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Spectral manifestations of nonradiative processes in azulene
JOURNAL
OF
MOLECULAR
Spectral
SPECTROSCOPY
41, 297-301
(1972)
Manifestations of Nonradiative in Azulene’
Processes
ROBIN M. HOCHSTRASSER AND TA-YUEN LI Department of Chemistry, and Laboratory for Research on the Structure of Matter, The University of Pennsylvania Philadelphia, Pennsylvania 19104
The linewidt,hs for S1 + SO (at 14 652 cm-l) and S2 + SO (at 28 048 cm-l) of azulene in a naphthalene host crystal at 1.2’K are presented along with measurements of the line narrowing due to perdeuteration of the azulene. The results are related to current ideas of nonradiative processes in azulene and azulene-ds . INTROnUCTION
The well known anomalous fluorescence of azulene (1, 2) has been qualitatively interpreted by a number of investigators (S-6). It is widely accepted that the electronic relaxation out of t’he fluorescent’ state Sr is much slower t,han out of the nonfluorescent, state 6’1. The state S1, but not X2, is understood to be nonadiabatically coupled strongly enough to nearby vibronic levels of lower states such that the fluorescence lifetime (7) is severely shortened compared with the natural radiat’ive lifetime. This is in cont#rast to the usual situation in aromatics that fluoresce from their lowest states X1 with lifetimes close to their natural lifetimes, and hhat are expected to radiate from nearby upper stat’es SZ with much longer lifetimes than the natural X2 e So lifetimes. Concomitant with radiative lifetime shortening there should occur a spectral line broadening of magnitude (%cT)-~ cm-‘. For the same reasons that deuteration is understood to cause a lifetime lengthening in aromat,ics, it, should also cause a spectral line narrowing. The purpose of this paper is t,o show that the linewidths in t)he spectra of azulene, 291+- So and X2 + SO, are consistent with these notions, and we present quant,itative data on t,he coupling mat’rix elements for the nonradiative processes originating at) S1, and on t,he deuteration narrowing of spectral lines. EXPERIMENTAL
RESULTS
The experiments were done with azulene in a naphthalene host crystal with guest concentrations in the range 10-5-10-6 mole fract,ion. The spectra of bot’h 1This research was supported in part by a 11epartment. of Health Grant GM 12592, and in part by Laboratory for Research on the Structure of Matter t)hrough an Advanced Research Projects Agency contract to the University of Pennsylvania. 297 Copyright
@ 1972 by Academic
Press, Inc.
HOCHSTRASSER
298
AND
LI
S1 c So and S, c So are known from the work of Sidman and ;IIcClure (7). We have rest,udied only the zero-zero bands of these transitions in carefully grown crystals at, 12°K. At this temperature the linewidth depends negligibly on the temperature, but there were variations of linewidth from sample to sample, and at higher concentrations the spectra were more complex. The spectra were recorded using a Jarell-Ash Czerny-Turner-Fastie spect rorneter with slit widths from m-20 P (red bands) to 40 P (violet, bands). The violet system was recorded in t’he 16th order at a practical resolution of 0.15 cm’, the red system was recorded in the 8th order at a practical resolution of 0.06 cm-‘. Care was taken to optimize the opt’ical density in order to minimize light scattering errors. For the curves shown the peak optical densities are known to within 1 O;#thereby introducing an error of 1”; into the half-width determination. For detection we used a 9558Q photomultiplier in conjunct)ion with a lockin amplifier. The spectra were recorded in polarized light. The spectra of azulene-hs and -& are shown in t’wo figures: I’ig. 1 shows the violet s,ystem at 2S 04s cm-’ (h,), and Fig. :! shows t,he red system at 14 652 cm’ (//8). The deuteration shifts for t,hc violet and red systems are 90 cm1 and 61 cml, respectively. A comparison of the figures shows clearly that. the violet qstem is considerably sharper, and the detailed results are given in Table 11 in terms of half-width (full width at) half peak optical densit,y). We do not believe that there are any significant inst’rumrntal contributions to these spectral line profiles. We also made some measurements in naphthalene-& with no significant change in t,he linewidth results. We have obtained accurate measurements for the perdeuteration shifts and for the site shifts of proto vs. deutero host lat,tices. These are as follows for S1 c- So : nzulrne-h.8 in napht,halene& 14 653 cm-‘; & in hg , 14 713 cm-’ (deuternt,ion shift, 61 cm’) ; h, in & , 14 657 cm-’ ; d8 in d8 14 719 (deuteration shift 62 cm’). There is a difference of 5 cm-’ between the energy of S, (11,) in proto
AZULENE ZERO-ZERO BANDS
CM-' FIG. 1. Absorption
and azldene-ds
spectrum of the 0,O band of the violet in naphthalene at 1.2”K.
