J . Phys. Chem. 1!J94,98, 2515-2519
2515
Photoassociation and Photoinduced Charge Transfer in Bridged Diary1 Compounds. 6. Intramolecular Triplet Excimers of Dicarbazolylalkanes and Their Comparison to an Intermolecular Triplet Excimer of Carbazole Jknjian Cai and Edward C. Limo*' Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received: October 5, 1993; In Final Form: December 9, 1993.
A time-resolved emission study of intramolecular triplet excimer formation has been carried out for dicarbazolylmethane (DCM) and dicarbazolylpropane (DCP) in fluid solution at room temperature. The triplet excimer formation was deduced from the comparison of the phosphorescence with the corresponding emission from the intermolecular triplet excimer of carbazole. It has been found that whereas the triplet excimer formation in DCP is evident in both polar and nonpolar solvents, the excimer formation in DCM is observed only in polar solvents at longer delay times. The result indicates that the conformation favored by the triplet excimer is more readily attainable in D C P than in DCM. The enhancement of the triplet excimer formation by polar solvent, which is also observed for carbazole, suggests that the triplet excimers are stabilized (at least in part) by charge resonance interactions. Comparison of the temporal characteristics of the normal delayed fluorescence of DCP with thoseof the corresponding excimer phosphorescence suggests that the delayed fluorescence a t long delay times is produced by bimolecular annihilation of the intramolecular triplet excimers. This in turn implies that the excited singlet-state species produced by bimolecular annihilation of the triplet excimers is unstable and rearranges into monomeric (Le., non-interacting) conformation prior to its decay by emission of radiation.
Introduction Over the past several years we havebcenstudying photophysical proprties of bridged diary1compoundsofgeneral structure M-XM, in which two identical aromatic moities (M) are joined to each other through a single bridging group (X = CH2,0, NH, etc,). The purpose of these studies was to probe the intermoiety interactions between an electronically excited moiety and the pendant ground-state moiety, leading to the formation of intramolecular excimers. The results for the bridged dinaphthyl compounds indicatethat the intermoiety interactionsvarystrongly with the nature of the bridging group. Thus, dinaphthylmethanes (1,l' and 2,2') and dinaphthyl ethers (1,l' and 2,2') exhibit the intermoiety interactions only in the lowest triplet state,1.2 while in dinaphthylaminesthe interactions occur in the excited singlet state,3 as well as in the lowest triplet state.' In earlier publications of this series,1~2Jwe have used timeresolved absorption and emission to study the dynamics of intramolecular triplet excimer formation in fluid solutions of dinaphthylmethanes, dinaphthyl ethers, and dinaphthylamines. It was shown that the triplet excimers can be identified by their characteristic phosphorescence and triplet-triplet absorption, as well as by the delayed fluorescence that arises from the bimolecular annihilationof the intramolecular triplet excimers. Comparison of the temporal characteristics of the triplet-triplet absorption (monomer as well as excimer) with those of the delayed fluortsctnce (monomer and excimer) provide8 compellingevidence for the formation of intramolecular triplet excimers in these molecular systems. In this paper, we extend the time-resolved emission studies of the intramolecular triplet excimers to 1,l -dicarbazolylmethane (DCM) and 1,3-dicarbazolylpropne (DCP). It is shown that the triplet excimer formation in DCM, as deduced from the appearance of the excimer phosphorescence, occurs only in polar solvents. For DCP with a longer methylene chain, the excimer formation is observed in both polar and nonpolar solvents. These Holder of the Goodyear Chair in Chemistry at The University of Akron. Abstract publiahcd in Aduance ACS Absrracrs. February 1, 1994.
