formation, so the intermediate decays radiationlessly or radiatively back to the ground state_ The net result is that this energy is not available to be transferred to an isolated 2OHA molecule and thus no fluorescence is observed upon X-trap excitation. The fact that both 0, and 0, sites show the same behavior in this regard is initially a bit puzzling since only the Og site appears to be photochemistry active. If, however, we imagine that both sites are in so-me way associated

with the X-trap

and can form

the in-

termediate state mentioned above, then the results can be explained by asstiming that only the Og site can, ar room temperature, go from this intermediate state to a stable dimer. The implication is that the hydroxyanthracene in the 09 site is less rigidly constrained *&an that in 0, or alternatively, that the 09 molecule is already closer to a stable dimer co&gun+ tion with respect to one of its neighbors. It is tempting to assume that this intermediate state is an extimer since, as we have discussed above, anthracene and its derivatives are known to form a wide range of excimeric species. In conclusion we have conf%rmedthe earlier fmdings of Craig and Rajikan [S] concerning site-selective heterodimer formation invoMng anthmcene and 2OB-L From the low temperature excitation spectra

Volume 57. number 2

czIiEb%IcAL PXYSICS LETTERS

of the two 20HA sites, we have been able to infer the presence of an intermediate state, perhaps an extimer that inhibits energy transfer from the X-trap to the 20HA molecule. Acknowledgement We thank J-0. Williams and J. Ferguson for stixnulating discussions. References [l] J.K.S. Wan, R.N. McCormick, E.J. Baum and J.N. Pitt%. J. Am. Chem. Sot. 87 (1965) 4409. I21 EJ. Bowen and D.W. Tannen, Trans. Faraday Sot. 51
15 July 19?8

(5) D.P. Craig axedJ. Rajikan, Chem. Phys. Qtters 47 (1977) 20. [6] NJ. Bridge and D. Vincent, J. Chem. Sot. Faraday II 68 (1972) 1522; A. Brilkinte,DS. Craig, A.W.H. Nau and J. Rajikan, Chem. Pliys. Letters 31<1975) 215. [7] 1.0. Williamsand B-P. Clarke, 1. Chem. Sot. Fzaday lI 73 (1977) 1371. [S] N. Radomska, R. Radon&i and K. Pigon. NoL Cryst. Liquid Cryst. 18 (1972) 75. [9 ] L.E. Lyons and L J. Warren, Austnlian J. Chem. 25 (1972) 1411. [lo] J.N. Thomas and J-0. Williams,Progr. Solid State Chem. 6 (1971) 119. [ll] GM. Parkinson,Ph.D. Thesis,Universityof Wales, Aberystwyth, 1978. 1121J.B. Birks and J.B. Aladekemo, Photochem.PhotobioL 2 (1963) 415; J.S. Bradshaw, N.B. Nielsen and D.P. Rees, J. Org. Chem. 33 (1968) 259; J. Ferguson andA.W.H. Nail, Mol. Phys. 27 (1974) 3771 (131 E.A. Chandross,J. Fergusonand E.G. NcRae, J. Chem. Phys. 45 (1966) 3546; E.A. Chandrossand J. Ferguson45 (1966) 3554,3563. [14] J. Ferguson,private communication;J. Am. Chem. So=., submittedfor publication. [lS] P-F. Jonesand N. N&l, 1. Chem. Phys. 48 (1968) 5440.

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