system
(8~ +
SU) of a,z~dene
NON-RADIATIVE
PROCESSES IN AZULENE
-0.6
ClOD8 AZULENE
-0.5
c v,
. .
%-so ZERO-ZERO
Cd-b
2
;
-0.3
:
;T
:
,’ 0
:’ .b :’ : W8
-0.2
-0.1
*... *’
:
BANDS . . .
*_ .
14654
. .
* .
.
: cm-’ i
_:’ _:’
‘;. i ‘\
I 14652
. .
. . * :
:YKFt
._..J I
:‘.
. *
-04
ti u
299
.-%.....
‘-. .,,.,.
. . . . . .-a
14650
...a
:.. X.
.,*: ,
14715
I 14713
-0..
*..
1471 I
CM-’
FIG. 2. Absorption spectrum ds in naphthalene at 1.2”K.
of the red system
TABLII: ENERGIES
AND
(S1 + So) 0,O band of azulene and azulene-
I
HALF-WIDTHB FORAZULENES, +So B.\NDS
Transition
s1 +-se Sf +- so
AND&+S~
TR.~NSITIONS:ZERO-ZERO Halfwidth (cm-‘)
fi (cm-‘)
14 652 28 048
118
as ____~
2.08 0.47
1.77 0.50
and deuteronaphthalene, and 6 cm-’ for & (&). This is roughly what is expected from the variation of energy denominator in the dispersion shift. SOURCES
OF LINE BROADENING
IN MIXED
CRYSTAL
SPECTRA
Considerable experience with mixed crystal spectra of aromatics and their derivatives has demonstrated that 1 cm-’ plus or minus cu. 0.5 cm-l is an expected linewidth for the lowest energy electronic transition of the guest molecule. Linewidths in neat crystal spectra or isotopically mixed cryst,als can be considerably sharper than this. We have concluded from a number of experiment,s that the most likely cause of linebroadening in mixed crystals is the inhomogeneity of the sample: it turns out to be very difficult to arrange to have all guest molecules in precisely the same environment. If this were possible the spectral lines still would not have the radiative width unless this exceeded the coupling of the guest state with host phonon states via the local strain. Unfortunately, these effects have not yet been measured and inhomogeneity appears to dominate the existing spectral line profiles in mixed crystals. The spectra of higher excited states of a guest are frequently broader and be-
300
HOCHSTRASSER
AND
LI
cause of the smearing out effect of phonon interact’ions the linewidths, but not the nonradiative lifetimes, can be readily measured with medium resolution techniques. In azulene the cont’rary is true: XS +- So linewidths arc much narrower t’han those in S1 +- So. The radiative lifetime of & -+ So is cu. 5 X lo--” set (8); thus the spectral linewidth should be 10P4 c6’ for an isolat.ed molecule. In this case the 0.5 cm-’ linewidth must be crystal imposed: the difference between 0.50 cm-’ (c&8)and 0.47 cm-’ (h8) is smaller than t)he probable error and is disregarded in what follows. The absorption spectra of azulene do not indicate t,hat there are significant changes of equilibrium geometry on excitation of either & or & . Stark effect measurements have shown that the dipole moments of the two st,at,es S1 and SsI? are about the same (9). The gas to mixed-crystal shift is considerably larger for & +- So (-710 em-‘) than for & +-- So (+375 cm-‘) (20). These fact.s incline us to the view that the 0.5 cm-’ spectral width of X2 +- So is a reasonable upper limit int#ernal standard for the inhomogeneity contribution to the S, +- So linewidth. QUAKTITATIVE CONSIDERATIO?~TS Three main results emerge from these studies: (I) The linewidth of S1 +-- So is larger than that of St c- S,, for nzulene-hR and 48 in a naphthalene host. at 1.201<. The zero-zero bands of these transit,ions are considered. (2) The linewidth of S1 +- So for azulene-(28 is significantly smaller than for azulene-h8 in naphthalene. (3) The line profile of A%+- SO for azulene (zero-zero band) is significant13 different from a Gaussian curve, and quite closely follows a Lorenzian over about four half-widths. This comparison is shown in Fig. 3. These results establish that the X1 +- So transilion is broadened by interaction with :L nearly uniformly spaced set, of vibronic stst,es, and the line\vidtbs in 11, AZULENE
14654
FIG. 3. Comparison of azulene 0,O having the proper half-width.
