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J. Phys. Chem. 1993, 97, 3480-3485
Intramolecular Photoassociation and Photoinduced Charge Transfer in Bridged Diary1 Compounds. 4. Temporal Studies of Intramolecular Triplet Excimer Formation in Dinaphthylmethanes and Dinaphthyl Ethers Steven H. Modiano,? Jozef Dresner,* Jianjian Cai, and Edward C. Lim’+o Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received: September 30, 1992; In Final Form: January 6. 1993
Comparisons of the temporal characteristics of the transient absorption with those of the delayed fluorescence for dinaphthylmethanes and dinaphthyl ethers provide compelling kinetic evidence for the formation of intramolecular triplet excimers.
Introduction In the first paper of this series,’ time-resolved absorption and emission spectra of photoexcited 2,2’-dinaphthylmethane and 2,2’dinaphthyl ether were presented which provide evidence for intramolecular triplet excimer formation, involving the association of a naphthalene moiety in its triplet manifold with the groundstatemoietyjoined toitbythebridginggroup. Itwasalsoproposed that bimolecular annihilation of the triplet excimers leads to the formation of an intramolecular singlet excimer with a characteristic delayed fluorescence spectrum.’.* These molecular systems provide an interesting photophysical case in which the singlet excimer can be produced only by photoassociation in the triplet manifold and subsequent bimolecular annihilations of the triplet excimers. In this paper, we present the temporal characteristics of the transient absorption and delayed fluorescence for the bridged dinaphthyls, which provide compelling kinetic evidence for the formation of intramolecular triplet excimers. Experimental Section The time-resolved transient absorption and delayed fluorescence spectra were measured with the gated diode-array laser spectrometer described previously.’ The fluid solutions of the samples, degassed by continuous bubbling of Nz gas, were excited by an excimer (XeCl) laser and the spectral and temporal characteristics measured at room temperature, as detailed in ref 1. The temporal characteristics of the absorption and emission were determined by plotting the integrated absorbance and the integrated emission intensity as a function of delay times. 1 ,l‘-Dinaphthyl ether (1,l’-DNE) was synthesized according to the procedure of Radionow and M a n ~ o wwhile , ~ 1,l’-dinaphthylmethane ( 1 ,l’-DNM) was prepared by following the procedure previously used for the synthesis of 2,2’-dinaphthylmethane (2,2’DNM).’ The compounds were purified by column chromatography and repeated recrystallization from 95% ethanol. GC/ M S analyses revealed no detectable impurity, which places an upper limit of the possible impurities at much less than 0.1% for the compounds investigated. Baker Photorex grade hexane and cyclohexane were used as received. They exhibited negligible emission and absorption when excited under the conditions employed in these experiments. Results and Discussion Figure 1 shows the time-resolved transient absorption spectra and time-resolved emission spectra of 2,2’-DNM in n-hexane at room temperature. As discussed previously,’ the transient Present address: U S . Army Research Lab, AMSRL-WT-PC, Aberdeen Proving Ground, MD 21005-5066. 1 Present address: Institute of Physics, PAN, AI. Lotnikow 32/46,02-668 Warszawa, Poland. 5 Holder of the Goodyear Chair in Chemistry at The University of Akron.