0022-3654/94/2098-25 15$04.50/0
results indicate that the conformation favored by the triplet excimer is much more readily attained by DCP than by DCM. The enhancement of the triplet excimer formation by polar solvents, which is also observed for carbazole, suggest that the excimer formation is facilitated, at least in part, by intermoiety charge resonance interactions. These behaviors are in sharp contrast to those in the related 1,l-di-a-naphthylmethaneand 1,3-di-a-naphthylpropanein which the intramolecular triplet excimer formation is observed independent of solvent polarity and the length of the methylene linkage (n = l a ) . $ Comparison of the temporal characteristics of the monomeric delayed fluorescence at long delay times with those of the excimer phosphorescence in DCP shows that the former emission decays with a rate which is 2 times greater than that of the latter emission at long delay times. This indicates that the delayed fluorescence arises mostly from the bimolecular annihilationof intramolecular triplet excimers. This in turn implies that the excited singletstate species produced by the bimolecular annihilation of the triplet excimers rearranges into monomeric (or non-interacting) conformation prior to its decay by an emission process.
Experimental Section The apparatus and procedureused to measure the spectral and temporal characteristics of the laser-excited emission (fluorescence, delayed fluorescence, and phosphorescence) have been described in detail.' Briefly, the fluid solution of the sample at room temperature was deaerated by continuous bubbling with argon and was excited by the unfocused 308-nm output (- 10 Hz) of a Lambda-Physik EMG-50 excimer laser (pulse width 20 ns). The emission from the sample was detected by a gated diode-array spectrometer, consisting of an ISA HR-320 monochromator and a PI intensified diodc-array detector. The temporal characteristics of the emission were determined by measuring the integrated intensity as a function of the delay time between excitation and observation. The maximum temporal resolution of the intensified diode-array apparatus is 5 ns. 1,l-Dicarbazolylmethane(DCM) and 1,3-dicarbazolylpropane (DCP), prepared by the method of Johnson? were the gift of
-
Q 1994 American Chemical Society
Cai and Lim
2516 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994
John Masnovi of Cleveland State University. Both samples, as received, contained carbazole as a major impurity. In addition, the DCM sample also contained N-(hydroxymethy1)carbazole as a secondaryimpurity. The carbazole impurities were removed from the DCM sample by repeated recrystallization in acetone, but this simple procedure did not remove carbazole from the DCP sample, DCP was therefore purified as follows: The sample was dissolved in a mixed solvent containing methylene chloride and pyridine. Acetyl chloride was then added dropwise at 0 OC. After the mixture was stirred at room temperature for 4 h, a precipitate (DCP) formed which was filtered and washed with water. The DCP precipitate was recrystallized 5 times using ethyl ether. No carbazole was detected by GCIMS, The only impurity detected in DCP was 1,4-dicarbazolylbutane (DCB) which was present at the 0.2% level, whereas DCM showed no detectable impurity of any kind. The upper limit of possible impurity (other than DCB in DCP) is 0.02% both for DCM and DCP, based on the GC/MS analyses. Carbazole from Aldrich was purified by column chromatography (silica gel) followed by repeated recrystallization from methanol. The only detectable impurities were 2-methyl- and 3-methylcarbazole which were present at 0.07% and 0.21%,respectively. Optima grade solvents from Fisher were used from newly opened bottles, and they exhibited negligible emission under the experimental conditions. For DCM and DCP, the sample concentrations were purposely kept low in order to distinguish intramolecular triplet excimers from possible intermolecular triplet excimers that might form in concentrated solutions. This, combined with small gate widths (necessary for accurate temporal measurements) used in the experiments, yielded an inferior signal-to-noise ratio. The photochemicalstabilityof all three compoundswas checked during and after measurements by comparing the intensity and spectral shape of the prompt fluorescence with those of the emission at the very beginning of the experiment. Under the experimental conditions employed, no detectable change was observed, indicating that the compounds were photostable.
Results and Discussion The rate processes associated with the formation and decay of triplet excimers (intra- and inter-molecular) can bedescribed by: ki
T1-
so
(1)
k,
T,+T,+S,+So
k4
T’,
T‘,
+ T,
ks
T’,
T, + So
S,(or SI) + S’, (or So)
+ T’,
k6
+
SI S’,
(3) (4)
(5)
(6) (7)
k
SI S‘,
(9)
Here T I is the triplet monomer, T’I is the intermolecular triplet
Carb in IS0
300
400
500
Carb in MeCN
300
4b0
500 nm
Figure 1. Time-resolveddelayed emission spectraof carbazolein isooctane (left) and acetonitrile (right), measured with a gate width of 5 M. Concentration of the solutions was 1 X 10-4 M, and laser energy was 2.5 mJ. The spectra have been normalized to the same peek height and artificially offset vertically to facilitate the comparison of the spectral shape.