ZERO-ZERO
BAND
I4652
I4650
bnnd (81 t
S,,) with Gaussian and Lorenzian
fits
NON-RADIATIVE
PROCESSES
IN AZULENE
301
and dg are large enough to be consistent with t’he absence of fluorescence, X1 -+ So , of significant quantum yield. They also establish the line narrowing effect of deuteration. APPROXIMATE CONSIDERATIONS
If the amount of 0.5 cm-’ is adopted as the inhomogeneity contribution to the &+-So width, we conclude that AE,,, (H) = 1.61 cm-’ and AE,/z(0) = 1.27 cm-l. It follows that the ratio of the nonradiative transition probabilities (W) is given by W,/W, = 1.27. DISCUSSION
In view of our observation of a Lorenzian profile and a deuterium isotope effect it would follow that azulene should fall in the Engleman-Jortner weak coupling limit. This most likely means that the radiationless process involves X1-X0 coupling rather than XI-Tl coupling. The singlet-triplet gap is about 3000 formula (6): cm-l. If we use the Engleman-Jortner
2@!_- YH UC-D
)I
WC-H
to calculate the ratio WH/WD we get best agreement with experiment for parameters YH = 0.133, yD = 0.135 which we consider to be reasonable values. Our observation of a Lorenzian line having an effective width of 1.61 cm-’ indicates that azulene should radiate from t,his state displaying exponential decay with a mean lifetime of not less than 3.3 psec (h,) or 4.2 psec (d8). These predictions can be compared with the 300 K measurements by Rentzepis of azulene in benzene solution (II ) : Rentzepis finds a radiat,ive lifet,ime of 8.3 psec for hg . ACKNOWLEDGMENT
We wish to thank Dr. R. H. Clarke for his help in obtaining some of the preliminary results for this study. RECEIVED: July 30, 1971 REFERENCES 1. M. BEER AND H. C. LONGUET-HIGGINS, J. Chem. Phys. 23,139O (1955). 2. G. VISWANATH AND M. KASHK, J. C%hem. Phys. 24, 574 (1956). S. G. R. HUNT, E. F. MCCOY AND I. G. Ross, Aust. J. Chem. 18, 591 (1962). 4. G. W. ROBINSON AND R. P. FROSCH, J. Chem. Phys. 37, 1962 (1962); 38, 1187 (1962). 5. J. P. BYRNE, E. F. MCCOY AND I. G. Ross, Aust. J. Chem. 18, 1589 (1965). 6. R. EN~LEMAN AND J. JORTNF,R,Mol. Phys. 18, 145 (1970). Y. J. W. SIDMAN AND D. S. MCCLURE, J. Chem. Phys. 24, 757 (1956). 8. R. C. DHINGRA .\NDJ. A. POOLE, J. Chem. Phys. 48,4829 (1968). 9. R. M. HOCHSTR~SSER:~ND L. J. NOE, J. Chem. Phys. 60, 1684 (1969). 10. G. R. HUNT AND I. G. Ross, J. Mol. Spectrose. 9, 50 (1962). 11. P. M. RENTZEPIS, Chem Phys. Lett. 3, 717 (1969).