absorption spectrum at the earliest time delay (1 ps) is composed of a feature in the region of 350-450 nm and a feature in the region of 500-700 nm. The absorption in the 350-450 nm region is composed of a relatively sharp absorption (with intensity maximum a t about 420 nm) and a broad feature at slightly longer wavelengths. The sharp feature is very similar to the triplettriplet absorption spectrum of 2-methylnaphthalene (see the dashed curve at the top), and we assign it to the triplet-triplet absorption spectrum of the noninteracting conformations of the molecule in the triplet state. At longer time delays (>SO ps), only the broad absorption is apparent in the spectral region of 380-460 nm. Since the ratio of the intensity of the broad 380460 nm feature to that of the 500-700 nm absorption remains independent of the time in the late gated spectra (with delay longer than about 50 ps), and since the 500-700 nm feature is absent in the transient absorption spectrum of 2-methylnaphthalene, both of these absorption features have been assigned’ to the transient absorption of the intramolecular triplet excimer of 2,2’-DNM. There is also a corresponding time evolution of the delayed emission spectra. Thus, at short time delay (-300 ns), the spectrum consists of a single emission, which is spectrally identical to the normal, prompt fluorescence (see top right). The intensity of this delayed emission however depends quadratically upon the laser fluence, indicating that the emission is the delayed fluorescence arising from the triplet-triplet annihilation of the monomeric (Le., noninteracting) conformation of the molecule in the triplet state.s With increasing time delay, a new redshifted emission appears superimposed on the delayed monomer (normal) fluorescence, and it becomes the dominant delayed emission at long delay times (>100 ps). Sine the intensity of this new emission is also proportional to the square of the laser intensity, we have assigned it to the delayed excimer fluorescence arising from the bimolecular annihilation of intramolecular triplet excimers.’.-’ As expected, the excitation spectrum of the delayed excimer fluorescence is identical to the absorption spectrum of 2,2’-DNM.6 Figure 2 presents corresponding time-resolved absorption and emission spectra for 2,2’-DNE. The only significant difference from 2,2’-DNM is that the transient absorption of 2,2’-DNE in the region of 380-460 nm appears to be mostly due to noninteracting conformers, as judged by its temporal characteristics (vide infra). The intensity of the delayed excimer fluorescence is again proportional to the square of the laser intensity, as shown in Figure 3. The results of 1 ,l’-DNM and 1.1’-DNE are qualitatively very similar to those of 2.2’-DNM and 2,2’-DNE, and they are presented in Figures 4 and 5 without remarks. The transient absorption and structured delayed fluorescence also appear in acetonitrile, dichloroethane, isooctane, and various alcohols. Figure 6 compares the time dependence of the triplet-triplet
0022-365419312097-3480%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3481
Photoassociation in Bridged Diary1 Compounds
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A
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Figure 1. Time-resolved transient absorption spectra, concentration = 5 X M (left) and time-resolved delayed fluorescencespcctra, concentration = 5 X 10 5 M (right) of 2,2’-DNM in hexane at room temperature. The delay times are as indicated. The transient absorption spectrum of 2-methylnaphthalene in hexane, concentration = 1 X IO-) M, at 100 ps delay is shown for comparison as the dotted line in (left). The delayed fluorescence spectrum of 2-methylnaphthalene in hexane, concentration = 1 X IO-) M, at 100 FS delay is shown for comparison as the dotted line in (right). Laser energy = I mJ; gate width = 5 ps. The spectra have been drawn normalized to the same peak height to facilitate the comparison of
their shape.
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Figure 2. Time-resolved transient absorption spectra, concentration = 5 X 10 M (left) and time-resolveddelayed fluorescencespectra, concentration = 5 X IO M (right) of 2,2’-DNE in hexane at room temperature. The delay times are as indicated. The transient absorption spectrum of 2-methoxynaphthalene in hexane,concentration = I X IO 3 M, at l00asdelay isshown for comparison as thedotted line (left). Thedelayed fluorescence spectrum of 2-methoxynaphthalene in hexane, concentration = 1 X 10 3 M, at 100 ~s delay is shown for comparison as the dotted line (right). Laser energy = I mJ; gate width = 5 ~ s The . spectra have been drawn normalized to the same peak height to facilitate the comparison of their shape.
absorption with that of the delayed fluorescence for 2,2’-DNM in cyclohexane at room temperature. A similar comparison is made for 2,2’-DNE in Figure 7. Some caveats are in order for the interpretation of these data. First, a rise time in the time dependence of the transient absorption of 2,2’-DNM is less noticeable for the spectral region of 380-460 nm than for the 500-700 nm region, due to the overlap of the excimer absorption with the monomer absorption in the shorter wavelength spectra.