excimer, S’1 and S‘Oare respectively,the excited singlet and ground singlet states of the dimeric species associated with T’I, and SI and SOare the lowest excited singlet state and the ground state of the monomer, respectively. Equations 1,4, and 7-9 describe the first-order decay processes, q s 3,5,and 6 denote bimolecular annihilation processes, and q s 2 and 4 represent the formation and the back-dissociation of the intermolecular triplet excimer, respectively. Figure 1 presents the time-resolved emission spectra of carbazole taken in isooctaneand acetonitrileat room temperature. In both solvents, the early-gated ( < l o p s ) delayed emissionspectra are identical to the corresponding prompt fluorescence (0 ps) of the monomeric species, indicating that the triplet-state species involved in the bimolecular annihilation process (leading to the appearance of the delayed fluorescence) is the monomeric carbazole (q3). At longer delay times (>20pa), however, the delayed emission spectra display, in addition, a red-shifted structure delayed fluorescence at about 400 nm. This delayed emission (as well as the related emission in dibenzothiopheneand dibenzofuran) was previously assigned’ to the excimer delayed fluorescence, which arises from thc: bimolecular annihilation of the intermoleculartriplet excimers of carbazole (eq 6). Consistent with this assignment,2*8 the structured delayed fluorcscence exhibits a rise time which is very similar to the decay time of the monomer delayed fluorescence, as shown in Figure 2. The time required for the structured emission to reach maximum intensity, t,,,, follows the expected relationship9 between it and the decay rate of the monomer delayed fluorescence k M and the decay rate of the excimer emission &E, viz., tmrx= (&M - &E)-, ln(kkt/ka). These results indicate that the triplet monomer of carbazole (from which the monomer delayed fluorescence originates) is the precursor of the structured emission we attribute to the bimolecular annihilation of the intermolecular triplet excimer of carbazole. This structured delayed fluorescence appears with substantial intensity even at very short delay times, as illustrated in Figure 3. These observations indicate that the structured emission is not a sensitized delayed fluorescence of an impurity, induced by energy transfer from a carbazole triplet state to an impurity triplet state. The spectral feature of the emission is definitely not that of the fluorescence of known impurities (methylcarbazoles-see Experimental Section) in the sample.
The Journal of Physical Chemistry, Vol. 98, No. 10, I994 2517
Charge Transfer in Bridged Diary1 Compounds 8.5
324-348nm A 402-495nm
DCM in is0
I
DCM in MeCN
A A
A
I
A
A
A
I
I
,I;
I
4
I
I
3.54 0
5
15
10
20
I
25
Time (microseconds)
Figure 2. Temporalcharacteristics of the monomer delayed fluorescence (square) and those of the excimer delayed fluorcscence (triangle) of carbazole in isooctane, measured using a gate width of 0.2 PS. Concentration of the solution was 1.5 X lk3M,and laser energy was 1.2 mJ. Carbazole in IS0
= 10 us 5 us 2 us 1
300
350
400
450
500
300
400
500
300
400
560 nm
Figure A Time-resolved delayed emission spectraof dicarbazolylmethane (DCM)in isooctane (left) and acetonitrile(right),measured with a gate width of 5 p. Concentration of the solutions was 8 X l k 5 M,and laser energy was 5.5 mJ. The spectra have been normalized to the same peak height on the monomer delayed fluorescence and artificially offset
vertically.
