Naphthalene triplet state interactions: Concentration and polarization dependence of mixed crystal phosphorescence line shapes

1 May 1971

CHEMICAL PHYSICS LETTERS

Volume 9, number 3

NAPHTHALENE

TRIPLET

C~~~~~~TRAT~U~ OF MIXED

STATE

INTERACTIONS:

AND POLARIZATION

CRYSTAL

DEPENDENCE

PHOSPHORESCENCE

LINE

SXAPES

*

C. L. BRAUN** and H. C. WOLF’

3 P&i&&s&es

Institut, Universitat Stuftgurt, Germcny

Stuttg-iati,

Received 12 January 1971

High resolution phosphorescence apeotra in the region of tbe zero-zero transition are reported ior naphthalena ClOH9 in a CLuD3 host with guest coucontration ranging from 0.2 to 10 mote percent. A concentration-dependent fine structure present io the spectra fnr guest concentrations above 1% is interpreted as arising from the intaractioa of nearest-neighbor ClOHg molecules.

1, INTR~DltrCTKON Observation af the phosphorescence spectrum CIoH3’a.s a guest in a cIDf)g host crystal under high resolution (0.5 cm-11 reveals a concentr:ition-depsndent fine structure which is eopecially obvious in the zero-zero transition. The obserlfed concentration and polarization dependences rule out site spfitting II] and isotopic impurity transitions as the source of the structure, which appears instead to result from the

of.naphthalene

i&?ra&iCIn

Of neaI%St-neighbor

Cl $8

paiFS.

Phosphorescence was excited in cieaved singIe crystals immersed in liquid He using light from an H330-5OO’iampfiltered with 10 cm of Ii@, a Corning 7-64 color filter and a Schott 310 nm reflection filter. Excitation entered normal to the crystal cleavage face and phosphorescence transmitted through the crystal wss imaged on the 15~ slits of an RSV spectrograph with reciprocal dispersion of 4 A/mm. Most frequently, spectra were, recorded photoelectrically by means of an exit slit and photcn%itiplier assembly that was scanned along the speetromoter focal plane. Fig. 1 is a composite ieproduction of phosphorestzepce spectra in the region of the zero‘zen,‘transition far crystals, that are 0.2, 2, 5, ,’

* Work su&.uM.ed by the Deutsahen Forachukgsge-

meinachaft.

?* Permanentaddress: Departntent of Chemky. Dartmaqth

Coliagi,

: . 03755;.USA.

i60”

Hanover, NevlHam&&ire

and 10 mole % in CL&. A 0.1% crystal has a spectrum essentially identical with that shown for a 0.2% crystal. Because for both of these dilute-guest crystals the transitions at +1.3 and +3 cm-1 from the main, sc polarized, zerozero line are also dc polarized and have intensities of 4 to 5% of the main tra.)sition intensity, it is reasonable to assign them as the zero-zero transitions of (r and /313Gsubstituted CIOH3 Th&assignment of the +3 cm-l transition agrees with that of Hanson [2,3]. It should be noted that the deuteronaphthalene transitions observed ir? the eero-zero spectral region by Hanson [2] did not appear in the present work, presumably because potassium-fusion purllication [3,4] was carried out separately for ClOHg and C+8. At concentrations of 1% and above, satellite tran@tions appear as shoufders located on either side of the isolated-gue& zero-aero line $. Hanson has recently reported [3] a high-resolu$ FOF 5 and10%crystals, the intense vibronic bands

located [ZJ at 512 snd 1381.6 en& aleo rewal in their marked asymmetry the presence of a concentmtiou-dependeat, lower energy phosphorescence origin, Shoulders are not very ,well-defined in these v&tonic transitfons, hoxwver, because even at low guest concentration, Prhere the observed .iera_zero line wfdtb is determined by spectrometer resoluff on;

the‘above vihronictransition& have a real widthof abwt 0.5 cm-l,’ whichmaybe a measureof the rate of v&rationalretaxatiod.in the ground state. In what fol tows, ,p3 shall for convenience refer to molecules ~oeoupying any of th&e six sites Ss neareot-nefs?3-

~ ,bora in spite of the fact that two modestly @fewat separation dis+xzes arq invotved. I .” _’ .. :. .’