OF
MOLECULAR
Spectral
SPECTROSCOPY
41, 297-301
(1972)
Manifestations of Nonradiative in Azulene’
Processes
ROBIN M. HOCHSTRASSER AND TA-YUEN LI Department of Chemistry, and Laboratory for Research on the Structure of Matter, The University of Pennsylvania Philadelphia, Pennsylvania 19104
The linewidt,hs for S1 + SO (at 14 652 cm-l) and S2 + SO (at 28 048 cm-l) of azulene in a naphthalene host crystal at 1.2’K are presented along with measurements of the line narrowing due to perdeuteration of the azulene. The results are related to current ideas of nonradiative processes in azulene and azulene-ds . INTROnUCTION
The well known anomalous fluorescence of azulene (1, 2) has been qualitatively interpreted by a number of investigators (S-6). It is widely accepted that the electronic relaxation out of t’he fluorescent’ state Sr is much slower t,han out of the nonfluorescent, state 6’1. The state S1, but not X2, is understood to be nonadiabatically coupled strongly enough to nearby vibronic levels of lower states such that the fluorescence lifetime (7) is severely shortened compared with the natural radiat’ive lifetime. This is in cont#rast to the usual situation in aromatics that fluoresce from their lowest states X1 with lifetimes close to their natural lifetimes, and hhat are expected to radiate from nearby upper stat’es SZ with much longer lifetimes than the natural X2 e So lifetimes. Concomitant with radiative lifetime shortening there should occur a spectral line broadening of magnitude (%cT)-~ cm-‘. For the same reasons that deuteration is understood to cause a lifetime lengthening in aromat,ics, it, should also cause a spectral line narrowing. The purpose of this paper is t,o show that the linewidths in t)he spectra of azulene, 291+- So and X2 + SO, are consistent with these notions, and we present quant,itative data on t,he coupling mat’rix elements for the nonradiative processes originating at) S1, and on t,he deuteration narrowing of spectral lines. EXPERIMENTAL
RESULTS
The experiments were done with azulene in a naphthalene host crystal with guest concentrations in the range 10-5-10-6 mole fract,ion. The spectra of bot’h 1This research was supported in part by a 11epartment. of Health Grant GM 12592, and in part by Laboratory for Research on the Structure of Matter t)hrough an Advanced Research Projects Agency contract to the University of Pennsylvania. 297 Copyright
@ 1972 by Academic
Press, Inc.
HOCHSTRASSER
298
AND
LI
S1 c So and S, c So are known from the work of Sidman and ;IIcClure (7). We have rest,udied only the zero-zero bands of these transitions in carefully grown crystals at, 12°K. At this temperature the linewidth depends negligibly on the temperature, but there were variations of linewidth from sample to sample, and at higher concentrations the spectra were more complex. The spectra were recorded using a Jarell-Ash Czerny-Turner-Fastie spect rorneter with slit widths from m-20 P (red bands) to 40 P (violet, bands). The violet system was recorded in t’he 16th order at a practical resolution of 0.15 cm’, the red system was recorded in the 8th order at a practical resolution of 0.06 cm-‘. Care was taken to optimize the opt’ical density in order to minimize light scattering errors. For the curves shown the peak optical densities are known to within 1 O;#thereby introducing an error of 1”; into the half-width determination. For detection we used a 9558Q photomultiplier in conjunct)ion with a lockin amplifier. The spectra were recorded in polarized light. The spectra of azulene-hs and -& are shown in t’wo figures: I’ig. 1 shows the violet s,ystem at 2S 04s cm-’ (h,), and Fig. :! shows t,he red system at 14 652 cm’ (//8). The deuteration shifts for t,hc violet and red systems are 90 cm1 and 61 cml, respectively. A comparison of the figures shows clearly that. the violet qstem is considerably sharper, and the detailed results are given in Table 11 in terms of half-width (full width at) half peak optical densit,y). We do not believe that there are any significant inst’rumrntal contributions to these spectral line profiles. We also made some measurements in naphthalene-& with no significant change in t,he linewidth results. We have obtained accurate measurements for the perdeuteration shifts and for the site shifts of proto vs. deutero host lat,tices. These are as follows for S1 c- So : nzulrne-h.8 in napht,halene& 14 653 cm-‘; & in hg , 14 713 cm-’ (deuternt,ion shift, 61 cm’) ; h, in & , 14 657 cm-’ ; d8 in d8 14 719 (deuteration shift 62 cm’). There is a difference of 5 cm-’ between the energy of S, (11,) in proto
AZULENE ZERO-ZERO BANDS
CM-' FIG. 1. Absorption
and azldene-ds
spectrum of the 0,O band of the violet in naphthalene at 1.2”K.
