J . Phys. Chem. 1990, 94, 142-147
142
Trlplet-Trlplet Fluorescence and Spin Polarization. Diphenylcarbene and Dibenzocy cloheptadienylidene K. W. Haider, Matthew S. Platz,* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
Alain Despres, Violaine Lejeune, and Eva Migirdicyan* Laboratoire de Photophysique Moleculaire du CNRS, Universite de Paris-Sud, Orsay Cedex, France (Received: March 13, 1989)
The laser-induced fluorescencespectrum of diphenylcarbene (DPC) obtained from four independent precursors is blue-shifted in low-temperature glasses relative to liquid solution. The spectrum of the less conformationally mobile ethano-bridged carbene dibenzocycloheptadienylidene(DBC) is virtually the same in solution and in glasses. The fluorescence spectra of 'DPC* and 3DBC* in Shpol'skii matrices have been recorded between 4.2 and 77 K. While DBC exhibits only one well-resolved spectrum, DPC presents a sharp spectrum, plus a broad blue-shifted one. The fluorescence decays of 'DPC* and 'DBC* measured on the sharp spectra were found to be nonexponential and influenced by the presence of a weak external magnetic field in a manner similar to that of the m-xylylene biradicals. The fluorescence decay of 3DPC*recorded on the broad spectrum is exponential and remains unchanged under this field.
Introduction
0
N7
It has long been a source of concern that the geometry of triplet diphenylcarbene ('DPC) in low-temperature solids differs from its geometry in liquid solution because of the constraints imposed by the rigid environment.' The few reports concerning the geometry of DPC are based on electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy of 3DPC substitutionally incorporated in benzophenone or 1 , l -diphenylethylene single crystals at 2 and 77 K.* Recently, there has been a renaissance in the study of triplet carbenes by fluorescence spectroscopy. Scaiano and Weir have found that the liquid-phase fluorescence of excited 'DPC is redshifted relative to the value observed in matrices. They concluded that the geometry of 'DPC in either the ground or excited state differs with environment, the species in the rigid matrix being unable to relax to its preferred liquid-phase structure.' In this paper, we confirm the ScaianwWeir results and present further data supporting their interpretation by studying a rigid carbene, dibenzocycloheptadienylidene('DBC). Information on the spin multiplicity of the carbene emitting level is provided by fluorescence decay measurements. The results show that the decays of 'DPC and 'DBC trapped in Shpol'skii matrices observed on sharp lines are nonexponential, as in the case of triplet-triplet fluorescence of m-xylylene b i r a d i ~ a l s . ~ Experimental Section
Laser-Induced Fluorescence (Columbus). Carbene precursors 3 and 4 were a generous gift from Professor G. W. Griffin.5 Compounds 1 and 5 were prepared as described elsewhere.6 Dichlorodiphenylmethane and 3-methylpentane were purchased from Aldrich Chemical Co. and used without further purification. ( I ) (a) Nazran, A. S.; Lee, F. L.; Gabe, F. J.; Lepage, Y.; Northcott, D. J.; Park, J. M.; Griller, D. J. Phys. Chem. 1984, 88, 5251. (b) Nazran, A .
S.; Gabe, E. J.; Lepage, Y.; Northcott, D. J.; Park, J. M.; Griller, D. J. Am. Chem. Sac. 1983, 105, 2912. (2) (a) Brandon, R. W.; Closs, G. L.; Davoust, C. E.; Hutchison, C. A,, Jr.; Kohler, B. E.; Silbey, R. J . Chem. Phys. 1965,43, 2006. (b) Anderson, R. J. M.; Kohler, B. E. J . Chem. Phys. 1976, 65, 2451. (3) (a) Turro, N. J.; Aikawa, M.;Butcher, J. A., Jr.; Griffin, G. W. J . Am. Chem. Sac. 1980, 102, 5128. (b) Scaiano, J. C.; Weir, D. Chem. Phys. Left. 1987, 141, 503. (C) Scaiano, J. C.; Johnston, L. J.; McGimpsey, W. G.; Weir, D. Ace. Chem. Res. 1988, 21, 22. (d) Scaiano, J. C.; Weir, D. Can. J. Chem. 1988, 66, 49 I . (4) (a) Lejeune, V.: Despres, A.; Fourmann, B.; Benoist d'Azy, 0.;Migirdicyan, E. J . Phys. Chem. 1987, 91, 6620. (b) Despres, A,; Lejeune, V.; Migirdicyan, E. J . Phys. Chem. 1988, 92, 6914. (5) (a) Griffin, G . W. Carbenes;Jones, M., Jr., Moss, R. A,, Eds.; Wiley: New York, 1973; Vol. I, pp. 305-349. (b) Wasacz, J. P.; Joullie, M. M.; Fuss, 1.; Griffin, G. W. J. Org.Chem. 1976, 41, 512. (6) Jones, W. M.i Joines, R. C.; Myers, J. A,; Mitsuhashi, T.;Krajca, K. E.; Waali. E. E.: Davis, T.L.; Turner. A . B. J. Am. Chem. Sac. 1973, 95, 826.