Second, the near-identity of the decay rates of the delayed monomer fluorescence and the delayed excimer fluorescence of 2,2’-DNM at longer times (>SO ps) is an artifact caused by the spectral overlap of the monomer emission with the much stronger excimer emission. Similar spectral overlapoccurs in the transient absorption of 2,2’-DNE in the 380-450 nm region and in the delayed fluorescence of 2,2’-DNE in the region of 32s-350 nm. Thus, comparison of the temporal characteristics of the delayed
Modiano et al. Equations 1, 7, 8, and 9 describe the first-order rate processes, eqs 3, 5 , and 6 denote bimolecular triplet-triplet annihilations, and the formation of the intramolecular triplet excimer from the noninteracting triplet molecule (Le., triplet monomer) and the dissociation of the excimer into the monomer are described by eqs 2 and 4, respectively. The populations of the singlet and triplet states are given by
7.00 6.00 h
Y=-9.27+2.08X
g : 3 5.00
R SqUared=O.W2
d
., 3.00
1.00 4 5
I
5.5
8.5
8
7
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In of Iasa Energy Figure 3. Log plot of the intensity of the delayed fluorescence versus the excitation intensity for 2.2’-DNE in hexane, concentration = 5 X 10 M, delay = 200 ps. emission (monomer or excimer) with those of the triplet-triplet absorption of the excimer should be based on the excimer absorption spectrum in the region of 500-700 nm. Reliable deconvolution of the shorter wavelength absorption to the monomer and excimer bands was possible only for 2,2’-DNM. The wavelength region of the absorption and emission, its assignment as monomer or excimer, and their temporal characteristics are summarized in Table I. Inspection of Figures 6 and 7 and the results in Table I lead to the following major conclusions: (1) Both the transient absorption and the delayed fluorescence, due to triplet excimers, exhibit a rise time to a maximum intensity followed by a decay, while those due to the triplet monomers do not exhibit a rise time. (2) The rise time of the delayed excimer fluorescence is very similar to the decay time of the delayed monomer fluorescence. (3) Both the rise and decay times of the delayed excimer fluorescence are approximately half those of the triplet-triplet absorption of the excimer. The above results can be rationalized by using the rate processes represented by the following kinetic scheme:
Equations 14-17 assume that bimolecular decay channel of TI or TI’ can be neglected compared to the unimolecular decay channel of the respective species. For the present systems, this assumption is justified since the decays of TI and TI’ are exponential (vide infra). Assumption 18 invokes that bimolecular annihilation of an excimer triplet and a normal triplet is much more important than back dissociation of excimer triplets into normal triplets. Unimportance of the back dissociation at early times is consistent with the fact that there is no evidence of equilibration between TI and TI‘, as evidenced by their differing decay rates. With assumptions 14-18 and the initial conditions that [TI] = [Tl]oand [Tl’lo = 0, the solution of the differential equations leads to
kj
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-
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h5
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-
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ro
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Here, TI is the noninteracting conformation of the molecule in its triplet state, TI’is the intramolecular triplet excimer, and SI andSl’are the excitedsinglet stateof the noninteracting molecule and that of the interacting conformation, produced by triplettriplet annihilation of monomers and excimers, respectively.
where A I = k l
+ kz and
A2
= A,
+ k7.
Photoassociation in Bridged Diary1 Compounds
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The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3483
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Figure 4. Time-resolved transient absorption spectra, concentration = 5 X lo" M (left) and time-resolved delayed fluorescence spectra, concentration = 5 X lo" M (right) of 1,l-DNM in hexane at room temperature. The delay times are as indicated. The transient absorption spectrum of 1 -methylnaphthalenein hexane, concentration = 1 X lo-' M, at 100 @delay is shown for comparison as the dotted line (left). The delayed fluorescence spectrum of 1-methylnaphthalenein hexane, concentration = 1 X lo-' M, at 100 ps delay is shown for comparison as the dotted line (right). Laser energy = 1 mJ; gate width = 5 1s. The spectra have been drawn normalized to the same peak height to facilitate the comparison of their shape.
