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Electronic and vibrational energy transfer and relaxation in crystalline ϱ-dibromodenzene
Chemi&I Physics 27 (19781.345-353 0 North-Holland Publishing Co&pany
.ELECTR&IC AND VIBRATIONAL ENERGY TRANSFER AND RELAXATION IN CRYSTALLINE p-DIBROMOBENZENE Teresa L. MUCHNICK * and Steven D. COLSON Sterling Chemistv Laboratory. Yale Wnivemiy, New Haven, Connecticut 06520, USA
Received 29 July 1977
Electronic and v&ational energy &sfer and relaxation mechanisms have been studied in crystalline p-dlbromobenzene. Variations in linewidths in the phosphorescence spectrum have been related to the vibrational exciton density-of-states functions. Migration by ground state vibrational excitons is found to be generally more probable than by triplet excitons in these crystals. Broadening in the absorption spectrum has been attributed to fast vibrational relaxation (= IO-l3 s) enhanced byexciton-phonon coupling. The phosphorescence quantum yield is found to be constant throughout the SO-+ T1 absorption spectrum. External spin-orbit coupling has been found to inhance the activity of the totally symmetric fundamentals, whereas Herzberg-Teller viironic coupling dominates in the forbidden part of the spectrum. The Henberg-TelIer active bzg mode is assigned as shifting in frequency from 277 cm -’ in the ground state to 104 cm-’ in the excited state.
1. Introduction Studies of the electronic spectroscopy of molecules in the ccndensed phase can yield information about the mechanisms involved in the dissipation of excitation energy. Line widths at low temperatures can be related to rates of vibrational and electronic relaxation [l-3]_ The temperature dependence of trapped exciton phosphorescence intensity allows for the determi-
nation of the concentration and trapping rates of localized states [4]. Resonant exchange interactions responsible for vibrationaI and/or electronic excitation energy migration result in exciton band formation. These bands can be studied by band-to-band transition methods [S] where the emission linewidth can be related to the density-of-states of the vibrational and/or electronic bands involved in the transition. .Within this context, the electronic spectroscopy of crystalline pdibromobenzene (DBB) is of interest. While many aspects of absorption [6,7], phosphorercence [4,6-8] vibrational [9, lo] and phosphorescence microwave [ll ,123 spectroscopy have been studied in * Present address: American Cyanamid Company, 1937 West Main Street, Stanford,
Connecticut
06904, USA.
great detail, a number of questions still remain. The vibrational analysis of the phosphorescence is fairly complete, however the wide range of linewidths observed [6,7] has not been fully explained. Both exciton and defect trap emission have been observed [4]However, the temperature dependence of the line widths and intensities has not been studied in detail. Furthermore, in contrast to theoretical expectations [13], the strong bZg progression forming mode which dominates the phosphorescence has been correlated [6,7] to a weak feature of nearly the same frequency in the excited state. In this paper, spectroscopic studies of the SO*T, transitions in crystalline DBB have been analyzed and the results applied to several of the questions raised in the preceding paragraphs. The phosphorescence of neat DBB has been observed at 1.8 K and 4.2 K. Lowering the temperature effects the ratio of exciton to trap emission, but has no significant effect on the emission liiewidths which vary throughout the spectrum from 1 to 12 cm-t _These observations, along with mixed crystal studies, lead to the conclusion that the variation in linewidths in the phosphorescence is related to the density-of-states of the vibrational exciton bands. The So + T, absorption spectrum of neat DBB has been
: T.L. Muchnick, S.D. Colson/EEergy trhsferend rehzzion in pdibromobenzene. : .. L _ Photoelectric scans were recorded on.maguetic tape .. observed at 1.8 K, 4.2 K and, in addition, absorption by a PDP-12 computer. . and phosphorescence excitation spectra of naphthaThe source used in the absorption and photoexcitalene-doped DBB at 1.5K have been recorded. The tion spectra was a Hanovia 959X98 500 W xenon lamp. origin line is found to be much sharper and more ternAbsorption spectra were recorded at 4.2 Ktid-1.8 K, perature sensitive than any other band-in the spectrum. in the usual manner. For the spectra taken at. 1-5 K, an The essential identity of the absorption and excitation Air Products Displex cryogenic refrigerator fitted with spectra shows that the broadening of the T, vibronic an optical dewar was used. With this apparatus, the levels can be attributed to rapid vibrational relaxation light source was first dispersed (after pre-filtering with to a common emitting levei, the T, state origin. CuSO4) and then focused on the sample. The light The lack of absorption--emission mirror image symwhich passed through the sample was detected by an metry in electronic spectra indicates the existence of RCA !P28 phototube to determine the absorption strong vibronic coupling (tibronic spin-orbit couspectrum. Absorption by DBB was also followed by pling for triplet states). In the Herzberg-Teller HT detecting the phosphorescence of the naphthalene imcase, it is expected that the promoting mode will purity, whose emission origin is to the red of the DBB change significantly in frequency from the ground state origin. This signal was focused on the photocathode of to the excited state [14]. Our analysis results in a rea Centronix Q4249BA phototube, filtered by a Hoya vised assignment for the excited state baa progression Y-48 UV cutoff filter to eliminate scattered exciting forming mode which is consistent with tks expectation. light. Variations in emission intensity as a function of In contrast, comparison of neat and mixed crystal exciting wavelength gave the phosphorescence excitaspectra show the totally symmetric modes to be actition spectrum of DBB. These two spectra, as welI as vated by extemai spin-orbit coupling. the incident light intensity, were detected simultaneously and stored on magnetic tape. For some of the ex2. Experimental periments a Lambrect Double-Glan-Air DGTYA-02 polarizer was used to obtain polarized absorption and Crystalline samples of zone-refined DBB were prephoto-excitation spectra. Peak positions were deterpared from Aldrich lot 10,422-l. For phosphorescence mined to f OS cm-l by calibration of the spectromexcitation spectra, a small amount of (< 1%) of naphthaeter scan drive with an Fe standard. lene was added to the DBB crystals. In addition DBB was studied as a guest (
T.L. Muchnick.SD. ColsonlEnergv
.I
tmsfer
cmd
relaxation
in
paibromobenzene
341
mode at 277 cm-l. III addition, several other weak, non-totally symmetric modes can be assigned. However, most of the intensity not involving the b2Kprogression forming mode is found in the ag modes. Almost ah the other lines in the spectrum can be assigned to simple combinations of the observed fundamentals Of particular interest in the analysis of the phosphorescence of DBB is the wide variation in linewidths observed. Table 1 is a list of some of the strong fundamentals and combinations in the spectrum. Symmetries of the fundamentals are given in parentheses. The halfwidths which vary from < I cm-l to > IO cm-l, do not change significantly with temperature. In addition, the relative intensity of the corresponding features is fairly insensitive to temperature. Note that, because ‘of their low intensity, half-widths and intensities could not be measured for three of the defect lines even though the transitions were observed. The lines in the bzg progression are broad and increase in width with increasing quantum number. Combinations involving this bag mode are broader than the other fundamentals. As a way of determining the significance of the observed variation in linewidths, the emission spectrum of 0.5% DBB in @chlorobenzene_d4
4.2’K f
Fig. 1. Comparison of the phosphorescence of p-dibromobenzene at 4.2 K and 1.8 K. Peaks are,labeledas follows,with a prime iadicating defect emission: (a) 0,O; (h) 211 cm-‘; (c) 377 cm-‘;(d) 211+ 277: (e) 2 X 277: (0 683 cm-‘; (g) 709 cm-‘;(h) 2 X 277 + 211; (i) 3 X 277.
