The phonon structure, triplet exciton splitting and predissociation of p-diiodobenzene

Volume 36, number 2

THE PHONON

CHEMICAL PHYSICS LEmRS

STRUCTURE,

TRIPLET

EXCITON

SPLITTING

1 November

1975

AND PREDISSOCIATION

OF p-DIIODOBENZENE Donald

M. BURLAND

IBM Researclr Laboratory,

Sm Jose. Cdifomia

95193. USA

Received 14 May 1975 Revised manuscript received 27 June 1975

The singlet-triplet

abcorption

spectrum

of puic pdiiodobenzcnc

crystals is shown to be dominated

by phonon

side-

bands. ?C’heintensity and extent of this progression is altributed to predissociation of the lowest triplet state. In view of thcsc rindinps, it does not seem necessary to assume, as has been done previously, that pdiiodobcnzene has 3 large triplet csciton

bandwidth.

1. Introduction The triplet exciton properties of crystalline p-diiodobenzene (DIB) have been somewhat of an enigma. While p-dichloroand p-dibromobenzene have no resolvable factor group splitting, indicating an extremely narrow exciton band (less than 0.2 cm-l for the O-O band) [l]. DIB has previously been assumed to have an exciton bandwidth more than a factor of ten times larger than triplet exciton bandwidths in any other molecular crystal so far studied. Castro and Hochstrasser [2] deduced a total factor group splitting of about 970 cm-’ from a strong vibronic coupling analysis of the crystal absorption spectrum. The extraordinarily large triplet factor group splitting is even more amazing when one notes that p-dichloroand p-dibrgmobenzene have almost the same crystal structure as DIB. One obtains the orthorhombic DIB structure (Pbca) [3] from the monoclinic p-dichloro or dibromobenzene structures (P21/a) [4] by only a slight rotation of the molecules. In this paper we will present experimentd evidence indicating that the unusual effects in the DIB singlettriplet absorption spectrum are not directly.related to the triplet exciton bandwidth but rather are manifestations in the crystal of the predissociation of DIB in the triplet state. In short we will show that the spectrum is dominated by phonon sidebands and that one

need not assume a large triplet exciton understand the absorption spectrum.

splitting

to

2. Experimental Considerable

care was taken in the purification and The raw DIB (Eastman 1102) was recrystallized twice from absolute ethanol, sublimed and then chromatographed over alumina and charcoal using absolute alcohol as the eluent. Finally, the DIB was extensively zone-refined. Crystals were grown in a Bridgman furnace. All of these steps, except for the sublimation, were carried out in the dark since it is well-known, as we shall discuss iater, that DIB photodissociates when it is molten or in solution

growth of the DIB crystals.

VI. The absorption spectra were recorded photoelectrically using a 3/4 m Spex spectrometer and 2 4000 W dc Xe arc lamp. The spectrometer resolution was 0.4 ,& in first order, the order in which spectra were taken. The DIB crystal was held in a strain fr2e mount insi& a variable temperature liquid helium cryostat. The temperature of the crystal was monitored by a Ge resistance thermometer located next to the crystal. The absorption spectra showed no significant sharpening below about IO K. Above ‘&is temperature‘the fine structure, to which we will allude shortly, disappeared. 247

Volume 36, number 2

CHEMICAL

PHYSICS

1

LETTERS

! 31

3500

3550

3650 Wavclenglh

3700 (.-I)

3750

3800

about 0.5 mm and oriented coniscopically.

3. Restilts

In fig. 1 we see the b-polarized DIB absorption spectrum at 5.5 K. There is a steadily rising baseline in the spectrum, indicated in the figure by the dotted line, due to the spectral characteristics of the Xe lamp in this region. The major features o$ the spectrum

agree well with the earlier work of Castro and Hochstrasser 12). In this paper we are concerned with the regular progression of lines superimposed on the gross structure. These features are accentuated by vertical lines in the figure. Fig. 2 is an illustration of the first broad absorp tion band in both b and c polarizations. Two features should be note& First, the regular progression is present in both polarizations and secortd there is, as Castro and Ho&s&&r [2] noted, a shift of about 115 cm-l absorption

tions. -2%;:.