0022-3654/90/2094-0142$02.50/0
3
2
1
4
5
Solutions of the precursors in 3-methylpentane having an optical density of 1 .O at the laser line (249 nm) were used to obtain all of the laser-induced fluorescence (LIF) spectra. Ambient temperature fluorescence spectra were recorded by decomposing the precursors and exciting the nascent carbene with a single pulse of a Lumonics Model TE-861M-4 excimer laser (KrF line; 249 nm; 80 mJ/pulse; 10 ns pulse width). The LIF from the sample was directed onto an Allied Analytical systems spectrograph through a fiber-optics cable. The dispersed fluorescence spectrum was collected via a PARC Model 1460 optical multichannel analyzer (OMA). The OMA was activated 5 ns following the laser pulse, for a 100-ns time window. LIF at 77 K was recorded similarly by immersing the sample cell in liquid N, prior to inserting into the precooled sample compartment. Alternatively, fluorescence spectra at 77 K were recorded with a Perkin-Elmer Model LS-5 spectrofluorometer. In this case, precursors were decomposed to DPC using a Rayonet photoreactor ( 5 RPR-2540 bulbs). Fluorescence spectra of DPC obtained in this manner showed excellent agreement with those recorded in the LIF experiment at 77 K. In the case of precursor 4, the LIF experiment produced no signal for DPC, but the desired fluorescence spectrum was easily obtained at 77 K (0.01 M 2 in 2-methyltetrahydrofuran) using the conventional apparatus described above. Fluorescence Spectra and Decays of Carbenes in Crystals (France). Diphenylcarbene ('DPC) and dibenzocycloheptadienylidene ('DBC) were generated "in situ" by photolysis of the corresponding diazo compounds dispersed in n-hexane and n-heptane (Merck Uvasol) at 15-30 K with 315-nm radiation (isolated from a 150-W Osram high-pressure xenon lamp through a silica prism Jobin-Yvon monochromator). Crystals of benzophenone (Janssen) containing diphenyldiazomethane were grown from solution in methanol (Merck Uvasol). The photolysis of these mixed crystals at 17 K with the 337-nm radiation of a nitrogen
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 143
T-T Fluorescence and Spin Polarization
TABLE I: Matrix and Liquid Solution Fluorescence Maxima (A,,,.) of Diphenylcarbene in 3-Methylpentane
505nm
&la,,
precursor
glass, 77 K
1
482 476 478 482
2 3 4
nm
solution. 298 K 505 a
504 505
"Not observed. I
A
9
I
..
,
I
I
483nm
I I
3 00
400
500 X(nm)
600
472.3nm I
Figure 1. Ambient temperature liquid phase and 77 K glassy matrix fluorescence spectra of 'DPC generated from 1.
laser produces jDPC. Two "Air Liquide" cryostats (A) or (B), which use either helium gas (A) or liquid helium (B) as coolant, allow studies of slowly (A) or fast (B) cooled samples. Fluorescence spectra were analyzed with a T H R 1500 JobinYvon spectrometer. Fluorescence decays were measured by using equipment and techniques as previously d e ~ c r i b e d .Coumarin ~ 480 dye from Exciton Co. was used in the dye laser to obtain 466-nm excitation radiation. The pulse width at half-maximum was 6 ns. In our experimental device, the applied magnetic field is limited to 1000 G in cryostat A and to 220 G in cryostat B. The fluorescence decays of the carbenes trapped either in nalkanes or in benzophenone crystals were excited with laser bandwidths of 4-6 cm-l. The decays were measured at selected spectrometer wavelengths with narrow but variable spectral widths. Results I . Fluorescence Spectra. A . Diphenylcarbene (3DPC). The previously reported TI To fluorescence spectra of 3DPC were obtained under very different experimental conditions. Trozzolo and Gibbons' and subsequent investigatorss-" irradiated diphenyldiazomethane 1 in rigid media at 77 K and obtained the fluorescence spectrum of the photogenerated 3DPC in a spectrofluorometer using a scanning monochromator. Turro et al. and Scaiano and Weir obtained their fluorescence spectrum of jDPC very d i f f e r e n t l ~ .They ~ exposed solutions of 1 to a short intense pulse from an excimer laser (308 nm). The laser pulse was sufficiently intense both to decompose the diazo precursor and to excite the nascent 3DPC. The fluorescence of 3DPC* could also be obtained by excitation of the carbene with a second laser operating at 337.1 nm which was pulsed shortly after the first one. The resulting emission was dispersed with a spectrograph and collected with an optical multichannel analyzer (OMA). Because fluorescence spectra can be influenced by details of instrument configuration and sample, we decided to obtain the solution and glassy matrix fluorescence spectrum of 3DPC* using the same experimental apparatus at both temperatures. A solution of 1 in 3-methylpentane at 298 K was decomposed by a 249-nm KrF laser pulse and the resulting emission obtained
-
(7) (a) Trozzolo, A. M.; Gibbons, W. A. J . Am. Chem. SOC.1%7,89,239. (b) Moritani, 1.; Murahashi, S.;Nishino, M.; Kimura, K.; Tsubomura, H. Tetrahedron Letf. 1966, 373. (8) Haider, K.; Platz, M. S.J . Phys. Org. Chem., in press. (9) Anderson, R. J. M.;Kohler, B. E.; Stevenson, J. M. J . Chem. Phys. 1979, 71, 1559.