The vibrational activity of the defect emission is identical to that observed for the exciton or&m and the results of the analysis will not be reported here. In addition, no significant change in vibrational intervals has been detected. The spectrum is dominated by a long progression in the b,, out-of-plane bromine
Table 1 Comparison of half-widths and intensities of some features in the TI -+ So spectrum ofp-dibromobenzene
9)
Half-width (cm-‘)
b)
Relative integrated intensity d)
(cm-‘) exciton c,
defect a)
exciton c)
defect a)
I2 100 10 70 4 10 12 150 30 40 110 15 20 120 210 30 90 130
10 100 8 60
21O.S(ag) 277.2&g)
4 6
4 6
210.5 + 277.2 2 X 271.2 683.3@zg) 708.7 (ag)
78 2 3
;
2 X 277.2 + 210.5
9
9
3 X 271.2 277.2 + 708.7 1064.8 lag) 4 X 277.2 1176.O(ag) 2 X 277.2 + 708.7 217.2 + 1064.8 5 X 277.2 1570.7(ag) 2 x 277.2 + 1064.8 6 X 271.2
9 5 4 10 2 7 5 11 1 9 12
9
a) Spectrum at 1:8 K.
b) Full width at half height.
_
4 4 10 7 5 10 1 8 11 c) Spectrum at 4.2 K.
-
7 150 3.5 50 110 25 125 220 40 70 140
d) Ictensity relative to bzg fundamental at 277 cm-‘.
348
T.i Muchni&-SD:
Colson;Ene&
rransferandreleration in p:dib
Fig. 3. The So -T1
:
_:
.:
: ..
transition ofp-dibromobetiene.at 15 K.
(A) Phosphorescenceexciiation spectrum. (B) fo - r(see text).
Fig. 2. Comparison of neat and mixed crystal phosphorescence in p-dibromohenzene at 1.8 K. (A) DBB in p-dichlorobexuene&. (8) Neat DBB. Corresponding peaks are labeled with the same letter, with the prime indicating defect emission. Tracing A has been displaced slightly in absolute ener@ to show the correlation between the two tracings. (a) 277.7 + 2105 + 1064.4;
(b) 3 X 277.2; (c) 1570.7; (d) 1590.5; (e) 2 X 277.2 + 1064.8; (f) 6 X 277.2.
at 1.g K was obtained. A comparison of corresponding regions of the mixed and pure crystal spectra is shown in fig. 2. Note that each feature in the neat crystal spectrum is a doublet consisting of a strong defect line and a weaker exciton line to its left. While neat crystal half-widths vary from 1 to 12 cm-l, alI of the lines in -the mixed crystal spectrum are approtimately the same width, 2 cm-l. The progression in thi b,, mode shows the same alternation in intensity in the mixed crystal as in the neat, but the progression in the mixed crystal falis off faster, i.e. more of the intensity.is in the first quantum. In contrast, all of the totally symmetric features.are weaker in the mixed crystal than in the neat crystal. Note, for example the behavior of the 1570 cm-l mode C in fig. 2. Similar effects can be seen in the phosphorescence spectrum of DEB inp-xylene [12]. 3.2. The S, + TI spectrum ofp-dibromobenzene The S, + T, transition of DBB has been studied by absorption and phosphorescence excitation spectroscopy_ Fig. 3 shows photoelectric scans taken at 15 K. In this figure, the excitation spectrum is compared to a spectrum which follows the light absorbed by the sample as a function excitation energy (i.e. 1, - I).
With the exception of Small instrumekal
variations,
the two spectra are virtually identical. This indicates that the DBB emission quantum yield is a constant over the wavelength range studied. The vibrational analysis of this transition is given & table 2 with the exception of the features at 385; 467. and 514 cm-l which have been left unassigned. Line positions have been tabulated-along with pertinent ground state frequencies given in brackets. Of most interest in this analysis is the identification of the bzg mode which is very active in the emission spectrum. Previous workers [6,7] assigned this mode as a very weak feature at’= 278 cm-l_ However, this interval was not involved in any combinations nor was a progression in that mode observed in the absorption spectrum. In the case of pdichlorobenzene, it has been shown [I 5] that the bag progression forming mode in that system undergoes a large redudtiori in frequency from the PIound state to the excited state
(298 cm-l to 96 cm-l). Since it is well established [6] that many aspects of the spectroscopy of DBB and DCB are similar, a change in the b,, mode frequency in DBB would be expected. One striking difference between DBB and.DCB involves the strength of excitonYphonon.caupling in the SO +.T1 transition. In DCB, ph&&i sidebands are observed, but the molecular transitions are well resolved [15]. However, in DBB (fig. 4) the pliorion coupling is so strong that it is not significatitly quenched even below = 2 K. When the absorpti& spectrum is observed with a pbiarized Ii& sou&, the feature at 105 cm-l has a ptilarization opposite to that of the . 0,O and.of the lattice modes at.22-65cm-I: These -.
-
..
g.& Muchnick, SD. Col.~n/Energy transfer and relaxation in pdibromobenzene
.:-...-.. ._ .:. of the So --t T1 spectrum of pdibromobenzene
T&h2 Ana.l~ti
_’ .-
“S VW
.:
w -m. m s. m m W m W. VW vi In -yw VW W In In W W W In s W W W W W m VW VW W
3582.40 3581.15 3579.41 3577.85 jj76.61 3575.48 3574.30 3269.02 3563.22 3556.56 3555.00 3545.30 3538.57 3533.59 3529.30 3526.87 3524.75 3523.38 3517.66 35 14.42 3510.62 3509.12 3502.78 3501.78 3497.55 3496.31 3445.24 3489.15 3479.44 3460.95 3453.97 3450.68 3448.68
349
at .15 K
27966.3
27916.0 27929.6 27941.8 27951.5 27960.3 279695 28010.9 28056.5 28109.1 28121.4 28198.3 28251.9 28291.7 28326.1 28345.6 28362.7 28373.7 28419.8 28446.1 28476.9 28489.1 28539.6 28548.8 28583.3 28593.4 28602.1 28652.1 28732.0 28885.4 288943.8 2897 L-4 28988.2
results confirm those of previous workers [6]. In addition, this feature undergoes a significant shift with temperature (96 cm-1 at 4.6 K versus 105 cm-1 at 1.5K). It has @en shown [16] that strong coupling between molecular and lattice vibrations can cause the molecular mode to bioaden and shift with temperature. Thus, the temper+ure induced shift in frequency is consistent with a molecular mode assignment of this feature. On-the basis of these observations tbe 105 cm-l feature has been assiged as correlating tq the 277 b2, piogressioti fdrming mode. The rest of the assignment basically follows that of the jrevious authors [6,7] except for overtones and combinations involving the bzg mode. A summary of the assigried fundamental fre-
0
9.7 23.3 35.5 45.2 54.0 63.2 104.6 150.2 202.8 215.1 292.0 345.6 385.4 419.8 439.3 456.4 467.4 513.5 539.8 570.6 582.8 633.3 642.5 677.0 687.1 695.8 745.8 829.7 979.1 1037.5 1065.4 1081.9
W 9.7 23.3 lattice 35.5 lattice 45.2 lattice 54.0 lattkc 63.2 &trio: 104.6 bg fundamental 12771 104.6 + 45.2 + 0.4 lattice 2C2.8 ag fundamental 12111 2 x 104.6 + 5.9 292.0 b3g fundamental 13151 292.0 + 54.0 - 0.4 lattice 3b5.4 2 x 104.6 + 202.8 + 7.8 385.4 f 54 + 0.3 lattice 3S5.4 c 63.2 + 7.8 lattice 467.4 5j3.5 539.8 bsg fundamental [622] 5 13.5 + 54 + 3.1 lattice 2 x 292.0 - 1.4 633.3 bzg fundamental 16831 642.5 as fundamentJ [709] 633.3 + 45.2 - I.5 Iattice 642.5 + 45.2 - 0.6 Iattice 642.5 + 54.0 - 0.7 lattice 642.5 + 104.6 - 1.3 829.7 bzg fundamental [940] 9iY.l ap fundamental [IO651 979-l+ 54 + 4.4 Iattice 1065.4 ag fundamental [ 11771 1081.9 979.1 f 104.6 - 1.8
quencies and the? ground state v&es is Iisted in table 3.