-I,

I

I

0

3710

3: 10

3850

The crystals we used were cleaved to a thickness of

the

I

3630

,

3600

Fig, I. The b-polarized pdiiodobenzene SO-‘T1 absorption spectrum 3t 5.5 K. The doTted line is a rough indication of the behavior of the Xr lamp in this spectral region. The short vertical lines above the spectrs emphasize the position of the fine structure in the spectra.

between

I 367c)

Wavelength (A)

I-

1

3

1975

NOVembei

-...

maxima

in the two

polariza-

Fig. 2. Absorption in the region of the Iirst broad maximum in the pdiiodobcnzene spectrum at 6.5 K. (a) c-polarization, (b) b-polarization. in both cwcs a 3 5 time constant was used with a 2.5 A/min scan rate.

If we measure the progression frequency in both 6 and c polarizations we find an average separation of 34cm-’ in the b polarized spectrum and 33.5 cm-l for the c polarized absorption. While the average frequencies in the two polarizations are in good agreement with each other, one should note that there is a relatively larger scatter (? 10 cm-l) in the enerm separation between any pair of adjacent peaks in the spectra. In table 1 the DIB Raman frequencies have been listed. Note that the observed 34 CITI-~ interval is in the region of the DIB lattice frequencies. This strongly suggests that what we are seeing here is not an absorption to an exciton

origin but rather

ar, intense

and extensive

phonon

sideband. Further support for this interpretation can be found by comparing the spectra in fig. 2 with absorption spectra in which strong phonon sidebands are known to be present. ExBmples that look remarkably like the

spectra presented here can be found in the 4.2 K luminescence excitation spectrum of ST centers in KI [6 1, the S,-,+ S1 absorption spectrum of naphthalene in biphenyl [7], the absorption spectrum of Yb2+ in ‘Kerr at 10 K [S] and the absorption spectra of p-cl-do.-aniline and p-brornoanilink in biphenyl. ai 4.2 .K [9]. All of these spectra share the characteristic that the ‘.

Volume 36, number 2 Table 1 Ramnn active opt&!

IlSi __. Assignment

phonon

Ag

33.0 36.9 39.1 44.7

B3g % %

for p-diiodobenzenc

Frequency (cm-l)

21.9

Ag

frequencies

._.. . ___._

Bgg Bl,o Big B3g “,D B2g

CHEWCAL PHYSICS LETTERS

31.3

47.9

135.8 140.9 142.6 158.3

regular structure is narrowest on the Iow-enerEy side of the line becoming progressively broader with increasing energy, almost disappearing on the high ener~JJ side. Note that the examples sited above aU involve absorption to a gcest molecule or ion in a host crystal and the fine structure either has been or may be interpreted as arising from localized or pseudo-localized lattice modes [lo]. The reason for the similarities between these mixed crystal absorptions and the pure crystal DIB absorption reported here will be discussed below.

4.

Discussion

The unusual aspect of the DIB triplet absorption spectrum is the extent to which the phonon sidebands dominate the spectrum. In most cases af absorption into a triplet exciton band, the zero-phonon line is the most intense feature [ 111. Sidebands of the type shown here are more commonly observed, as we noted earlier, in the absorption spectrum of a guest molecule interacting wiith host crystal phonons. The peculiarities of the DIB triplet absorption spectrum can be understood by recalling that DIB dissociates in its triplet state with an effective quantum yield nearly equal to unity [ 121. Marchetti and Kearns [ 121 have speculated that the predissociation might