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Volume 9. number 3

1 May 1971

CHEMICAL PHYSICS LETTERS

concentration*. Theoretical calculations (51 predict that significant (exchange) interactions arise only between nearest-neighbor molecules which lie in the same a6 plane. In this plane, a CloK8 molecule is surrounded by four interchange equivalent and hvo translationaIly equivalent nearest-neighbor sites. The observed concentration dependeace of the satellite intensity is roughly consistent with detailed considerations of the statistics for occupation of the nearest-neighbor sites by other Cl&I6 molecules. For example, with a 5% crystal the total satellite intensity is roughly 35% of the center band tntensity. Site occupation statistics indicate that a Cl&j molecule is 0.36 times as likely to have at least ane other Cl6H6 molecule as a nearest neighbor as to be isolated (i.e. without nearest CloH8 neighbors). The corresponding probability ratio for at least one interchange-equivalent nearest neighbor is 0.23. These comparisons should not be t&en to indicate that interaction between translationally Fig. 1. Zerc-zero phosphorescence spectra of ClOH8 in a Cl& host crystal. From bottom to top with arbitrary displacement of tbe baseline: 0.2, 2, 5, 10 and 10 mole per cent CloH8. The top spectrum is for a crystal at l.S°K; the remainder are at 4.2°K. The original spectra were digitized and then scaled both vertically and horizontally before computer plotting of tbe above results.

The vertical

scaling allows the center

band, “isolated guest”, peak intensity to be equal in all plots in order to make obvious the growth of satellite intensity with ~10H8 concentration. The horizontal scaling was necessary in order to allow approximate correction for non-linearity in the original wavelength scale. Tbe “isolated guest” transition energy was measured photographically to be 21208.9 cm-l for a 2% crystal and the other spectra are arbitrarily centered at the same value. tion phosphorescence spectrum for a 1.4% crystal which is similar to but less well-resolved than the spectrum of a 2% crystal illustrated in fig. 1. He tentatively assigns transitions at +1.3 and -1.2 cm-l, relative to the isolated guest transition, to the resonance splitting of nearestneighbo: interchange equivalent pairs of C!I$I6 molecules. .The results we report allow a test of this assignment although it ma be noted at the outset that the lower-energy lEC$I6 transition identified above should make an important contribution to the intensity observed at il.3 cm-l in a 1.4% crystal.

2. CONCENTRATION DEPENDENCE Fi& 1 illustrates that relative to the isolatedguest intensity, the satellite transition intensities at k(l.2*0.2) cm-l grow rapidly with guest

equivalent

molecul.zs

is necessarily

suf-

ficient to contribute to satellite band intensity, but rather only that considerations based solely on nearest-neighbor interactions are mea.ningfuL Truly quantitative interpretation af the concentration cfependence results is complicated not only by the obvious problems of spectral resolu-

tion and 13CCgR6 interference but also by the fact that for 5 and 10% crystals all origin levels

are in thermal equilibrium wirith one another. The latter observation is exemplified by the top two

curves in fig. 1. where lowering the crystal temperature from 4.2 to l.B°K shifts sufficient population into the lower energy transition to allow its resolution. In addition, as will be discussed further, the satellite transitions are polarized differently from the center transition. Thus, on the basis of concentration dependence alone, it does not seem possible to decide whether the interaction between translationally eqGvalent gueets is large enough to contribute to the satellite band intensity. 3. POLARIZATION

DEPENDENCE

A pair of interchmtge equivalent guests is ex* A tracking error in the wavelength

scanning device produced a reproducible non-Linearity in the scan rate. Variations %ere approximakly corrected for by recording He and Ne calibration lines in the vicinity of the napbthalene zero-zero transitions. The quoted error limits (95% co&deuce level) reflect the nkcesaarily approxfmate nature of the wavelength corrections made for about 20 spectra from crystal’s of varying concentration.