4. Discussion
The linewidths observed for the vibronic transitions in the So +T, spectrum are much broader than those of either the absorption origin or most of the emission lines. A suggested photochemical channel involving predissociation along the b2g normal coordinate [I 7] or the onset of some other non-radiative triplet deactivation process could possibly account for this line broad-
350
XL.
Muchnick, SD.
ColsonfEnetgytransferand rerajratitin in pdibromobetuene-
Table 3 Comparison of mode frequencies: p-dibromobenzene Ground state a) vF(cm-' )
%
brg bag
%
212 IQ7 1063 1176 1565 3061 811 271 685 934 b) 306 625 1292 1.552 3055
a) Ref. [IO]. b) Ref. [9].
210.5 708.7
202.8 642.5
1064.8
979.1
1176.0 1570.7 -
1065.4 104.6 633.3 829.7 292.0 539.8 -
271.2 683.3 939.6 314.7 621.9 cj -
c) Ref. [7].
ening. However, the simultaneous detection of the absorption and phosphorescence excitation spectrum (fig. 3) and their near identity rules out these explana-
tions. If vibrationally activated radiationless depopulation or photochemistry were responsible for the broadening, the excitation spectrum would be very different, showing little, if any vibronic intensity because
of the reduced emission quantum efficiency of the vibronic levels. We are therefore left to conclude that the line broadening is caused by vibrational relaxation to a common emitting level. In contrast to the extremely sharp origin (< 1 cm-l), the linewidths of most of the other features in the So * Tl spectrum are between 10 cm-l and 40 cm-l. If all other broadening mechanisms are neglected, this would correspond to a Tl state vibrational relaxation time of the order of IO-l3 s. This is much faster than that found for similar systems such as the naphthalene T1 state. However, the phonon coupling is much stronger in the DBB crystal and may explain the differenceStrictly speaking there are three possible contributions to these observed linewidths; intramolecular relaxation, intermolecular relaxation and inhomogeneous energy broadening. There is no evidence for strong vibronic contributions to the inhomogeneous energy tidih of an excited state level in molecular crystals
and none is expected. Thus,_the inhomogeneous con: tribution-to the vibronic lines$ome.of whichhare nearly two orders of magnitudibroader than the origin line, must be small. Whether or’not the relaxation pro-. cesses responsibIe for the lme width are primarily interor intramolecuIar may be subject to question. However, interinolecular excitation exchange interactions are, to first order, zero for vibronic levels involving nontotally symmetric vibraiions and are proportional to the intensity of totally symmetric modes. No evidence is seen for this behavior in the spectrum. Furthermore, the excitation exchange interactions for the origin band are known to be very small (< 1 cm-r) [6]. Thus, the reIaxation route is likely to be primarily intramolecular even though energy conservation requires that intermolecular processes be involved in transferring the excess energy to the lattice modes. The strength of this coupling of the vibronic exciton states to the lattice modes may be the main factor responsible for the differences in these observed linewidths and the very narrow lines seen in the Tl + So spectrum of other crystals like naphthalene
where the exciton phonon coupling is much smaller. Careful measurements of the line shapes in the DBB spectrum may be interesting in this regard [l-3] _ However, in fig. 3 it can be seen that the absorption between the main vibronic bands never really goes to zero, i.e. each line is superimposed on a structured continuum caused by lattice and other modes. In such a case, the interpretation of the observed line shapes will not be straightforward: 4.2. Vibronic intensities The vibronic analysis of the So + Tl transition of DBB presented in this work differs from previous ones [6,7] in the assignment of the b2g mode. A shift in frequency of this mode from 277 cm-l in the ground state.to 105 cm-l in the excited state is consistent with a similar shift observed in DCB [lS] and with the predictions of Herzberg-Teller vibronic coupling theory 1141. The asymmetry in absorption and emission spectra can be directly related to the vibronic coupling mechanism active in the induced part of the spectrum [14]. In the Herzberg-Teller HT limit, the promoting mode is expected to shift in-frequency from the ground to the excited state. In addition, the induced intensity
T.L. Muchnick,S.D. Colson IEnergy transferand relaxationin p-dibromobenzerre
will be stronger in emission than in absorption by a factor of about w&b, the ratio of the promoting mode frequencies in the ground and excited states. In contrast, for cases where the frequency shift is small, contributions from non-B0 couphng can lead to a greater intensity in absorption than in emission [14] _ Thus, the-frequency shift of the bZg mode in DBB is consistent with the HT limit. The “forbidden” vibronic activity in the emission of DBB is much stronger than that observed for the absorption. While limited resolution of the origin in absorption prevents exact measurements, we estimate that the ratio of the relative intensity of the first quanta of the bzg mode in emission to that in absorption is roughly 3: 1, which is consistent with the asymmetry predicted by the ratio of frequencies. In the absence of vibronic coupling, a FranckCondon argument would predict a progression in the even’overtones of the b,, purely by virtue of the frequency shift [ 131. However, even with a frequency shift the order of 3: 1, the intensity of the 0,O would be predicted to be 14 times greater than the sum of the intensities in the totally symmetric portion of the progression. Thus, the strength of the even quanta in the progression cannot be accounted for by a simple FranckCondon argument. Instead, we suggest that these overtones are involved in external spin-orbit coupling in contrast to the odd overtones which are enhanced by internal spin-orbit coupling. In this manner, the alternation of intensity in the progression could be accounted for in the absence of the previously proposed [6] nuclear displacement effects. This revised interpretation is supported by the analysis of the mixed crystal spectra. In contrast to the behaviour of the by progression, the intensity of the observed totally symmetric fundamental modes is very sensitive to the environment (i.e. mixed versus neat crystal) becoming considerably weaker in a DCB orp-xylene host crystal. This would indicate that these modes are indeed active in external spirorbit coupling which is known [18] to selectively enhance totally symmetric modes. The strength of this coupling would depend upon the details of the crystal structure and the chemical composition of the host. ‘Ihis effect is illustrated in fig. 2 where the sharp aa mode C is of moderate strength in the neat crystal and becomes quite weak in the DCB host crystal.
351
4.3. Ground state vibrational energy migration atzd relwation
Variations in the emission spectral linewidths and their dependence on the environment can be used to study inter- and intramolecular energy transfer and relaxation processes. In addition to energy transfer and relaxation processes, crystal inhomogeneities could contribute to the observedlinewidths. This effect is primarily electronic in nature because the electronic gas-to-crystal shift is much greater than that of the molecular vibrational levels. Thus, the sharpness of the origin in the neat DBB spectrum indicates that this broadening mechanism is not an important factor. The increase in linewidth with increasing quanta in the b2s progression could be due to increased intramolecular vibrational relaxation. However, relaxation is not expected to be as sensitive to the environment as are the Iinewidths in DBB. (The same lies are not preferentially broadened in the T, + So spectrum of DBB in a DCB host crystal.) Furthermore, the mixture of broad and sharp features observed over a small frequency range (see fig. 3) indicates that the dominant deactivation process is highly selective, an unlikely restriction for intramolecular relaxation in large molecules. The large variation in linewidths and the lack of absorption-emission mirror image symmetry (a factor which is now resolved) resulted in an earlier suggestion [7] that the emission spectra was due to two types of DBB molecules. Howeverl a more detailed study of the p-dichlorobenzene emission spectra led the same authors to a different interpretation of a similar observation in that system. They concluded [I 51 that the variation in line widths reflected differences in the ground state vibrational exciton density-of-states functions. It will be seen that these new data support the latter interpretation, for DBB. The results in table 1 can be summarized by noting that transitions involving a bZg mode have widths of 5-12 cm-r while ah others have widths less than 4 cm-r It has been noted in other sysiems [191 that out-ofplane vibrational modes (such as the DBB b_zgmode) frequently have larger excitation exchange mteractions than in-plane, low amplitude modes (such as the DBB ag modes). This results in a shorter excitation transfer time for the vibrational exciton and a wider exciton band width for the out-of-plane modes. The band width
352;. ..