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Figure 5. Time-resolved transient absorption spectra, concentration = 5 X l o " M (left) and time-resolved delayed fluorescence spectra, concentration = 5 X l o " M (right) of I,I-DNE in hexane at room temperature. The delay times are as indicated. The transient absorption spectrum of 1 -methoxynaphthalenein hexane, concentration = 1 X IO-' M, at 50 ps delay is shown for comparison as the dotted line (left). The delayed fluorescence spectrum of 1-methoxynaphthalenein hexane, concentration = 1 X lo-' M, at 50 ps delay is shown for comparison as the dotted line (right). Also
shown in the bottom window (right) is the spectrum (dotted line) obtained from the subtraction of the 1 ps spectrum from the 80 ps one. Laser energy = 1 mJ; gate width = 5 ps. The spectra have been drawn normalized to the same peak height to facilitate the comparison of their shape. the excimer absorption (XI). Furthermore, the assumption of XI When XI > k7, eq 20 predicts that the time-dependent triplet > k7 is also borne out by the greater buildup rate (XI) of the concentration [Tl'(t)] shouldexhibit a rise timeof l/X, anddecay time (i.e., mean lifetime) of 1 /k7, whereaseq 19 shows that triplet excimer absorption relative to itsdecay rate (k7). Forthedelayed monomer, [Tl(r)], decays exponentially with mean lifetime of which can arise from normal (or monomer) fluorescence from SI, both the triplet-triplet annihilation of noninteracting molecules ]/XI. Both of these predictions are consistent with the time dependence of the triplet-triplet absorption of 2,2'-DNM and (eq 3) and the triplet-triplet annihilation of one noninteracting 2,2'-DNE, which shows that the decay rate of the monomer molecule with an interacting one (eq S), eq 21 predicts that the absorption (XI) is approximately the same as the buildup rate of decay is of biexponential form when k3(XI - k,) > ak2kS (the
Modiano et al.
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
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a rate constant of 2XI,while the other decays with a rate constant of Xz. This prediction is supported by theexperimental observation that the short-component decay rate (2XI)of the delayed normal fluorescence is nearly 2 times greater than the buildup rate (XI) of the triplet excimer absorption. Finally, for the delayed excimer fluorescence, which is produced by the triplet-triplet annihilation of excimers, as well as by the T I TI' heteroannihilation, eq 22 shows that the emission is composed of three major components. One component is a decay term with a rate (2k7),i.e. twice the decay rateofthe triplet excimer absorption, but theidentity (decay or buildup) of the two other components cannot be discerned without considering the signs of the respective pre-exponential factors. Experimentally, the time dependence of the delayed excimer fluorescence is characterized by a building up to a maximum intensity followed by an exponential decay (see Figures 6 and 7). Since the observed decay rate is approximately 2 times greater than that of the excimer absorption, the first term in eq 22 can be associated with the excimer annihilation by comparison with eq 20. The second term in the equation has the same exponential dependence as the short-component decay term in eq 21, and it could account for the observed rise or buildup in the delayed excimer fluorescence (the rate of which is observed to be very similar to the short-component decay in the delayed monomer fluorescence) if the pre-exponential factor is negative. This requires that (1 - a)(Xl - k7)k5 > kzk6 (the denominator is a large positive number since kd > k7). As ks and kg are the rate constants for the diffusion-controlled bimolecular annihilation process, they are expected to be similar in magnitude. Assuming once again 1 - a 1 (Le., a 0), the condition (1 -.)(XI - k7)k5 > k2k6 therefore requires that XI - k7 > kz,which in turn implies k l > k7 (note that XI = kl k~).Although k l is not known separately in our experiment, the fact that X = kl kz is an order of magnitude greater than k7 (Table I) suggests that kl is greater than k7. Similar arguments lead to a conclusion that the preexponential factor for the third term in the bracket of eq 22 is small compared to the pre-exponential factor for the first term. We may therefore attribute the rise and the decay of the delayed excimer fluorescence to the second and the first term of eq 22, respectively. As noted earlier, the assumption 1 - a z 1 (or a cz 0) isequivalent toassuming that theTl +TI' heteroannihilation (eq 5 ) makes a major contribution to the delayed excimer fluorescence, but not to the delayed normal fluorescence. This is a reasonable assumption since the excited singlet state of the noninteracting molecule (SI),produced by the TI + TI' annihilation, can rapidly change into SI'by SI-SI' energy transfer (i.e., SI SO' SO SI'). This energy transfer is favored by (1) close distance between the annihilation-created SIand So' and (2) a large overlap between the fluorescence band of SIand the absorption band of SO'(ref 3). The fact that the observed buildup rate of the delayed excimer fluorescence is approximately twice that of the excimer absorption and nearly equal to the short-component decay rate of the delayed normal fluorescence is therefore consistent with the predictions of eqs 19-22. The results also confirm that it is the intramolecular singlet excimers, and not intermolecular singlet excimers, which give rise to the delayed excimer fluorescence. Intermolecular singlet excimers formed by bimolecular annihilation of triplet monomers would decay exponentially (with no rise time) with a rate which is identical to that of the delayed monomer fluorescence and twice the decay rate for the monomer triplet. The conclusion that the delayed excimer fluorescence originates from the intramolecular singlet excimers (formed by bimolecular annihilation of intramolecular triplet excimers) is also supported by the fact that the structured delayed excimer fluorescence is spectrally very different from the broad, structureless excimer fluorescence observed from naphthalene and related compounds in fluid solutions. We believe that the inability of the triplet-triplet annihilation of the noninteracting conformations of D N M and D N E to produce
+
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Figure 7. Temporal characteristics of the transient absorption (a) and delayed fluorescence (b) for 2,2'-DNE in cyclohexane at room temperature. Concentration = 5 X IO a M,laser energy = I mJ. Regions of integration as indicated.