1 November

1975

phase. When the iodine atom is released, it is not free to assume any energy but is restricted to those energies corresponding to normal modes of the lattice, and the continuum thus corresponds to some appropriatsly weighted density of states. Ano’Lher way to iook at this is to note that the potential energy of recoil of the iodophenyl radical can be exchanged with the lattice only through excitation of lattice vibrational quanta. Of course, onz does not see permanent evidence of the dissociation in the crystalline state. The iodine atom and the iodophenyl radical. are held near each other by the crystal lattice and quickly recombine. Such recombination also occurs in solution, but in solution there is a much greater probabi5ty that the two frawents will escape from each other [ 141. This predissociation manifests itself in the crystal absorption spectrum not just by a broadening of the spectrum, as it wouid in gas or liquid phases, but also by an extensive phonon sideband structure. Normally one would not expect to see a progression in a single phonon frequency in a pure crystal phonon sideband since there is no k = 0 seiection rule for the phonon wave vector in this exciton plus phonon excited state. The 34 cm-l progression that we do observe is, as we have noted, characteristic of a progression in a pseudolocalized mode. The DIB triplet state thus appears to be so different in its electronic distribution from the ground state that it acts much as an impurity would act in modifying the phonon modes in its vicinity. An aspect of the spectrum that remains to be explained is the I1 5 cm -1 shift in the first absorption maximum between b and c polarizations. One might attribute this shift to the exciton splitting within a strong or intermediate coupling regime [ 151. This of course would get us back to the problem of explaining the anomalously large exciton splitting. Although this possibility certainly cannot be ruled out, there are other p!ausibIe explanations. One explanation for the shift that we advance here can be obtained by assuming that the entire c poIarized spectrum is built from a false origin. Going back to table 1, we note that there are Ran-tan active modes in the region 136-l 58 cm-l. These modes correspond

involve the radiationless decay of the triplet xr* state

to libration about the I-I molecular axis. If we imag ine that the transition is primarily in-plane polarized,

into a dissociate nut continuum, 2 classic example of predissociation [ 131. In a crystal, the dissociative continuum is not a uniform continuum as it is in the gas

as Castro and Hochstrasser found [2], and if we further note that in the crystal the molecular planes are nearly perpendicular to the b-direction; then it is pos-

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I November

CHEMICAL PHYSICS LETTERS

sible for such a EbrationaI optical phonon to bring int&ity to the b-polarized spectrum by rotating the j!anes of the DIB molecules. The 115 cm-l shift may be due to the building of the b-polarized spectrum ‘from such a~ optical phonon false origin. The discrepancy between the 115 cln-r shift and the 136 cm- 1 phonon frequency can be attributed to our inability to measure the shift accurately, since we do not know the position of the phononless origin in either polarization nor can we be sure that all of the maxima in the spectrum correspond to exactly the same number of 34 cm-l phonon quanta. Furthermore, although these high frequency librational modes might be expected to have less dispersion than the lower frequency ones, dispersion would tend to mask any correlaticn between Ramzn modes (k z 0 modes) and maxima in the absorption spectrum where there is no k selection rule for the phonons. A reasonable analysis of the entire spectrum in both !I and c polarizations can be made assuming such a false origin without the necessity of 2ssumin g 2 large triplet excitdn splitting*.

1975

decay into the dissociating nu* state has been estimated to be 101r3 s [‘12]. This means that, unless the exciton dispersion in each of the vibronic states is on the order of 100 cm-!, no energy transfer can occur within the lifetime of the trip!et state and exciton states cannot be established. There is no reason to believe that these spectral consequences of predissociation in a crystal are unique to DIE. It would be interesting to look at the low temperature spectrurm of the iodotoluenes, iodobenzene and other compounds where predissociation may occur in the region of the lowest triplet state. Likewise, pseudolocalized lattice modes in pure crystals might occur whenever the excited state electronic distribution

differs significantly

from the ground state distri-

bution and should be observable in other systems. For example, they may be present in charge transfer crystals where the excited

state is much more ionic than

the ground state.