.I ;-. .._..:’
- :::..-:. _.;. T.i: &f&hnfck;:~+D_~ C&&/E&$ ._._;.e_ .. -..
&&r&d :. -. ‘..I
&&t&& _‘ _.c
~~~i~j&&j&$&~~, ‘.- -1 -- .:“..: . ... .;;~
.._’.: .I_ ‘:, I. ’ ‘,--’ .I _’ ..:.I:[ C’_ :;-... ; ._ _:_~;+_ ..,
i :.tals &eie exci~~~-~phonbn.coupling i&>&u&ker;1’ ’ ’ The lnrgc difference between thk vibr@c:&I ~I-&&~: .. relaxation rates is however consistent v$th,$revious, ‘1 ..
‘of the_-.i%al$ate ,iri the_DBB-e&ton phosphorescence _ .. 1$Tl~~~~:ih~~~~[6]‘an6.~e.def~ct en&oh &&i&a@
~fro&‘~_$&&zed &ate_ Thus, there will .b&no restriction.
work [.l~!]~. -..I.rf,:-:$-.,.~.:; 1.;_-; -;:;-l~.-‘::!; _~.~~2’:,?,‘_c_-_ The,line_wid$ in the phq~~hdreS~~nce.spe~~m-~ .~ of neatbB@show a,huge variatial-rangilfr~‘~~~~l~_ ti, -12 cx$~j:%$s+ widths:are.s~~~,io~l;e~~a;sed by resoirtince intermole&lar. transfer of the ground state:: ~~brationalex&&i~energy by~+i$a;is&with : -.mixed &ystal sp&iiai For numerous v&rational levels, the rate of.vibrationd.energy transfer is found to. :be-much larger. tharrthat for-the transfer-of pure (vi-‘: brationless) electroriiL triplet. state energy. This,is alsoin-contrast-to systems &here elect;omcexcitation-phon@ coupling is weaker_ y -. ~’ _’ f ,_-.’
.-frd;li3he’&L.0 selection -rule [2O];.and_the emission. : l$r&idths~%$ represent the’vibrational exciton’den- ...sity{ofistates function: This interpreta;ion is sugported’ ;bythe~observation that, irrthe mixed crystal, the emis.--.si&hrres (whidh now_involve localiied-vibration,al states) all have linewidths -which are essentially the ‘-: same as the .origin line (2 cm-l); mostly .due to in- .. homogeneous broadening. Note that the defect vibra-: tional levels observed in the neat crystal spectrum have the same linewidths as the exciton emission. This o& curs in DBB since the trap vibrational levehhave the ~samefrequency as the-corresponding host levels and are therefore within the host vibrationalkxciton btids. On the basis of this discussion it can be concluded. that the variations in linewidth m.the DBB phosphorescence spectrum retiect.differences in the vibrational exciton density-of-states functions.of the observed
The,detailed vibronic analysis pf-the T, Y+Su.:absorption spectrum is r&red and found to be consistent with strong internal HT-vibro&spinlrrbit ~ou@ling? with some contributions-from external spin-orbit coupling as is the phosphorescence spectrum. -- .-
modes. T&e fact that modes of varying linewidths
found in close proximity
are indicates thtic Fermi resonance
.. : .-- : Acknowledgement .. -_ -Financial support from the National Science Founda-
(molecular plus crystal-induced) is small. Note that the vibrational’bands resulting from the excitation of one or more quanta of the b2a mode are wider than the origin band [6] for the T, electronic state. Thus, the migration of such vibrational excitation energy in crystalline DBB will be much-faster than the migration-of excitation energy in the triplet origin exciton band.
tion is gratefully-acknowledged.
References
.~ : .__
:
This .follows directly from the- differen&.in the electronic and vibrational band widths. The excitation ex-
[l ] H. de Vries and D.A. !&sni&‘hys.
change interaction determines both the band widths and the migration rate (iump frequency).
[2] [3] [4]
5. Conclusion
[S] In this paper, the triplet state absorption and phos[6]
phorescence spetitra,have been used to determine the relative importance of the energy transfer and relaxation processes active iu crystalline pdibromobenzene. The phosphorescence quantum yield is fo?utd to be
[7]
is caused by vibrational relaxation to the Tl. state origin level. This vibrational relaxation time’(= 0.1 ps) is found to be:significantly.shorter than ,thbse (= S-ps.[i]j recently measured for vibronic limes in mole&rIar crys-
Rev. Letters 36 (i976)91. IT.J. msma end D.A: W&ma, Chem. F’hys. Letters 42. (1976)520. A.H. Zewail and’T.E. Or&ski, Chem. ihys. kt&& (1977) 389. GA. George and G.C. M&k, J..@e&. phys. 54 :;971) 815. S.D. Colson, D-M. Hanson, R. K&e&n and. G.W. Robinsen; J. Chem; Phys.48 (1968) 2215. .’ G. Castro and R.Mi Hochstrasser, J..Cbem. Phys. 46 I (1967) 3617.... S.D. Colsou and B.W.G+h, J, &I& P&+($72)
.3048.
..
.:___
:
[S] GiA: Ge&e and G.C:Morris, Moi. Cr&nd
.*&:15.(1g71)&
constant throughout the Tl‘+So absorptionspectrum, indicating that the observed vibronic line- broadening
:
:
.;:
Liquid
.:I:>:.:- . . . -:::::-; ., ~_ -, ..
[9] PN:_Gates, K, ~d@~e.~~d ~~,$~e$e,.sP;~~t@hi+ ..;z..
., ‘A+25A(J969).,507;- .- . .:. :, . . .::i :: : .‘l .---f:: : 25A (l$+‘)- .,;- .. ,lo~7__: ,._: -,.:.,y. ’ ._;:.,:’ -?;I;:’ ;:_. :;.‘. f’-y:: r i ..; -.. [iii-G;.Kdthtidgarn-&; H.J; &I&md-D.\V:Pi&t,J. Cherrt. ,.~_ [lo] _M.Suzirki ““d M. Jto;8~e;;&+&-Ac&
:
l%ys.$l<197~) ~q~s:~_.~..;.
.;.: :: i :-:. .:--. ,I ..
:
~::
(._.
..
~_
-:
._
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:.. .‘T.L. &ckhick,
SD. Cokm/finer~~ trmrfer&d -.
‘[ii] -G.-K~thanaaram.~ar;dD._S. Tinti. &em. Phys. Letters ‘o (1973) 225; ._ : -[~3~.$~ e&b@%g,Ele&nid s&Zra of polyatomic molecules ‘No&z&d. Princetoii, 1966): &at&i&d E.C. Lim. Chem. P&s. Letters 35 [l!J:N.K_ ~~~ (1975)303;. [w] B.W. C&h, D.B. IMbnan and S.D. Colson, Cbem. Pbys. ‘. l(1972) 191. 61 PIN. P&arl and R. Kopeban, J. Chem. Phys. 58 (1973) L-
:.
relcmtion in p-dibromob&ene
353
[17j R.M. Hocbstrasserand D.A. Wiersma, Israel J. Chem. 10 (1972)X7. [18] B.W. Gash and S.D. Colson, J. Chem. Phys. 59 (1973) 3528. [19] A.R. Gee and G.W. Robinson, J. Chem. whys. 46 (1967) 4847; 1201 S.D. Colson, R. Kopebnan and G.W. Robinson, J. Chem. Pbys. 47 (1967) 27; 47 (1967) 5462; S.D. Colson, D.M. Hanson, R. Kopeiman and G.W. Robinson, J. Chem. Phys. 48 (1968) 2215.