third term with decay rate of ko is too fast to be pertinent to our temporal studies in the microsecond time scale). This inequality implies that the triplet-triplet annihilation of the noninteracting molecule with excimer is relatively unimportant in producing the delayed normal fluorescence (Le., a 0). Under this condition, one component of the delayed normal fluorescence decays with
=
+
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= +
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Photoassociation in Bridged Diary1 Compounds
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3485
TABLE I: Temporal Characteristics of Transient Absorption and Delayed Fluorescence for 2,2'-DNM and 2,2'-DNE, As Monitored at the Specified Wavelength Region' transient absorption
species 2,2'-DNM
2,2'-DNE
monomer (380-415 nm)h no rise 100 ps decayd largely monomer (380-450 nm)(' no rise -40 ps decay
-
excimer (420-460 1 1 ps rise 120 ps decay
delayed fluorescence excimer (500-700 nm) 11 ps rise I20 ps decay excimer (500-700 nm) 20 ps rise 55 ps decay
-
monomer (320-350 nm)" -7 ps decay 50 ps decay largely monomer (320-350 nm). no rise 20 ps decay
-
-
excimer (350-450 nm) 6 ps rise 55 ps decay excimer (350-450 nm) 11 ps rise 30 ps decay
Where possible, assignments of bands to monomer and excimer are indicated. See captions of Figures 6 and 7 for experimental conditions. Results of deconvolution. Fit to a biexponential decay function. See the remarks in the text. N o reliable deconvolution of the monomer and excimer bands was possible due to the strong spectral overlap.
intermolecular singlet excimers is a consequence of the steric effects posed by the second chromophore. Presumably, the additional naphthalene chromophore in D N M or D N E hampers the attainment of the sandwich configuration of two naphthalene moieties which is required for the formation of the intermolecular singlet excimers. In summary, the observed temporal characteristics of the transient absorption and the delayed fluorescence, attributed to the triplet excimers, are consistent with the general kinetic scheme expected of the molecular systems undergoing formation and decay of intramolecular triplet excimers. The results therefore strongly support the formation of intramolecular triplet excimers in the dinaphthylmethanes and dinaphthyl ethers. Observation of similar spectral and temporal characteristics of transient absorption and delayed emission in other related molecular systems suggests that the formation of intramolecular triplet excimers
may be a general phenomenon for bichromophoric compounds linked by a single bridging group.'
Acknowledgment. This work was supported by Division of Chemical Sciences, Office of Energy Research, US.Department of Energy under Grant DE-FG02-89ER14024. References and Notes ( I ) Modiano, S. H.; Dresner, J.; Lim, E. C. J . Phys. Chem. 1991, 95, 9144. (2) Cai, J.; Lim, E. C. J . Phys. Chem. 1992, 96, 2935. (3) Modiano, S.H.; Dresner, J.; Lim, E. C. Chem. Phys. Lett. 1992,189, 144. (4) Radionow, W. M.; Manzow, S.J. SOC.Chem. Ind., London (Frans.) 1923, 42, 509. (5) See, for example, Birks, J. B. Phorophysics of Aromatic Molecules; Wiley-lnterscience: London, 1970; pp 372-402. ( 6 ) Cai, J.; Lim, E. C. Unpublished results. (7) Cai, J.; Lim, E. C. In preparation for publication.