(IO) (a) Ono, Y . ;Ware, W. R. J . Phys. Chem. 1983,87,4426. (b) Ware, W. R.; Sullivan, P. J. J . Chem. Phys. 1968, 49, 1445. ( 1 I ) (a) Graham, D. J.; Wohler, K. W. Chem. Phys. Lett. 1985,116,497. (b) Graham, D. J.; Chia-Ling Wang J . Chem. Phys. 1986,85, 4441.
I
I
470
480
1
X(nm)
Figure 2. Fluorescence spectra of 'DPC in benzophenone at 17 K (A) and in n-heptane (B) at 14 K (curve I), 77 K (curve II), and 10 K (curve 111). The spectra were excited with 315-nm radiation.
in a manner similar to that described by Scaiano and Weirs3 A fresh sample of the same stock solution of 1 was then cooled to 77 K and the fluorescence spectrum of the carbene was obtained with the same apparatus used at ambient temperature. As shown in Figure 1 the spectra of jDPC generated in liquid solution and in a low-temperature glass are quite different even when measured with identical equipment. The fluorescence origins (Amax) peak at 482 and 505 nm respectively at 77 and 298 K. Similar results were obtained with two independent precursors (2 and 3) of jDPC. Although 4 can be photolyzed to produce jDPC at 77 K by prolonged exposure to 254-nm radiation,8 fluid solution laserinduced fluorescence (LIF) could not be detected with this precursor upon excitation with a single pulse of 249-nm light at 298 K. Regardless of the precursor, the A,, observed for jDPC in the matrix was at substantially shorter wavelength than that observed in the liquid solution (Table I) and this shift is not dependent on the details of the experiment. The data suggest that the geometry in the matrix of either ground-state jDPC or electronically excited jDPC* or both species is distorted from its geometry in liquid phase (vide infra). Fluorescence Spectra in Polycrystalline Matrices. Fluorescence spectra of 3DPC generated by photolysis of 1 with 315-nm radiation in polycrystalline matrices are displayed in Figure 2. In a benzophenone matrix at 17 K, the spectrum shown in Figure 2A exhibits a broad band (half width AD = 300 cm-') peaking around 475 nm and two sharp lines at 471.2 and 472.3 nm. This structure is similar to that observed by Anderson, Kohler, and Stevenson9 who have attributed the broad band to a phonon side band, since it is displaced to the red in emission and to the blue in absorption with respect to the sharp zero-phonon line. By use of narrow-band laser excitation, these authors have shown that the broad sidebands are not due to inhomogeneous broadening. In agreement with results from EPR and ENDOR experiments,2b the spectra are consistent with )DPC occupying substitutional sites
144
The Journal of Physical Chemistry, Vol. 94, No. I , 1990
of the benzophenone host crystal, the observed structure being due to the strong excitation-phonon coupling. Such a strong coupling can be explained by geometrical changes induced upon excitation of the nonrigid D P C 9 I n a n-heptane Shpol'skii matrix from 14 to 77 K, the fluorescence spectra of jDPC present two maxima at 483 and 503.7 nm, whose relative intensities and shapes depend on the freezing rate of the sample, the excitation wavelength, and the concentration of 1. All the spectra presented in Figure 2B have been excited with 315-nm radiation. Curve I corresponds to a sample frozen within 1 h from 298 to 14 K; curves I1 and 111 correspond to samples quickly frozen to 77 and 10 K, respectively. The rapid cooling is expected to isolate the precursor and consequently DPC in substitutional sites of the matrix. Despite the use of a Shpol'skii matrix, the band at 483 nm remains broad (AF z 700 cm-l) even at 14 K. The band at 503.7 nm is sharper with a half width of AF r 65 cm-I at 10 K and AS r 175 cm-I at 77 K. The position of this sharp band is very similar to that observed for DPC in liquid solution and may therefore correspond to a matrix site containing a relaxed DPC. The relative intensity of the two bands changes significantly under excitation with 315and 320-nm radiation, indicating that the bands at 483 and 503.7 nm are the fluorescence origins of two distinct species which could be )DPC constrained in two different geometries. In addition, the relative intensity of these two bands changes drastically by varying the concentration c of precursor 1 over 4 orders of magnitude. This concentration effect has been studied at 77 K with fast-cooled samples. At the lowest concentration c z M, only the broad spectrum similar to curve I in Figure 2 is detected. At the highest concentration c r M, the observed spectrum corresponds to the