us
550 nm
Figure3. Time-resolveddelayed emissionspectra (monomer and excimer) of carbazole in isooctane, measured with a gate width of 0.2 @. The experimental conditions were the same as in Figure 2. The spectra have been normalized to the same peak height on the monomer delayed
fluorescence and artificially offset vertically. Since an impurity level in excess of 10% is required to account for the appearance time and intensity of the emission,we conclude that the red-shifted structured emission is the excimer delayed fluorescence of carbazole produced by bimolecular annihilation of intermoleculartriplet excimen. It should be noted from Figure 1 that the intensity of the excimer delayed fluorescence relative to that of the monomer delayed fluorescence is substantially stronger in the polar solvent acetonitrile than in the nonpolar solvent isooctane. Under intense illumination conditions or in concentrated solutions, as in the case for Figure 1, the excimer phosphorescence (eq 7) is not apparent in the delayed emission spectra due to the very strong delayed fluorescence. However, time-resolved emission spectra taken with a lower sample concentration and with a greatly reduced laser fluence clearly display the excimer phosphorescence with an intensity maximum at about 480 nm, as shown in earlier papers? A very similar phosphorescence, also attributed to theexcimer, has beenobserved from N-ethylcarbazole in solution10 and in polystyrene,lI as well as DCP in polystyrene.11 Figure 4 compares the time-resolved emission spectra of DCM in isooctane and acetonitrile. The delayed emission spectra in isooctane are spectrally identical to the prompt fluorescence (0ps delay) of DCM for all delay times, indicating that the triplet-
state speciesresponsible for the delayed fluorescence(via triplettriplet annihilation, eq 3) is the non-interacting conformation of DCM, in which there is little attractive interaction between the photoexcited carbazole moiety and the pendant ground-state moiety. The photophysical behavior of DCM in acetonitrile at shorter delay times is also very similar to that in isooctane, and only the normal delayed fluorescenceis evidentin the time-resolved spectra. At longer delay times, however, the delayed emission spectra display an additional emission with intensity maximum at about 450 nm. Sincetheoverall spectral featureof this emission is very similar to the excimer phosphorescence from carbazole and DCP (vide infra), we assign this emission to an intramolecular triplet excimer of DCM. Consistent with this assignment, the intensity of the 450-nm emission is proportional to the first power of the laser fluence, as contrasted to the quadratic dependence of the intensity of the 360-nm delayed fluorescence on the laser fluence. Because of the weaknessof the spectral features assigned to the triplet excimer, it was not possible to carry out reliable temporal measurements of the emissions for DCM. The observation of an important solvent-polarity dependence of the triplet excimer formation in carbazole and DCM, which is not evident in the related naphthalene and DNM, may be an indication of charge-resonance stabilization of triplet excimers in these compounds. The electronic energy gap (eV) between the charge-transfer (CT) triplet state (M+M-) and the lowest triplet state (M*M), in hexane, can be estimated from the equation E(M+M-) - E(M*M) =
ElI2(M+/M-) - El12(M-/M) + 0.15 where ElIz(M+/M) and Elp(M-/M) are the redox potentials of an aromatic molecule M, and an assumption is made that the CT triplet state is degenerate with the correspondingCT singlet state. For N-ethylcarbazole, the M+M-- M*M energy gap so deduced is 1.03eV, which is substantially smaller than the corresponding gap of 1.72 eV for naphthalene.13 The contribution of CT forces to the formation of triplet excimer is therefore expected to be more important for carbazole than for naphthalene. Since the energy gap will decrease substantially with solvent polarity, the CT force could be an important source of binding energy for the
Cai and Lim
2518 The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 DCP in IS0
DCP in MeCN
450
1
300
400
500
300
400
560
nm
Figure 5. Time-resolveddelayedemission spectra of dicarbazolylpropane
(DCP) in isooctane (left) and acetonitrile (right), measured with a gate M, and laser width of 5 ps. Concentration of the solutions was 1 X energy was 22 mJ. The spectra have been normalized to the same peak height on the monomer delayed fluorescence and artificially offset vertically.