system
(8~ +
SU) of a,z~dene
NON-RADIATIVE
PROCESSES IN AZULENE
-0.6
ClOD8 AZULENE
-0.5
c v,
. .
%-so ZERO-ZERO
Cd-b
2
;
-0.3
:
;T
:
,’ 0
:’ .b :’ : W8
-0.2
-0.1
*... *’
:
BANDS . . .
*_ .
14654
. .
* .
.
: cm-’ i
_:’ _:’
‘;. i ‘\
I 14652
. .
. . * :
:YKFt
._..J I
:‘.
. *
-04
ti u
299
.-%.....
‘-. .,,.,.
. . . . . .-a
14650
...a
:.. X.
.,*: ,
14715
I 14713
-0..
*..
1471 I
CM-’
FIG. 2. Absorption spectrum ds in naphthalene at 1.2”K.
of the red system
TABLII: ENERGIES
AND
(S1 + So) 0,O band of azulene and azulene-
I
HALF-WIDTHB FORAZULENES, +So B.\NDS
Transition
s1 +-se Sf +- so
AND&+S~
TR.~NSITIONS:ZERO-ZERO Halfwidth (cm-‘)
fi (cm-‘)
14 652 28 048
118
as ____~
2.08 0.47
1.77 0.50
and deuteronaphthalene, and 6 cm-’ for & (&). This is roughly what is expected from the variation of energy denominator in the dispersion shift. SOURCES
OF LINE BROADENING
IN MIXED
CRYSTAL
SPECTRA
Considerable experience with mixed crystal spectra of aromatics and their derivatives has demonstrated that 1 cm-’ plus or minus cu. 0.5 cm-l is an expected linewidth for the lowest energy electronic transition of the guest molecule. Linewidths in neat crystal spectra or isotopically mixed cryst,als can be considerably sharper than this. We have concluded from a number of experiment,s that the most likely cause of linebroadening in mixed crystals is the inhomogeneity of the sample: it turns out to be very difficult to arrange to have all guest molecules in precisely the same environment. If this were possible the spectral lines still would not have the radiative width unless this exceeded the coupling of the guest state with host phonon states via the local strain. Unfortunately, these effects have not yet been measured and inhomogeneity appears to dominate the existing spectral line profiles in mixed crystals. The spectra of higher excited states of a guest are frequently broader and be-
300
HOCHSTRASSER
AND
LI
cause of the smearing out effect of phonon interact’ions the linewidths, but not the nonradiative lifetimes, can be readily measured with medium resolution techniques. In azulene the cont’rary is true: XS +- So linewidths arc much narrower t’han those in S1 +- So. The radiative lifetime of & -+ So is cu. 5 X lo--” set (8); thus the spectral linewidth should be 10P4 c6’ for an isolat.ed molecule. In this case the 0.5 cm-’ linewidth must be crystal imposed: the difference between 0.50 cm-’ (c&8)and 0.47 cm-’ (h8) is smaller than t)he probable error and is disregarded in what follows. The absorption spectra of azulene do not indicate t,hat there are significant changes of equilibrium geometry on excitation of either & or & . Stark effect measurements have shown that the dipole moments of the two st,at,es S1 and SsI? are about the same (9). The gas to mixed-crystal shift is considerably larger for & +- So (-710 em-‘) than for & +-- So (+375 cm-‘) (20). These fact.s incline us to the view that the 0.5 cm-’ spectral width of X2 +- So is a reasonable upper limit int#ernal standard for the inhomogeneity contribution to the S, +- So linewidth. QUAKTITATIVE CONSIDERATIO?~TS Three main results emerge from these studies: (I) The linewidth of S1 +-- So is larger than that of St c- S,, for nzulene-hR and 48 in a naphthalene host. at 1.201<. The zero-zero bands of these transit,ions are considered. (2) The linewidth of S1 +- So for azulene-(28 is significantly smaller than for azulene-h8 in naphthalene. (3) The line profile of A%+- SO for azulene (zero-zero band) is significant13 different from a Gaussian curve, and quite closely follows a Lorenzian over about four half-widths. This comparison is shown in Fig. 3. These results establish that the X1 +- So transilion is broadened by interaction with :L nearly uniformly spaced set, of vibronic stst,es, and the line\vidtbs in 11, AZULENE
14654
FIG. 3. Comparison of azulene 0,O having the proper half-width.