.ELECTR&IC AND VIBRATIONAL ENERGY TRANSFER AND RELAXATION IN CRYSTALLINE p-DIBROMOBENZENE Teresa L. MUCHNICK * and Steven D. COLSON Sterling Chemistv Laboratory. Yale Wnivemiy, New Haven, Connecticut 06520, USA
Received 29 July 1977
Electronic and v&ational energy &sfer and relaxation mechanisms have been studied in crystalline p-dlbromobenzene. Variations in linewidths in the phosphorescence spectrum have been related to the vibrational exciton density-of-states functions. Migration by ground state vibrational excitons is found to be generally more probable than by triplet excitons in these crystals. Broadening in the absorption spectrum has been attributed to fast vibrational relaxation (= IO-l3 s) enhanced byexciton-phonon coupling. The phosphorescence quantum yield is found to be constant throughout the SO-+ T1 absorption spectrum. External spin-orbit coupling has been found to inhance the activity of the totally symmetric fundamentals, whereas Herzberg-Teller viironic coupling dominates in the forbidden part of the spectrum. The Henberg-TelIer active bzg mode is assigned as shifting in frequency from 277 cm -’ in the ground state to 104 cm-’ in the excited state.
1. Introduction Studies of the electronic spectroscopy of molecules in the ccndensed phase can yield information about the mechanisms involved in the dissipation of excitation energy. Line widths at low temperatures can be related to rates of vibrational and electronic relaxation [l-3]_ The temperature dependence of trapped exciton phosphorescence intensity allows for the determi-
nation of the concentration and trapping rates of localized states [4]. Resonant exchange interactions responsible for vibrationaI and/or electronic excitation energy migration result in exciton band formation. These bands can be studied by band-to-band transition methods [S] where the emission linewidth can be related to the density-of-states of the vibrational and/or electronic bands involved in the transition. .Within this context, the electronic spectroscopy of crystalline pdibromobenzene (DBB) is of interest. While many aspects of absorption [6,7], phosphorercence [4,6-8] vibrational [9, lo] and phosphorescence microwave [ll ,123 spectroscopy have been studied in * Present address: American Cyanamid Company, 1937 West Main Street, Stanford,
Connecticut
06904, USA.
great detail, a number of questions still remain. The vibrational analysis of the phosphorescence is fairly complete, however the wide range of linewidths observed [6,7] has not been fully explained. Both exciton and defect trap emission have been observed [4]However, the temperature dependence of the line widths and intensities has not been studied in detail. Furthermore, in contrast to theoretical expectations [13], the strong bZg progression forming mode which dominates the phosphorescence has been correlated [6,7] to a weak feature of nearly the same frequency in the excited state. In this paper, spectroscopic studies of the SO*T, transitions in crystalline DBB have been analyzed and the results applied to several of the questions raised in the preceding paragraphs. The phosphorescence of neat DBB has been observed at 1.8 K and 4.2 K. Lowering the temperature effects the ratio of exciton to trap emission, but has no significant effect on the emission liiewidths which vary throughout the spectrum from 1 to 12 cm-t _These observations, along with mixed crystal studies, lead to the conclusion that the variation in linewidths in the phosphorescence is related to the density-of-states of the vibrational exciton bands. The So + T, absorption spectrum of neat DBB has been
: T.L. Muchnick, S.D. Colson/EEergy trhsferend rehzzion in pdibromobenzene. : .. L _ Photoelectric scans were recorded on.maguetic tape .. observed at 1.8 K, 4.2 K and, in addition, absorption by a PDP-12 computer. . and phosphorescence excitation spectra of naphthaThe source used in the absorption and photoexcitalene-doped DBB at 1.5K have been recorded. The tion spectra was a Hanovia 959X98 500 W xenon lamp. origin line is found to be much sharper and more ternAbsorption spectra were recorded at 4.2 Ktid-1.8 K, perature sensitive than any other band-in the spectrum. in the usual manner. For the spectra taken at. 1-5 K, an The essential identity of the absorption and excitation Air Products Displex cryogenic refrigerator fitted with spectra shows that the broadening of the T, vibronic an optical dewar was used. With this apparatus, the levels can be attributed to rapid vibrational relaxation light source was first dispersed (after pre-filtering with to a common emitting levei, the T, state origin. CuSO4) and then focused on the sample. The light The lack of absorption--emission mirror image symwhich passed through the sample was detected by an metry in electronic spectra indicates the existence of RCA !P28 phototube to determine the absorption strong vibronic coupling (tibronic spin-orbit couspectrum. Absorption by DBB was also followed by pling for triplet states). In the Herzberg-Teller HT detecting the phosphorescence of the naphthalene imcase, it is expected that the promoting mode will purity, whose emission origin is to the red of the DBB change significantly in frequency from the ground state origin. This signal was focused on the photocathode of to the excited state [14]. Our analysis results in a rea Centronix Q4249BA phototube, filtered by a Hoya vised assignment for the excited state baa progression Y-48 UV cutoff filter to eliminate scattered exciting forming mode which is consistent with tks expectation. light. Variations in emission intensity as a function of In contrast, comparison of neat and mixed crystal exciting wavelength gave the phosphorescence excitaspectra show the totally symmetric modes to be actition spectrum of DBB. These two spectra, as welI as vated by extemai spin-orbit coupling. the incident light intensity, were detected simultaneously and stored on magnetic tape. For some of the ex2. Experimental periments a Lambrect Double-Glan-Air DGTYA-02 polarizer was used to obtain polarized absorption and Crystalline samples of zone-refined DBB were prephoto-excitation spectra. Peak positions were deterpared from Aldrich lot 10,422-l. For phosphorescence mined to f OS cm-l by calibration of the spectromexcitation spectra, a small amount of (< 1%) of naphthaeter scan drive with an Fe standard. lene was added to the DBB crystals. In addition DBB was studied as a guest (
T.L. Muchnick.SD. ColsonlEnergv
.I
tmsfer
cmd
relaxation
in
paibromobenzene
341
mode at 277 cm-l. III addition, several other weak, non-totally symmetric modes can be assigned. However, most of the intensity not involving the b2Kprogression forming mode is found in the ag modes. Almost ah the other lines in the spectrum can be assigned to simple combinations of the observed fundamentals Of particular interest in the analysis of the phosphorescence of DBB is the wide variation in linewidths observed. Table 1 is a list of some of the strong fundamentals and combinations in the spectrum. Symmetries of the fundamentals are given in parentheses. The halfwidths which vary from < I cm-l to > IO cm-l, do not change significantly with temperature. In addition, the relative intensity of the corresponding features is fairly insensitive to temperature. Note that, because ‘of their low intensity, half-widths and intensities could not be measured for three of the defect lines even though the transitions were observed. The lines in the bzg progression are broad and increase in width with increasing quantum number. Combinations involving this bag mode are broader than the other fundamentals. As a way of determining the significance of the observed variation in linewidths, the emission spectrum of 0.5% DBB in @chlorobenzene_d4
4.2’K f
Fig. 1. Comparison of the phosphorescence of p-dibromobenzene at 4.2 K and 1.8 K. Peaks are,labeledas follows,with a prime iadicating defect emission: (a) 0,O; (h) 211 cm-‘; (c) 377 cm-‘;(d) 211+ 277: (e) 2 X 277: (0 683 cm-‘; (g) 709 cm-‘;(h) 2 X 277 + 211; (i) 3 X 277.