formation of intramolecular triplet excimer of DCM, and intermolecular triplet excimer of carbazole, in solvents of large dielectric constant. Triplet excimer formation via the geminate recombination of ion pairdo can be ruled out for D C M and carbazole, as the transient absorption spectra do not reveal the presence of the radical cation, and the excimer emissions (phosphorescence and delayed fluorescence) of carbazole are observed even in nonpolar solvents. Figure 5 compares the time-resolved emission spectra of DCP in isooctane and acetonitrile with the corresponding fluorescence spectrum. The time evolution of the delayed emission spectra is very similar in both solvents. The prompt fluorescence spectrum (0-ks delay) of the compound in each solvent is composed of the normal monomeric fluorescencewith intensity maximum at about 360 nm and a broad shoulder a t about 420 nm, which can be assigned to the intramolecular singlet excimer of the compound, formed by the association of the photoexcited (singlet-state) moiety with the pendant ground-state moietyS6 The delayed emissions at very early delay times also consist of a spectral feature due to the monomeric emission and that due to the excimer emission. The intensity ratio of the excimer to the monomer components is approximately the same in the delayed emission and in the prompt emission, indicating that the triplet-triplet annihilations leading to the delayed fluorescence involve monomeric triplet-state species (i.e., non-interacting conformations of the molecule in the triplet state) at these early delay times (eq 3). This suggests that the triplet-state species produced by singlet triplet intersystem crossing of the intramolecular singlet excimer (and having confirmation identical to the singlet excimer) is unstable and dissociates into monomeric triplet-state species: and/or the quantum yield of the intersystem crossing is small for the excimer relative to the monomer. At longer delayed times, the spectrum is dominated by a structureless emission at about 480 nm which can be identified as the excimer phosphorescence. The lack of clear solvent-polarity dependence of triplet excimer formation (which contrasts the observation in carbazole and DCM) is not surprising since in DCP, with a long alkyl chain, the efficiency of the intramolecular excimer formation is expected to be influenced by chain conformations and by chain dynamics (associated with bringing two chromophoresclose together). The intramolecular character of the triplet excimer in DCP is -+
500
nm
550
Figure 6. Comparison of the excimer phosphorescence of carbazole in methylcyclohexane,DCM in acetonitrile, and DCP in isooctane. Concentration of the sample, laser energy, and gate width were 2.7 X 1V M, 21.5 pJ, and 2 ms for carbazole; 5.7 X M, 4.4 mJ, and 50 ps for
M, 5.4 mJ, and 50 ps for DCP. Delay times are
DCM; and 1.2 X as indicated.
A
338-350nm 481-562 nm
31
2
14 0
5
10
15
20 25 30 35 Time (microseconds)
40
45
I
50
Figure 7. Temporal characteristics of the excimer phosphorescence
(triangle) and those of the delayed fluorescence (square) of DCP in isooctane, measured with a gate width of 1 ps. Concentration of the solution was 1 X 10-5 M, and laser energy was 22 mJ. Regions of integration as indicated.
demonstrated by the observation that the intensity of the excimer phosphorescence (relative to that of the delayed fluorescence) is not sensitive to the variations in solute concentrations (< 10-5 M) and that the appearance time of the excimer phosphorescence is substantially shorter than that of the excimer delayed fluorescence of carbazole with much greater concentrations (compare Figures 1 and 5 ) . The excimer phosphorescence of carbazole, DCM and DCP are compared in Figure 6. Figure 7 presents the temporal characteristics of the normal (Le., monomeric) delayed fluorescence, excimer delayed fluorescence, and excimer phosphorescence of DCP in isooctane at room temperature. Three major time regimes can be recognized by comparison to the time-resolved emission spectra of Figure 5 : short, medium, and long. In the short ( < l O - ~ s )time range, the delayed emission spectra are composed almost exclusively of the monomer fluorescence and the excimer fluorescence (spectrally identical to the prompt excimer fluorescence) that arise from the bimolecular annihilation of monomeric triplet-state species. Consistent with this supposition, the temporal characteristics of the two emissions are identical in the short time range. In the intermediate (IO-30-4 time range, in which both the excimer phosphorescence and the monomer delayed fluorescence appear,