sharp fluorescence with an origin at 503.7 nm. At intermediate concentrations, the emission is a mixture of broad and sharp spectra, the relative intensity of the sharp fluorescence increasing with concentration. The results are interpreted by triplet-triplet energy transfer between the two species, the efficiency of which increases with concentration. Finally, we have found that the effect of a 220-G magnetic field is different on the sharp and on the broad bands; while the intensity of the latter remains constant, the field decreases by 30% the intensity of the sharp band. This field effect is related to that observed on the fluorescence decays. The fluorescence originating at 503.7 nm has been analyzed by using the vibrational modes and ground-state frequencies of benzenic compounds. Thus, the vibronic bands at 585,620,985, and 1570 cm-I have been assigned respectively to modes 6a, 6b, breathing mode 1, and mode 8a/8b of the Cb benzylic unit. This fluorescence cannot be attributed to the diphenylmethyl radical since we have observed that this radical emits at 522 nm in a n-heptane matrix at low temperature. If the sharp and broad fluorescence spectra are assumed to be due to respectively relaxed and nonrelaxed DPC, then the relative intensity of the two spectra is expected to change upon annealing the system, since this treatment will modify the proportion of the two species. The most spectacular results, presented in Figure 3, were obtained in the n-hexane Shpol'skii matrix where the annealing treatment was carried out at about 90 K overnight. Curve A corresponds to a concentrated sample (c z M) quickly frozen to 7 K. The observed fluorescence is again composed of a broad band at 483 nm and a sharp origin pointing at 502.8 nm followed by its vibronic bands. On curve B is presented the fluorescence measured at 15 K after the annealing treatment at 90 K. Comparison of curves A and B shows clearly that the broad band has vanished. Nevertheless, when the sample is cooled down to 5 K, a broad band appears again as shown in curve C. The thermal BCB cycle can be repeated several times without noticeable change. One possible interpretation of the data is the attribution of the broad fluorescence to )DPC in a distorted geometry to either the ground or the first excited state or in both states of the carbene. A continuum starting at 400 nm and presenting a maximum around 470 rim is superposed on the fluorescence spectra of DPC in curves A . B, and C of Figure 3. This continuum can be at-
Haider et al.
4b0
450
500 X(nm)
550
Figure 3. Fluorescence spectra of 3DPC in n-hexane at 5-15 K before (A) and after (B and C) the annealing treatment. The spectra were excited with 315-nm radiation. I
300
400
500
6 00
X(nm)
Figure 4. Ambient temperature liquid solution and 77 K glassy matrix fluorescence spectra of 'DBC generated from 5.
tributed to the dimer of DPC, the tetraphenylethylene, as suggested by Trozzolo and Gibbons.'" B. Dibenzocycloheptadienylidene (3DBC). If the environmentally induced shift of the fluorescence maxima of 3DPC* is due to a matrix-related change in carbene geometry, then this shift should diminish when a rigid carbene is studied. Laser-induced fluorescence spectra of )DBC (in 3-methylpentane) were obtained as per 3DPC a t 298 and 77 K. As shown in Figure 4 the A,, is nearly identical in the glass and in liquid solution. The matrix-induced shift in the fluorescence maximum of )DBC is only a fraction of that observed with )DPC, consistent with the idea that the matrix shift in the latter carbene is related to conformational changes. Our results on the LIF of )DBC at ambient temperature agree with those of Scaiano and Weir.) Fluorescence Spectra in Shpol'skii Matrices. The fluorescence spectra of 3DBC generated from 5 by photolysis with 315-nm radiation were obtained in frozen n-hexane and n-heptane matrices. Benzophenone was not used as a matrix for DBC since precursor 5 could not be dispersed in this crystal using the same procedure as for precursor 1. The quasi-line fluorescence spectrum of )DBC in n-hexane at 12 K obtained with a fast-cooled sample is displayed in Figure 5. This spectrum exhibits a sharp zero-phonon line ( A j zz 6 cm-I) at 502.9 nm accompanied by a structured phonon sideband. A
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 145
T-T Fluorescence and Spin Polarization
I
0.0
t IB
502.9 nm
T=12 K
I
\71cm-'
6 2 5 c m-l
I
J
Figure 5. Fluorescence spectrum of 'DBC in n-hexane at 12 K excited with 3 15-nm radiation.