ZERO-ZERO
BAND
I4652
I4650
bnnd (81 t
S,,) with Gaussian and Lorenzian
fits
NON-RADIATIVE
PROCESSES
IN AZULENE
301
and dg are large enough to be consistent with t’he absence of fluorescence, X1 -+ So , of significant quantum yield. They also establish the line narrowing effect of deuteration. APPROXIMATE CONSIDERATIONS
If the amount of 0.5 cm-’ is adopted as the inhomogeneity contribution to the &+-So width, we conclude that AE,,, (H) = 1.61 cm-’ and AE,/z(0) = 1.27 cm-l. It follows that the ratio of the nonradiative transition probabilities (W) is given by W,/W, = 1.27. DISCUSSION
In view of our observation of a Lorenzian profile and a deuterium isotope effect it would follow that azulene should fall in the Engleman-Jortner weak coupling limit. This most likely means that the radiationless process involves X1-X0 coupling rather than XI-Tl coupling. The singlet-triplet gap is about 3000 formula (6): cm-l. If we use the Engleman-Jortner
2@!_- YH UC-D
)I
WC-H
to calculate the ratio WH/WD we get best agreement with experiment for parameters YH = 0.133, yD = 0.135 which we consider to be reasonable values. Our observation of a Lorenzian line having an effective width of 1.61 cm-’ indicates that azulene should radiate from t,his state displaying exponential decay with a mean lifetime of not less than 3.3 psec (h,) or 4.2 psec (d8). These predictions can be compared with the 300 K measurements by Rentzepis of azulene in benzene solution (II ) : Rentzepis finds a radiat,ive lifet,ime of 8.3 psec for hg . ACKNOWLEDGMENT
We wish to thank Dr. R. H. Clarke for his help in obtaining some of the preliminary results for this study. RECEIVED: July 30, 1971 REFERENCES 1. M. BEER AND H. C. LONGUET-HIGGINS, J. Chem. Phys. 23,139O (1955). 2. G. VISWANATH AND M. KASHK, J. C%hem. Phys. 24, 574 (1956). S. G. R. HUNT, E. F. MCCOY AND I. G. Ross, Aust. J. Chem. 18, 591 (1962). 4. G. W. ROBINSON AND R. P. FROSCH, J. Chem. Phys. 37, 1962 (1962); 38, 1187 (1962). 5. J. P. BYRNE, E. F. MCCOY AND I. G. Ross, Aust. J. Chem. 18, 1589 (1965). 6. R. EN~LEMAN AND J. JORTNF,R,Mol. Phys. 18, 145 (1970). Y. J. W. SIDMAN AND D. S. MCCLURE, J. Chem. Phys. 24, 757 (1956). 8. R. C. DHINGRA .\NDJ. A. POOLE, J. Chem. Phys. 48,4829 (1968). 9. R. M. HOCHSTR~SSER:~ND L. J. NOE, J. Chem. Phys. 60, 1684 (1969). 10. G. R. HUNT AND I. G. Ross, J. Mol. Spectrose. 9, 50 (1962). 11. P. M. RENTZEPIS, Chem Phys. Lett. 3, 717 (1969).