Acknowledgement 5. Conclusions We have shown that :he DIB pure crystal triplet absorption spectrum is dominated by phonon sideband structure strongly reminiscent of structure normalIy artributed to localized or pseudolocalized phonon modes in doped crystals. We attribute the unusual appearance of these pseudolocalized modes in a pure cry&l spectrum to the consequences of molecular predissociation, where the dissociating fragments are ccnstrained by a crystal lattice. The pseudolocalized modes appear because of the large change in electronic distribution (i.e., intermolecular force constants) between the ground state and the disscciative excited state. The strength of the phonon sideband with respect to the unobserved zero-phonon line is due to the large change in the local lattice equilibrium configuration in the relaxed exl:ited state. The lifetime of the DEB m* triplet state before its

I would like to express my gratitude to .I. Duran for purifying and growing the crystals used here and J.-M. Turiet for obtaining some of the absorption spectra. Several interesting conversations with G. Castro, D. Haarer and R. Macfarlane have sharpened many of the points in this paper. Finally, I am indebted to the referee for several extremely useful comments.

References [l] G. Castro and R.M. Hochstrasser, [2j [3] [4]

[S]

* The iefcrea has pointed out that the 6=polarized false origin might also ti due to a bzO carbon-iodine out-of-plane bending mod& that ha$ shifted=fiom 241 *m-’ [ i6] in the pound state to i 15 IX-’ in the excited triplet state. Such il large shift .‘(from 307 cm-’ tc 96 cm-‘) has been reporled for this mode

in$=dichlorobenzene

[171...

[6]

J. Chem. Phys. 46 (1967) 3617. G. Castro and MI. Hochstmsser, Mol. Cryst. 1 (1966) 139. L. Dun-chai and Yu. T. Struchkov, Izv. Akad. Nauk. SSSR 3KhN 12 (1959) 2095. V. Croatto, S. Bezzi and E. Bua, Acta Crys!. 5 (1952) 825; S. Bezzi and V. Croatto, Gaze. Chim. Ital. 72 (1942) 318. W. Wolf and N. tiarasch, J. Org.Chem. 26 (1961) 283; N. Kharasch, W. Wolf, T.J. Erpe!ding, P-G. Naylor and L. Tokes, Chem..Ind. (1962) 1720; J.A. Kampmeier and E. Hoffmeister, J. Am. Chem. Sot. 84 (1962) 3787. K.K. Rebane z&d L.A. Rebane, Pure Appl. Chem. 37 (1974) 161.

Volume 36, number 2

CHEMICAL PHYSICS LETTERS

!7] P.H. Chereson, P.S. Friedman and R. Kopelman, 5. Chem. Phys 56 (1972) 3716. [8] hf. Wagner and 1V.E. Bron, Phys. Rev. 139A (1965) 223. [9] A.P. Marchetti, 5. Chem Phys 56 (1972) 5101. [lo] K-K. Rebane, Impurity spectra of solids (F%num Press, New York, 1970); A.A. Maradudin and G.F. Nardelli, eds., Eiumentary excitations in solids (Plenum Press, New York, 1968).

[ 11j

R.M. Aochstrasser and P.N. Prasad, J. Chem. Phys. 56

(1972) 2814. [ 121 A. Marchetti and D.R. Kearns, I. Am. Chem. Sot. 89

(1967) 5335; hf. Dzvonik, S. Yang and R. Bersohn, I. Chem. Phys. 61 (1974) 4408.

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1975

[13] G. Her&erg, Electronic spectra and electronic structure of polyatomic molecules (Van Nostrand. Princeton, 1966) pp. 455-482. [14] F.W. Lampe and R.M. Noycs, I. Am. Chem. Sot 76 (1954) 2140. [15] W.T. Sknpson and D.L Peterson, J. Chem. Phys. 26 (1957)

588;

R.L. Fulton and M. Goutcrman,

I. Chem. Phys. 41

(1954) 2280. 1161 A. Stojiljkovic

and D.H. Whiffcn, Specirochim. Acta 12 (1958) 47. [ 171 B.W. Gash, D.B. HeLlmann and S.D. Colson, Chem. Phys. l(1972) 191. [ 181 S. Cramer, D. Burland and B. Hudson, to be published.

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