100
0
200
300
t(ns)
"'I A
Figure 7. Fluorescence decays of )DPC in n-hexane at 30 K measured on the sharp fluorescence at 502.8 nm in the absence (A) and in the presence (B) of a 2 2 0 4 magnetic field. (C) Fluorescence decay of a dye measured at 502.8 nm at room temperature. Time resolution was 1 ns.
c
T=2 0 K
t(ns1 Figure 6. Fluorescence decays of 'DPC in n-hexane at 30 K measured on the broad fluorescence at 483 nm in the absence (A) and in the presence (B) of a 2 2 0 4 magnetic field. Time resolution was 1 ns.
2
u 200 400
0
600
800
t(ns) sharp line at 71 cm-l from the origin is superposed on the phonon sideband. Such a low frequency could likely be attributed to a hindered motion involving the ethano bridge of the seven-membered ring. The spectrum includes also vibronic bands at 625, 699 (625 74), 1042, 1172, and 1583 cm-l. Except for the last one which is attributed to the C=C-C stretching mode, these frequencies are higher than those observed in DPC, as expected for a stiffer aromatic unit. A full analysis of this spectrum will be given elsewhere. In an n-heptane matrix, the fluorescence spectrum of jDBC at 10-20 K is sharp (Av 16 cm-') and multisite for a fast cooled sample and broader with an origin at 506 nm for a slowly cooled sample. The latter spectrum is the envelope of the sharp multisite spectrum. In contrast with the above-described results obtained for DPC in Shpol'skii matrices, noteworthy is the absence of a blue-shifted broad band in the fluorescence spectrum of this rigid carbene. In the presence of a 220-G magnetic field, the fluorescence intensity of jDBC in n-alkanes decreases by about 20%. 11. Fluorescence Decays. The fluorescence decays of jDPC* and jDBC* in Shpol'skii matrices and of 3DPC in benzophenone were measured at 10-30 K in the absence and in the presence of a magnetic field. When trapped in n-alkane matrices, the carbenes were excited with 337-nm nitrogen laser radiation. Since benzophenone absorbs at 337 nm, jDPC in this crystal was directly excited by using dye laser radiation at 466 nm. Under these conditions, the sensitization of 3DPC fluorescence by benzophenone excitons was avoided. A. Diphenylcarbene (jDPC). The fluorescence decays of jDPC in n-hexane at 30 K measured on the broad band at 483 nm in the absence (curve A) and in the presence (curve B) of a 22043 magnetic field are displayed in Figure 6. Both curves can be fitted by a sum of two exponential functions with component lifetimes
+
Figure 8. Fluorescence decays of 'DPC in benzophenone at 20 K in the absence (A) and in the presence of a 1000-G magnetic field (B). The decays are excited with a laser line at 466 nm and measured on the sharp zero-phonon line at 472.3 nm. Time resolution was 10 ns.
of 10 and 135 f 5 ns. Among the two components, only the second one is attributed to DPC since the decays measured on the continuum at 465 nm indicate that the IO-ns component is due to tetraphenylethylene. Comparison of curves A and B show that the 220-G magnetic field does not influence the decay of jDPC when measured on the broad fluorescence. The fluorescence decays of jDPC in n-hexane at 30 K measured on the sharp origin at 502.8 nm in the absence (curve A) and in the presence (curve B) of a 22043 magnetic field are presented in Figure 7. The contribution of the continuum due to tetraphenylethylene, measured at 509 nm, has been subtracted from both decays. Curve A is nonexponential and significantly modified by the 22043 magnetic field. The decay consists, for the most part, of a component of about 20 ns in curve A, which decreases to about 13 ns in curve B. The decrease of this component lifetime, observed in the presence of the field, suggests the existence of a shorter-lived component that cannot be detected in our system. Curve C is the decay of a dye measured at 502.8 nm at room temperature. This curve gives an estimation of the time resolution of our experimental device. The fluorescence decays of 3DPC in benzophenone at 20 K measured on the sharp zero-phonon line at 472.3 nm in the absence (curve A) and in the presence (Curve B) of a 1000-G magnetic field are depicted in Figure 8. Here again, the decay is nonexponential and significantly altered by the magnetic field. Curve A can be analyzed as a sum of two exponential functions with component lifetimes of about 50 f 5 and 145 f 15 ns. The lifetime of the slow component here is very close to the lifetime
146 Ln
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 t
-
I
0
T=POK
C " ,
I
4 00
200
600
t(ns) Figure 9. Fluorescence decays of 3DBC in n-heptane at 20 K in the absence (A) and in the presence of 200-G (B),400-G (C), 750-G (D), and 1000-G (E) magnetic fields. The decays are excited with a laser line at 337 nm and measured on the fluorescence origin band at 506 nm. Time resolution was 10 ns.