The vibrational activity of the defect emission is identical to that observed for the exciton or&m and the results of the analysis will not be reported here. In addition, no significant change in vibrational intervals has been detected. The spectrum is dominated by a long progression in the b,, out-of-plane bromine
Table 1 Comparison of half-widths and intensities of some features in the TI -+ So spectrum ofp-dibromobenzene
9)
Half-width (cm-‘)
b)
Relative integrated intensity d)
(cm-‘) exciton c,
defect a)
exciton c)
defect a)
I2 100 10 70 4 10 12 150 30 40 110 15 20 120 210 30 90 130
10 100 8 60
21O.S(ag) 277.2&g)
4 6
4 6
210.5 + 277.2 2 X 271.2 683.3@zg) 708.7 (ag)
78 2 3
;
2 X 277.2 + 210.5
9
9
3 X 271.2 277.2 + 708.7 1064.8 lag) 4 X 277.2 1176.O(ag) 2 X 277.2 + 708.7 217.2 + 1064.8 5 X 277.2 1570.7(ag) 2 x 277.2 + 1064.8 6 X 271.2
9 5 4 10 2 7 5 11 1 9 12
9
a) Spectrum at 1:8 K.
b) Full width at half height.
_
4 4 10 7 5 10 1 8 11 c) Spectrum at 4.2 K.
-
7 150 3.5 50 110 25 125 220 40 70 140
d) Ictensity relative to bzg fundamental at 277 cm-‘.
348
T.i Muchni&-SD:
Colson;Ene&
rransferandreleration in p:dib
Fig. 3. The So -T1
:
_:
.:
: ..
transition ofp-dibromobetiene.at 15 K.
(A) Phosphorescenceexciiation spectrum. (B) fo - r(see text).
Fig. 2. Comparison of neat and mixed crystal phosphorescence in p-dibromohenzene at 1.8 K. (A) DBB in p-dichlorobexuene&. (8) Neat DBB. Corresponding peaks are labeled with the same letter, with the prime indicating defect emission. Tracing A has been displaced slightly in absolute ener@ to show the correlation between the two tracings. (a) 277.7 + 2105 + 1064.4;
(b) 3 X 277.2; (c) 1570.7; (d) 1590.5; (e) 2 X 277.2 + 1064.8; (f) 6 X 277.2.
at 1.g K was obtained. A comparison of corresponding regions of the mixed and pure crystal spectra is shown in fig. 2. Note that each feature in the neat crystal spectrum is a doublet consisting of a strong defect line and a weaker exciton line to its left. While neat crystal half-widths vary from 1 to 12 cm-l, alI of the lines in -the mixed crystal spectrum are approtimately the same width, 2 cm-l. The progression in thi b,, mode shows the same alternation in intensity in the mixed crystal as in the neat, but the progression in the mixed crystal falis off faster, i.e. more of the intensity.is in the first quantum. In contrast, all of the totally symmetric features.are weaker in the mixed crystal than in the neat crystal. Note, for example the behavior of the 1570 cm-l mode C in fig. 2. Similar effects can be seen in the phosphorescence spectrum of DEB inp-xylene [12]. 3.2. The S, + TI spectrum ofp-dibromobenzene The S, + T, transition of DBB has been studied by absorption and phosphorescence excitation spectroscopy_ Fig. 3 shows photoelectric scans taken at 15 K. In this figure, the excitation spectrum is compared to a spectrum which follows the light absorbed by the sample as a function excitation energy (i.e. 1, - I).
With the exception of Small instrumekal
variations,
the two spectra are virtually identical. This indicates that the DBB emission quantum yield is a constant over the wavelength range studied. The vibrational analysis of this transition is given & table 2 with the exception of the features at 385; 467. and 514 cm-l which have been left unassigned. Line positions have been tabulated-along with pertinent ground state frequencies given in brackets. Of most interest in this analysis is the identification of the bzg mode which is very active in the emission spectrum. Previous workers [6,7] assigned this mode as a very weak feature at’= 278 cm-l_ However, this interval was not involved in any combinations nor was a progression in that mode observed in the absorption spectrum. In the case of pdichlorobenzene, it has been shown [I 5] that the bag progression forming mode in that system undergoes a large redudtiori in frequency from the PIound state to the excited state
(298 cm-l to 96 cm-l). Since it is well established [6] that many aspects of the spectroscopy of DBB and DCB are similar, a change in the b,, mode frequency in DBB would be expected. One striking difference between DBB and.DCB involves the strength of excitonYphonon.caupling in the SO +.T1 transition. In DCB, ph&&i sidebands are observed, but the molecular transitions are well resolved [15]. However, in DBB (fig. 4) the pliorion coupling is so strong that it is not significatitly quenched even below = 2 K. When the absorpti& spectrum is observed with a pbiarized Ii& sou&, the feature at 105 cm-l has a ptilarization opposite to that of the . 0,O and.of the lattice modes at.22-65cm-I: These -.
-
..
g.& Muchnick, SD. Col.~n/Energy transfer and relaxation in pdibromobenzene
.:-...-.. ._ .:. of the So --t T1 spectrum of pdibromobenzene
T&h2 Ana.l~ti
_’ .-
“S VW
.:
w -m. m s. m m W m W. VW vi In -yw VW W In In W W W In s W W W W W m VW VW W
3582.40 3581.15 3579.41 3577.85 jj76.61 3575.48 3574.30 3269.02 3563.22 3556.56 3555.00 3545.30 3538.57 3533.59 3529.30 3526.87 3524.75 3523.38 3517.66 35 14.42 3510.62 3509.12 3502.78 3501.78 3497.55 3496.31 3445.24 3489.15 3479.44 3460.95 3453.97 3450.68 3448.68
349
at .15 K
27966.3
27916.0 27929.6 27941.8 27951.5 27960.3 279695 28010.9 28056.5 28109.1 28121.4 28198.3 28251.9 28291.7 28326.1 28345.6 28362.7 28373.7 28419.8 28446.1 28476.9 28489.1 28539.6 28548.8 28583.3 28593.4 28602.1 28652.1 28732.0 28885.4 288943.8 2897 L-4 28988.2
results confirm those of previous workers [6]. In addition, this feature undergoes a significant shift with temperature (96 cm-1 at 4.6 K versus 105 cm-1 at 1.5K). It has @en shown [16] that strong coupling between molecular and lattice vibrations can cause the molecular mode to bioaden and shift with temperature. Thus, the temper+ure induced shift in frequency is consistent with a molecular mode assignment of this feature. On-the basis of these observations tbe 105 cm-l feature has been assiged as correlating tq the 277 b2, piogressioti fdrming mode. The rest of the assignment basically follows that of the jrevious authors [6,7] except for overtones and combinations involving the bzg mode. A summary of the assigried fundamental fre-
0
9.7 23.3 35.5 45.2 54.0 63.2 104.6 150.2 202.8 215.1 292.0 345.6 385.4 419.8 439.3 456.4 467.4 513.5 539.8 570.6 582.8 633.3 642.5 677.0 687.1 695.8 745.8 829.7 979.1 1037.5 1065.4 1081.9
W 9.7 23.3 lattice 35.5 lattice 45.2 lattice 54.0 lattkc 63.2 &trio: 104.6 bg fundamental 12771 104.6 + 45.2 + 0.4 lattice 2C2.8 ag fundamental 12111 2 x 104.6 + 5.9 292.0 b3g fundamental 13151 292.0 + 54.0 - 0.4 lattice 3b5.4 2 x 104.6 + 202.8 + 7.8 385.4 f 54 + 0.3 lattice 3S5.4 c 63.2 + 7.8 lattice 467.4 5j3.5 539.8 bsg fundamental [622] 5 13.5 + 54 + 3.1 lattice 2 x 292.0 - 1.4 633.3 bzg fundamental 16831 642.5 as fundamentJ [709] 633.3 + 45.2 - I.5 Iattice 642.5 + 45.2 - 0.6 Iattice 642.5 + 54.0 - 0.7 lattice 642.5 + 104.6 - 1.3 829.7 bzg fundamental [940] 9iY.l ap fundamental [IO651 979-l+ 54 + 4.4 Iattice 1065.4 ag fundamental [ 11771 1081.9 979.1 f 104.6 - 1.8
quencies and the? ground state v&es is Iisted in table 3.