261

b

1 May i97i.

CHEMICAL PHYSICS LETTERS ,.

Volurrk 9, number 3

petted to give’ rise to ho opbositoly ‘polarized eiec$mic transitions approximately equally spaced above and below the “isolated” guest transition while a @air of tratrslat~~tnEly -equivalent guests give rise to a single tr~~iti~~n,displaced from,. but polarized, We, the isolated guest transition [3). For a 3.5% c~ystaI, fig. 2 indicates that the higher energy satellite transition is ac polarized whife the lower energy satellite is b polarizecj. S$milar polarization char&cteristics were observed in 2, 5, and 10% crystals as well, but the satellite -signal to noise was not so favorable .as in fig. 2. The polarization behavior is consistent with that observed for a Cl9H6 crysta1 in absorption when the higher energy Davydov corn: -ponenf is 8c and the lower Zrpolarized. Thus it appears ttiat the interchange equivalent interactions which give rise to the Davydov splitting in a pure crystat are also responsible for at least the major portion of the satellite intensity. Correspon~i~iy there appears to be little doubt that I.2 f 6.2 cm -1 can be identified with the interaction energy of an interchange equivalent pair of ClOH3 molecules. This conclusion is reinforced by the fact that it agrees very well with the 8.8 cm-l Davydov sp;itting (61 reported for the zerozero transition of a CloHS crystal where if nearest-neighbor interactions only are significant, the factor grolap splitting should be equal to eighi: times the interchange equivalent interaction energy. This vaiue is also consistent with ESR hata [?I *.

The relative intensities of the‘two satellite trarisi-lionsobserved in figs. 1 and ‘2 are not fully understood. From crystal, &sorptton spectra [ti], one expects that the higher-energy, ac potar&d component should be approximately four times as intense as the lower-energy, b potarized component. it does not appear that Boltzmann factor considerations alone are sufficient to explain the reversed relative intensities faund for 3.5, 5, and 10% crystals even though it should be realized that such a conclusion is made somewhat tenuous by spectral resolution limitations. No trace of the predominantly ac polarized intensi?y expected from tr~la~onaUy equivalent pairs is obvious in fig. 2. White these pairs could contribute to the higher energy satellite intensity, such contribution would only make rationalization cf the observed relative intensities more difficult. It does not seem likely either that translationally equivalent pairs contribute to the iower-energy satellite intensity as it is fairly cleanly b polarized, The tentative conclusion then is that the interaction energy of a translationally equivalent pair is not sufficient to allow obsWvation of the resuiting transition under the resolution conditions of these experiments.

ACKNWVLEDGEMENT The authors acknowledge substantial assistance from Mr. R. Sehmidberger in the early phase of experimental work and from Mr. R. Farber in computer processing of the spectral data. Supportfor Charles Braun from a Dartmouth College Faculty Fefbwshtp is also gratefully acknowledged..

+ MO&measurements and a more detailedthf?oretfCilt anatysis wilt be publishedsoon. t -I

.‘Q”, REFERENCES

Y

fW’)*

Y

(11 E. F. Znlewski and D. S. McClure. in: Molecular kmi-’ oescence, ed. E. C. Lini (Benjamin,NewYoric, 1969) p. 739. 121 D. M. Hnnson.J. Cbem.Phvs. 61 ci969t506% i3j I). M. Hnnaon; J. Chem, Ph$?. 52 il97Oj 3409. [4] D. M. Hanson and G. W. Robinson, J. Chem. Plays, 43 (?905) 4174. i5jJ.Jorbcr. S.A.XUce, J.L.KatzandS.Choi, J. Chem. Pbys. 42 (1965) 309. [SJ G. Castro and G.,W. Robinson, J. Chem.Phys. 50 (1969) 1159. [!I g,7Schwoerer and H. C. Wok Mot. Cryst. 3 (1967)

RW’P-.

‘Fig. 2. Polarized zero-zero‘phosphorescence spectra ‘at 4.241 for 3,5 mole % C.i&, in ClOD6: The isolated’ .. ‘.. guest transitionoccurs at Up

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