of 135 ns measured on the broad fluorescence at 483 nm for 3DPC in n-hexane at 30 K . The fluorescence decays of 3DPC have already been measured by Ware et al.IOin organic glasses at 77 K, by Graham et al.l] in benzophenone at 10-20 K, and by Scaiano et aL3 in liquid solution at room temperature. Ono and WareIoa mention that the decay curve of 3DPC in methylcyclohexane at 77 K consists, for the most part, of only a single exponential with a lifetime of 123 ns. This value is in good agreement with the lifetime of 135 ns measured here at 483 nm on the broad fluorescence. However, in their experimental conditions, these authors have not detected the red-shifted sharp fluorescence and its nonexponential decay influenced by the magnetic field. I n their study of DPC in benzophenone, Graham et al." have populated the excited triplet state of DPC either by direct means or by sensitization through benzophenone triplet excitons. Under direct excitation conditions, these authors have found that the fluorescence decay of 3DPC in benzophenone was exponential with a lifetime of 140 ns when measured at 25 K and of 320-360 ns when measured at I O K. In addition, they have indicated that the decay curve obtained at 10 K was not influenced by the application of a magnetic field. These results are in contradiction with ours observed in benzophenone under the same excitation conditions. However, the lifetime of 140 ns that they have determined at 25 K is very close to the long component lifetime of 145 ns that we observe at 20 K. The results presented here are the first report of the nonexponential character of 3DPC fluorescence decay at 20 K, under direct excitation conditions. B. Dibenzocycloheptadienylidene (3DBC). The fluorescence decays of 3DBC in n-heptane at 20 K measured on the origin band at 506 nm for a slowly cooled sample in the absence (curve A) and in the presence of a magnetic field varying from 0 to 1000 G (curves B, C, D, and E) are displayed in Figure 9. On this semilogarithmic scale, it is clear that the decays are nonexponential. Curve A can be analyzed as the sum of two exponential functions with component lifetimes of 50 f 15 and 130 f 15 ns. Upon application of the magnetic field, the fluorescence decay is significantly altered even for a field as weak as 200 G (see curve B). The fluorescence decay of 3DBC in n-hexane at 4.2 K measured on the sharp origin at 502.9 nm for a fast-cooled sample is nonexponential with component lifetimes similar to those obtained in the n-heptane matrix, within the accuracy of our experiments. In the presence of a 220-G magnetic field, the decay is significantly modified, as in curve B Figure 9. Discussion I. Fluorescence Spectra. The most reasonable interpretation of the phase-dependent shift in the fluorescence spectra is that
Haider et al. the rigid environment of the matrix prevents either ground-state 3DPC or excited-state 3DPC* from relaxing to its most stable geometry, which is presumably obtained in liquid solution. The exceptionally broad phonon sidebands observed by Anderson et aL9 in the absorption and fluorescence spectra of 3DPC in a benzophenone single crystal at 2 K have been attributed to a strong excitation-phonon coupling. This is in turn due to geometrical changes in the carbene induced upon photoexcitation. It is conceivable that the initial geometry of the carbene precursor predetermines the geometry of the ground state of the matrix-isolated species. One might imagine that the photogenerated 3DPC might be forced to adopt a geometry similar to that of the precursor due to constraints imposed by the rigid environment. Thus 3DPC may be generated in the matrix with an interplanar angle between the two phenyl rings, and a central phenyl-C-phenyl bond angle with values similar to those of the precursor. This is certainly true in the case of dimesitylcarbene because the EPR parameters of this carbene change dramatically upon annealing and indicate that this carbene is not formed in a linear geometry at 77 K, but becomes more nearly linear when the matrix is softened.' Four independent precursors to 3DPC give very similar matrix fluorescence spectra, all of which differ markedly from their liquid solution spectra. It does not seem plausible that all four precursors would predispose the geometry of nascent 3DPC in the same manner. Thus it would appear that it is the shorter lived excited state of 3DPC* which does not have sufficient time to relax to its equilibrium geometry prior to fluorescence. This interpretation requires that the geometries of 3DPC and 3DPC* be substantially different. A nonrelaxed geometry of 3DPC* explains the direction of the shift in the fluorescence maxima. Raising the energy of 3DPC* by geometric distortion will of course increase