4. Discussion
The linewidths observed for the vibronic transitions in the So +T, spectrum are much broader than those of either the absorption origin or most of the emission lines. A suggested photochemical channel involving predissociation along the b2g normal coordinate [I 7] or the onset of some other non-radiative triplet deactivation process could possibly account for this line broad-
350
XL.
Muchnick, SD.
ColsonfEnetgytransferand rerajratitin in pdibromobetuene-
Table 3 Comparison of mode frequencies: p-dibromobenzene Ground state a) vF(cm-' )
%
brg bag
%
212 IQ7 1063 1176 1565 3061 811 271 685 934 b) 306 625 1292 1.552 3055
a) Ref. [IO]. b) Ref. [9].
210.5 708.7
202.8 642.5
1064.8
979.1
1176.0 1570.7 -
1065.4 104.6 633.3 829.7 292.0 539.8 -
271.2 683.3 939.6 314.7 621.9 cj -
c) Ref. [7].
ening. However, the simultaneous detection of the absorption and phosphorescence excitation spectrum (fig. 3) and their near identity rules out these explana-
tions. If vibrationally activated radiationless depopulation or photochemistry were responsible for the broadening, the excitation spectrum would be very different, showing little, if any vibronic intensity because
of the reduced emission quantum efficiency of the vibronic levels. We are therefore left to conclude that the line broadening is caused by vibrational relaxation to a common emitting level. In contrast to the extremely sharp origin (< 1 cm-l), the linewidths of most of the other features in the So * Tl spectrum are between 10 cm-l and 40 cm-l. If all other broadening mechanisms are neglected, this would correspond to a Tl state vibrational relaxation time of the order of IO-l3 s. This is much faster than that found for similar systems such as the naphthalene T1 state. However, the phonon coupling is much stronger in the DBB crystal and may explain the differenceStrictly speaking there are three possible contributions to these observed linewidths; intramolecular relaxation, intermolecular relaxation and inhomogeneous energy broadening. There is no evidence for strong vibronic contributions to the inhomogeneous energy tidih of an excited state level in molecular crystals
and none is expected. Thus,_the inhomogeneous con: tribution-to the vibronic lines$ome.of whichhare nearly two orders of magnitudibroader than the origin line, must be small. Whether or’not the relaxation pro-. cesses responsibIe for the lme width are primarily interor intramolecuIar may be subject to question. However, interinolecular excitation exchange interactions are, to first order, zero for vibronic levels involving nontotally symmetric vibraiions and are proportional to the intensity of totally symmetric modes. No evidence is seen for this behavior in the spectrum. Furthermore, the excitation exchange interactions for the origin band are known to be very small (< 1 cm-r) [6]. Thus, the reIaxation route is likely to be primarily intramolecular even though energy conservation requires that intermolecular processes be involved in transferring the excess energy to the lattice modes. The strength of this coupling of the vibronic exciton states to the lattice modes may be the main factor responsible for the differences in these observed linewidths and the very narrow lines seen in the Tl + So spectrum of other crystals like naphthalene
where the exciton phonon coupling is much smaller. Careful measurements of the line shapes in the DBB spectrum may be interesting in this regard [l-3] _ However, in fig. 3 it can be seen that the absorption between the main vibronic bands never really goes to zero, i.e. each line is superimposed on a structured continuum caused by lattice and other modes. In such a case, the interpretation of the observed line shapes will not be straightforward: 4.2. Vibronic intensities The vibronic analysis of the So + Tl transition of DBB presented in this work differs from previous ones [6,7] in the assignment of the b2g mode. A shift in frequency of this mode from 277 cm-l in the ground state.to 105 cm-l in the excited state is consistent with a similar shift observed in DCB [lS] and with the predictions of Herzberg-Teller vibronic coupling theory 1141. The asymmetry in absorption and emission spectra can be directly related to the vibronic coupling mechanism active in the induced part of the spectrum [14]. In the Herzberg-Teller HT limit, the promoting mode is expected to shift in-frequency from the ground to the excited state. In addition, the induced intensity
T.L. Muchnick,S.D. Colson IEnergy transferand relaxationin p-dibromobenzerre
will be stronger in emission than in absorption by a factor of about w&b, the ratio of the promoting mode frequencies in the ground and excited states. In contrast, for cases where the frequency shift is small, contributions from non-B0 couphng can lead to a greater intensity in absorption than in emission [14] _ Thus, the-frequency shift of the bZg mode in DBB is consistent with the HT limit. The “forbidden” vibronic activity in the emission of DBB is much stronger than that observed for the absorption. While limited resolution of the origin in absorption prevents exact measurements, we estimate that the ratio of the relative intensity of the first quanta of the bzg mode in emission to that in absorption is roughly 3: 1, which is consistent with the asymmetry predicted by the ratio of frequencies. In the absence of vibronic coupling, a FranckCondon argument would predict a progression in the even’overtones of the b,, purely by virtue of the frequency shift [ 131. However, even with a frequency shift the order of 3: 1, the intensity of the 0,O would be predicted to be 14 times greater than the sum of the intensities in the totally symmetric portion of the progression. Thus, the strength of the even quanta in the progression cannot be accounted for by a simple FranckCondon argument. Instead, we suggest that these overtones are involved in external spin-orbit coupling in contrast to the odd overtones which are enhanced by internal spin-orbit coupling. In this manner, the alternation of intensity in the progression could be accounted for in the absence of the previously proposed [6] nuclear displacement effects. This revised interpretation is supported by the analysis of the mixed crystal spectra. In contrast to the behaviour of the by progression, the intensity of the observed totally symmetric fundamental modes is very sensitive to the environment (i.e. mixed versus neat crystal) becoming considerably weaker in a DCB orp-xylene host crystal. This would indicate that these modes are indeed active in external spirorbit coupling which is known [18] to selectively enhance totally symmetric modes. The strength of this coupling would depend upon the details of the crystal structure and the chemical composition of the host. ‘Ihis effect is illustrated in fig. 2 where the sharp aa mode C is of moderate strength in the neat crystal and becomes quite weak in the DCB host crystal.
351
4.3. Ground state vibrational energy migration atzd relwation
Variations in the emission spectral linewidths and their dependence on the environment can be used to study inter- and intramolecular energy transfer and relaxation processes. In addition to energy transfer and relaxation processes, crystal inhomogeneities could contribute to the observedlinewidths. This effect is primarily electronic in nature because the electronic gas-to-crystal shift is much greater than that of the molecular vibrational levels. Thus, the sharpness of the origin in the neat DBB spectrum indicates that this broadening mechanism is not an important factor. The increase in linewidth with increasing quanta in the b2s progression could be due to increased intramolecular vibrational relaxation. However, relaxation is not expected to be as sensitive to the environment as are the Iinewidths in DBB. (The same lies are not preferentially broadened in the T, + So spectrum of DBB in a DCB host crystal.) Furthermore, the mixture of broad and sharp features observed over a small frequency range (see fig. 3) indicates that the dominant deactivation process is highly selective, an unlikely restriction for intramolecular relaxation in large molecules. The large variation in linewidths and the lack of absorption-emission mirror image symmetry (a factor which is now resolved) resulted in an earlier suggestion [7] that the emission spectra was due to two types of DBB molecules. Howeverl a more detailed study of the p-dichlorobenzene emission spectra led the same authors to a different interpretation of a similar observation in that system. They concluded [I 51 that the variation in line widths reflected differences in the ground state vibrational exciton density-of-states functions. It will be seen that these new data support the latter interpretation, for DBB. The results in table 1 can be summarized by noting that transitions involving a bZg mode have widths of 5-12 cm-r while ah others have widths less than 4 cm-r It has been noted in other sysiems [191 that out-ofplane vibrational modes (such as the DBB b_zgmode) frequently have larger excitation exchange mteractions than in-plane, low amplitude modes (such as the DBB ag modes). This results in a shorter excitation transfer time for the vibrational exciton and a wider exciton band width for the out-of-plane modes. The band width
352;. ..