Charge Transfer in Bridged Diary1 Compounds the decay rates of the two emissions bear no simple relationship. This is to be expected if the monomer delayed fluorescence arises from the bimolecular annihilation of the monomer triplets, excimer triplets, as well as the heteroannihilation of the monomer triplets with excimer triplets (eq 5 ) . Finally, in the long (>30-~s)time regime, where the bimolecular annihilations of the long-lived triplet excimers is expected to make a major contribution to the intensity of the delayed fluorescence, the decay rate of the delayed fluorescence is twice that of the excimer phosphorescence. The observation of the monomeric delayed fluorescence arising from the bimolecular annihilations of the intramolecular triplet excimers of DCP suggests that the skewed singlet-state species produced from the annihilation process is unstable and rearranges into non-interacting (i.e., monomeric) conformation prior to decay by emission of radiation. 1,l'-Dinaphthylethane also exhibits mostly the monomeric delayed fluorescence,14 but 1,l-dinaphthylmethane displays structured delayed excimer fluorescence as well as the monomeric delayed fluorescence.l s 2 We conclude this section by elaborating on the significance of the observed solvent-polarity dependence of the triplet excimer formation in carbazole and DCM. It is well-known that singlet excimersof aromatic molecules are stabilized by exciton resonance and charge resonance. These two sources of binding energies are both largest for face-to-face arrangement of two aromaticmoieties, which accounts for the sandwich-pair geometry of the singlet excimers. For the triplet excimers, however, the exciton stabilization is expected to be negligibly small due to the extremely small transition moment of the singlet-triplet transition, whereas the charge resonance interaction is expected to be ineffective due to the high energy of the charge-transfer state relative to the lowest triplet state of the aromatic moiety. It was therefore proposedgJ5J6 that the triplet excimers of aromatic molecules are stabilized largely by van der Waals forces. Triplet excimers are therefore expected to be less stable and considerably different in geometry, as compared to the corresponding singlet excimer. An
The Journal of Physical Chemistry, Vol. 98, No. 10, 1994 2519 exception to this generalization could occur when the singlettriplet transition moment is unusually large (as, for example, in heavy-atom containing molecules) or when the energy of the CT state is unusually low. The observation of the solvent-polarity dependence of the triplet excimer formation in DCM and carbazole suggests that the compounds are the examples of the exception resulting from the relative low energy of the C T state. Acknowledgment. We are grateful to Dr. John Masnovi for the gift of the dicarbazolylalkane samples and to Dr. Klaas Zachariasse for pointing out the possible importance of charge resonance interaction in the stabilization of the carbazole triplet excimer. This work was supported by Grant DE-FGOZ89ER14024 from the Office of Basic Energy Sciences of the Department of Energy.
References and Notes (1) Modiano, S.H.; Dresner, J.; Lim, E. C. J . Phys. Chem. 1991, 95, 9144. (2) Modiano,S. H.;Dresner, J.;Cai, J.;Lim, E. C.J. Phys. Chem. 1993, 97, 3480. (3) Dresner, J.; Modiano, S. H.; Lim, E. C. J . Phys. Chem. 1992, 96, 4310. (4) Cai, J.; Lim, E. C. J. Phys. Chem. 1993, 97,6203. (5) Okajima, S.;Subudhi, P. C.; Lim, E. C. J . Chem. Phys. 1977,67, 4611. ( 6 ) Johnson, G. E. J . Chem. Phys. 1974, 61, 3002. (7) (a) Cai, J.; Lim, E. C. J . Phys. Chem. 1992, 96, 2935. (b) Cai, J.; Lim, E. C. J. Phys. Chem. 1992, 97, 3892. (8) Cai, J.; Lim, E. C. J. Phys. Chem. 1993, 97,8128. (9) Lim, E. C. Acc. Chem. Res. 1987, 20, 8 . (10) Haggquist, G. W.; Burkhart, R. D. J. Phys. Chem. 1993,97,2576. (1 1) Abia, A. A.; Burkhart, R. D. Macromolecules 1984, 17, 2739. (12) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1968, 72,257. (13) Zachariasse, K. A. Private communication. (14) Modiano, S. H.; Dresner, J.; Lim, E. C. Unpublished result. (15) Modiano, S.H.; Dresner, J.; Lim, E. C. Chem. Phys. Lerr. 1992,189, 144. (16) Lim, B. T.; Lim, E. C. J . Chem. Phys. 1993, 78, 5262.