the 3DPC*/3DPC energy gap and produce the observed blue shift in the matrix. Distorting the geometry of ground-state 3DPC by matrix packing forces without (hypothetically) distorting that of )DPC* would lead to a red shift, opposite to that which is observed. It is possible that the matrix has distorted the geometry of ground-state 3DPC, but such distortion must be less severe than that present in the excited state, 3DPC*. In the n-alkane matrices, the sharp fluorescence origins of 3DPC values that are very close to those observed and 3DBC have A, in liquid solution. This suggests that the carbenes are particularly well-fitted and relaxed in these matrix sites. In addition, 3DPC in Shpol'skii matrices exhibits a broad fluorescence which is blue-shifted with respect to the sharp one. The ,A, of this broad fluorescence is very close to that observed in the glass at 77 K for DPC generated from the same precursor (see Table I). This broad spectrum could possibly correspond to 3DPC constrained in one or several matrix sites where its geometry is distorted either in the ground and/or in the excited states. Such a broad spectrum has not been detected for the rigid )DBC in n-alkane matrices. I I . Fluorescence Decays. The fluorescence decays of 3DPC* and 'DBC* measured on the sharp spectra are nonexponential and influenced by the presence of a weak magnetic field. In contrast the fluorescence decay of 3DPC* recorded on the broad spectrum is exponential and remains unchanged under this field. EPR studies of 3DPC'2 and 3DBC'3 trapped in crystalline solutions at low temperatures provide strong evidence that the ground states of both carbenes are triplets. The analysis of the nonexponential decays reported here indicates component lifetimes ranging around 20-50 and 130-145 ns for 3DPC* as well as for 3DBC*. The corresponding transitions are therefore spin-allowed, and the emitting state of both carbenes is a triplet. Nonexponential decays have already been found for the triplet-triplet fluorescence of another class of biradicals: m-xylylene and its methylated derivative^.^ In the latter species, the decays are biexponential and are drastically modified in the presence of (12) (a) Murray, R. W.; Trozzolo, A. M.; Wasserman, E.; Yager, W. A. J . A m . Chem. SOC.1962, 84, 3213. (b) Brandon, R. W.; Closs, G. L.; Hutchison, C. A,, Jr. J . Chem. Phys. 1962, 37, 1878. (13) Moritani, I.; Murahashi, S . I.; Nishino, M.; Yamamoto, Y . :Itoh, K.; Mataga. N. J . A m . Chem. SOC.1967, 89, 1259.
T-T Fluorescence and Spin Polarization
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 147
a magnetic field. These results were attributed to the fluorescence from individual sublevels of the lowest excited triplet level which is the emitting state of m-xylylene biradicals. This requires that the fluorescence decay from the triplet sublevels be faster than the spin-lattice relaxation between the different sublevels. This condition is probably satisfied for the 30-1200 ns fluorescence of m-xylylene biradicals. Such a model involving spin polarization can also be used to explain the decay dynamics in the triplet-triplet fluorescence of 3DPC and 3DBC, measured under direct excitation conditions. The ground triplet state of these carbenes are split into T ~ rY, , and T~ components whose zero field splitting (ZFS) parameters are ID/hcl = 0.4055 cm-I and lE/hcl = 0.019 cm-' for 'DPC and ID/hcl = 0.3932 cm-I and IE/hcl = 0.017 cm-l for 'DBC. These large ID/hcl values result from spinspin interaction between the two unpaired electrons which are largely localized on the divalent , 7, carbon atom. The ZFS parameters between the T ~ T, ~ and sublevels have not been determined in the first excited triplet state TI. Theory predicts that only sx(To) 7,(T1!, Ty(To) T,,(T,), and 7,(TO) 7,(T1) transitions are electronically allowed and the corresponding transition moments are equaL4 In the absence of spin-orbit interaction and nonradiative decay, the fluorescence emitted from the T, state of these carbenes is expected to be exponential. By analogy with the m-xylylene model, the nonexponential decays measured on the sharp spectra of 3DPC and 3DBC would be consistent with fluorescence from independent TI sublevels at a rate faster than the rate of equilibration by spin-lattice relaxation. Furthermore, the decay must occur with distinct rates for each sublevel populated in the To T1 excitation. It is reasonable to assume that the first condition is satisfied for the spin-allowed TI To fluorescence of carbenes as per the m-xylylene biradicals, since the measured decay rates are on average relatively fast in the carbenes. In fact, the long component lifetimes observed in carbenes are shorter than those observed in m-xylylene biradicals. This interpretation