.I ;-. .._..:’
- :::..-:. _.;. T.i: &f&hnfck;:~+D_~ C&&/E&$ ._._;.e_ .. -..
&&r&d :. -. ‘..I
&&t&& _‘ _.c
~~~i~j&&j&$&~~, ‘.- -1 -- .:“..: . ... .;;~
.._’.: .I_ ‘:, I. ’ ‘,--’ .I _’ ..:.I:[ C’_ :;-... ; ._ _:_~;+_ ..,
i :.tals &eie exci~~~-~phonbn.coupling i&>&u&ker;1’ ’ ’ The lnrgc difference between thk vibr@c:&I ~I-&&~: .. relaxation rates is however consistent v$th,$revious, ‘1 ..
‘of the_-.i%al$ate ,iri the_DBB-e&ton phosphorescence _ .. 1$Tl~~~~:ih~~~~[6]‘an6.~e.def~ct en&oh &&i&a@
~fro&‘~_$&&zed &ate_ Thus, there will .b&no restriction.
work [.l~!]~. -..I.rf,:-:$-.,.~.:; 1.;_-; -;:;-l~.-‘::!; _~.~~2’:,?,‘_c_-_ The,line_wid$ in the phq~~hdreS~~nce.spe~~m-~ .~ of neatbB@show a,huge variatial-rangilfr~‘~~~~l~_ ti, -12 cx$~j:%$s+ widths:are.s~~~,io~l;e~~a;sed by resoirtince intermole&lar. transfer of the ground state:: ~~brationalex&&i~energy by~+i$a;is&with : -.mixed &ystal sp&iiai For numerous v&rational levels, the rate of.vibrationd.energy transfer is found to. :be-much larger. tharrthat for-the transfer-of pure (vi-‘: brationless) electroriiL triplet. state energy. This,is alsoin-contrast-to systems &here elect;omcexcitation-phon@ coupling is weaker_ y -. ~’ _’ f ,_-.’
.-frd;li3he’&L.0 selection -rule [2O];.and_the emission. : l$r&idths~%$ represent the’vibrational exciton’den- ...sity{ofistates function: This interpreta;ion is sugported’ ;bythe~observation that, irrthe mixed crystal, the emis.--.si&hrres (whidh now_involve localiied-vibration,al states) all have linewidths -which are essentially the ‘-: same as the .origin line (2 cm-l); mostly .due to in- .. homogeneous broadening. Note that the defect vibra-: tional levels observed in the neat crystal spectrum have the same linewidths as the exciton emission. This o& curs in DBB since the trap vibrational levehhave the ~samefrequency as the-corresponding host levels and are therefore within the host vibrationalkxciton btids. On the basis of this discussion it can be concluded. that the variations in linewidth m.the DBB phosphorescence spectrum retiect.differences in the vibrational exciton density-of-states functions.of the observed
The,detailed vibronic analysis pf-the T, Y+Su.:absorption spectrum is r&red and found to be consistent with strong internal HT-vibro&spinlrrbit ~ou@ling? with some contributions-from external spin-orbit coupling as is the phosphorescence spectrum. -- .-
modes. T&e fact that modes of varying linewidths
found in close proximity
are indicates thtic Fermi resonance
.. : .-- : Acknowledgement .. -_ -Financial support from the National Science Founda-
(molecular plus crystal-induced) is small. Note that the vibrational’bands resulting from the excitation of one or more quanta of the b2a mode are wider than the origin band [6] for the T, electronic state. Thus, the migration of such vibrational excitation energy in crystalline DBB will be much-faster than the migration-of excitation energy in the triplet origin exciton band.
tion is gratefully-acknowledged.
References
.~ : .__
:
This .follows directly from the- differen&.in the electronic and vibrational band widths. The excitation ex-
[l ] H. de Vries and D.A. !&sni&‘hys.
change interaction determines both the band widths and the migration rate (iump frequency).
[2] [3] [4]
5. Conclusion
[S] In this paper, the triplet state absorption and phos[6]
phorescence spetitra,have been used to determine the relative importance of the energy transfer and relaxation processes active iu crystalline pdibromobenzene. The phosphorescence quantum yield is fo?utd to be
[7]
is caused by vibrational relaxation to the Tl. state origin level. This vibrational relaxation time’(= 0.1 ps) is found to be:significantly.shorter than ,thbse (= S-ps.[i]j recently measured for vibronic limes in mole&rIar crys-
Rev. Letters 36 (i976)91. IT.J. msma end D.A: W&ma, Chem. F’hys. Letters 42. (1976)520. A.H. Zewail and’T.E. Or&ski, Chem. ihys. kt&& (1977) 389. GA. George and G.C. M&k, J..@e&. phys. 54 :;971) 815. S.D. Colson, D-M. Hanson, R. K&e&n and. G.W. Robinsen; J. Chem; Phys.48 (1968) 2215. .’ G. Castro and R.Mi Hochstrasser, J..Cbem. Phys. 46 I (1967) 3617.... S.D. Colsou and B.W.G+h, J, &I& P&+($72)
.3048.
..
.:___
:
[S] GiA: Ge&e and G.C:Morris, Moi. Cr&nd
.*&:15.(1g71)&
constant throughout the Tl‘+So absorptionspectrum, indicating that the observed vibronic line- broadening
:
:
.;:
Liquid
.:I:>:.:- . . . -:::::-; ., ~_ -, ..
[9] PN:_Gates, K, ~d@~e.~~d ~~,$~e$e,.sP;~~t@hi+ ..;z..
., ‘A+25A(J969).,507;- .- . .:. :, . . .::i :: : .‘l .---f:: : 25A (l$+‘)- .,;- .. ,lo~7__: ,._: -,.:.,y. ’ ._;:.,:’ -?;I;:’ ;:_. :;.‘. f’-y:: r i ..; -.. [iii-G;.Kdthtidgarn-&; H.J; &I&md-D.\V:Pi&t,J. Cherrt. ,.~_ [lo] _M.Suzirki ““d M. Jto;8~e;;&+&-Ac&
:
l%ys.$l<197~) ~q~s:~_.~..;.
.;.: :: i :-:. .:--. ,I ..
:
~::
(._.
..
~_
-:
._
‘..
:.. .‘T.L. &ckhick,
SD. Cokm/finer~~ trmrfer&d -.
‘[ii] -G.-K~thanaaram.~ar;dD._S. Tinti. &em. Phys. Letters ‘o (1973) 225; ._ : -[~3~.$~ e&b@%g,Ele&nid s&Zra of polyatomic molecules ‘No&z&d. Princetoii, 1966): &at&i&d E.C. Lim. Chem. P&s. Letters 35 [l!J:N.K_ ~~~ (1975)303;. [w] B.W. C&h, D.B. IMbnan and S.D. Colson, Cbem. Pbys. ‘. l(1972) 191. 61 PIN. P&arl and R. Kopeban, J. Chem. Phys. 58 (1973) L-
:.
relcmtion in p-dibromob&ene
353
[17j R.M. Hocbstrasserand D.A. Wiersma, Israel J. Chem. 10 (1972)X7. [18] B.W. Gash and S.D. Colson, J. Chem. Phys. 59 (1973) 3528. [19] A.R. Gee and G.W. Robinson, J. Chem. whys. 46 (1967) 4847; 1201 S.D. Colson, R. Kopebnan and G.W. Robinson, J. Chem. Pbys. 47 (1967) 27; 47 (1967) 5462; S.D. Colson, D.M. Hanson, R. Kopeiman and G.W. Robinson, J. Chem. Phys. 48 (1968) 2215.