involving spin-polarization applies only if the TI sublevels decay with different rates. However, as pointed out above, these rates should be equal in the absence of spin-orbit coupling (SOC). According to Salem and Rowland,14SOC should be substantial in carbenes for two reasons: the two singly occupied atomic orbitals are perpendicular in the triplet state and the average distance between the two unpaired electrons, mostly localized on the divalent carbon atom, is small. SOC mixes singlet and triplet levels having the same symmetry. Spectroscopic and theoretical studies15 performed on methylene, the prototype of all carbenes, indicate that there are a number of low-lying singlet states between the To and TI states. In the case of 'DPC, a singlet state lying about 1400 cm-I above the ground triplet has been reported by Turro and Eisenthal16 using a combination of chemical trapping and kinetic data. The mixing of singlet and triplet sublevels will destroy the equality of the three transition moments between the sublevels of To and TI states,
leading to bi- or triexponential radiative decay from TI sublevels. Alternatively, the TI sublevels can decay with different rates as a result of intersystem crossing induced by SOC. The efficiency of such a process involving a given triplet sublevel and a given singlet is predicted by symmetry rules. This radiationless transition in competition with the radiative decay could be responsible for the selective depopulation of one or two triplet sublevels out of the three. The consistency of this model with the nonexponential decay of carbenes is supported by the drastic modification of the decay curves in the presence of a magnetic field. Upon application of the field, the wave functions describing the T, sublevels are mixed. The depopulation rate of each mixed sublevel becomes a linear combination of the three zero-field rates, with weights proportional to the square of the mixing coefficient^.^^ A 220-G magnetic field is expected to mix two states separated by a few cm-'. The EPR data obtained for 'DPC1* and 'DBC13 indicate the presence, in the ground state To, of two sublevels separated by such a small energy (2IE/hcl = 0.038 cm-I for 'DPC and 0.034 cm-' for 'DBC). If the ZFS parameters are of the same order of magnitude in the first excited triplet state TI as in To, one can expect a modification of the fluorescence decays under such a small magnetic field. This could explain the magnetic field effect on the decays measured on the sharp fluorescence spectra of 3DPC and 'DBC. However a 220-G magnetic field induces no effect on the decay of 'DPC measured on the broad fluorescence. If the corresponding carbene is effectively distorted in the TI excited state, its ZFS parameters can be larger in TI than in To and a higher field would be necessary to influence the decays. Alternatively, the SOC between the singlets and the triplet sublevels can be less selective in the distorted carbene with respect to the relaxed one.
(14) Salem, L.; Rowland, C. Angew Chem., In?. Ed. Engl. 1972, 2, 92. (15) Davidson, E. R. Dirudiculs; Borden, W. T., Ed.; Wiley-Interscience: New York, 1982; pp. 73-105, and references therein. (16) Dupuy, C.; Korenowski, G.M.; Mc Auliffe, M.; Hetherington, W. M., 111; Eisenthal, K. B. Chem. Phys. Left. 1981, 77, 272.
Registry No. 1, 883-40-9; 2, 2051-90-3; 3, 401 12-59-2; 4, 470-35-9; 5, 6141-55-5; 'DPC, 3129-17-7; 'DBC,15306-40-8.
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Conclusion The matrix fluorescence spectra of 3DPC* generated from four different precursors are blue shifted relative to their fluorescence maxima in liquid solution. However, it is possible to observe DPC sites in Shpol'skii matrices whose emission maxima are similar to those in solution. The data suggest that the geometry of 3DPC* differs from that of ground-state 3DPC and is prevented by matrix packing from relaxing to its preferred fluid solution geometry. The data do not conclusively demonstrate that the geometry of matrix-isolated ground-state 3DPC differs significantly from its geometry in liquid solution. Thus the conclusions about the geometry of 'DPC inferred from matrix EPR work may still be relevant to liquid phase. The decays of 'DPC* and 'DBC* measured on the sharp fluorescence spectra are nonexponential and influenced by the presence of an external magnetic field. The data are highly reminiscent of previously reported studies of m-xylylene biradicals and are explained by a fluorescence decay in the carbene that is faster than spin relaxation within the triplet sublevels. Acknowledgment. The authors are indebted to Dr. J. C. Scaiano for communicating his results prior to publication.
(17) Hall, L.; Owens, D.; El-Sayed, M. A. Mol. Phys. 1971, 20, 1025.
