J Phys. Chem. 1990, 94, 6632-6637
6632
Triplet-Triplet Fluorescence and Spin Polarization of 1- and 2-Naphthylphenylcarbenes Alain Despres, Eva Migirdicyan,* Laboratoire de Photophysique MolPcuiaire du CNRS, Bdtiment 21 3, UniversitP Paris-Sud, 91 405 Orsay Cedex, France
Karl Haider, Vincent M. Maloney, and Matthew S. Platz Department of Chemistry, The Ohio State Uniuersity, I20 West 18th Avenue, Columbus, Ohio 43210 (Receioed: January 3, 1990)
The fluorescence spectra of 1- and 2-naphthylphenylcarbenes (I-NPC and 2-NPC) dispersed in liquid and solid solutions have been studied as a function of temperature, concentration, host, and sample history by laser flash photolysis and conventional spectroscopy. As with diphenylcarbene, 1- and 2-NPCs in Shpolskii matrices at low temperatures present broad and red-shifted sharp spectra. The concentration dependence of the relative intensities of these spectra indicates that energy transfer is very efficient in these systems. In the case of 2-NPC in n-heptane at 77 K, a light-induced interconversion between two fluorescence spectra is observed and tentatively attributed to conformational isomerization. The decays of 2-NPC in n-hexane at 7 K are recorded on the two sharp fluorescence origins at 588.7 and 600.1 nm. In the presence of a 2 2 0 4 magnetic field, the decay measured at 600.1 nm is altered while that measured at 588.7 nm remains unchanged. I n both cases, the decays are attributed to the fluorescence from independent T, sublevels at a rate faster than the rate of spin-lattice relaxation. The different field effect on the two fluorescence origins is discussed.
Introduction The past decade has witnessed a renaissance in the study of carbenes by direct physical methods.’ The dynamics of simple monoarylcarbenes such as 1- and 2-naphthylcarbenes ( I - and 2-NCs) and diarylcarbenes such as diphenylcarbene (DPC) have been elucidated by laser flash photolysis (LFP).2
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2
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/
1-NC
2-NC
1-NPC
1
DPC
These studies have shown that there is tremendous variation in both the absolute rates and reactivity patterns of carbenes with structure. Although, for example, 1- and 2-NCs and DPC are ground-state triplet species, the lifetimes of 31- and 32-NCs in alkane solutions at ambient temperature are more than I O times shorter3 than that of 3DPC under identical condition^.^ Furthermore, ?l-NC in the ground state has a lifetime of only a few minutes in Shpolskii matrices a t 4.2 K,S whereas 3DPC is indefinitely stable in the matrix between 77 and 4.2 K.6 The chemistry of 2-NC in alkanes in fluid solution is dominated by reactions proceeding through the low-lying singlet excited state,3 whereas the chemistry of DPC in this solvent proceeds exclusively through the ground triplet state.’ 1 -Naphthylphenylcarbene ( 1 -NPC) and 2-naphthylphenylcarbene (2-NPC) contain structural features common to both the naphthylcarbenes and the diphenylcarbene systems. ( I ) For leading references see: Recent Aspects of Carbene Chemistry. Platz. M. S. Tetrahedron 1985, 41, 1423-1612. (2) (a) Schuster, G . B. Adu. Phys. Org. Chem. 1986, 22, 31 I . (b) Platz, M. S.; Maloney, V. M. I n Kinetics and Spectroscopy of Carbenes and Biradicals; Platz, M. S . , Ed.; Plenum: New York, in press. (3) (a) Barcus, R . L.; Hadel, L. M.; Johnston, L. J.; Platz, M. S.; Savino, T.G.: Scaiano, J . C. J . Am. Chem. Soc. 1986, 108, 3928. (b) Horn, K. A,; Chateauneuf, J. E. Tefrahedron 1985, 41, 1465. (4) Hadel. L . M.; Platz, M. S.; Scaiano, J. C . J . Am. Chem. Sac. 1984, 106, 283. (5) Haider, K . W.; Platz. M. S.; DesprBs, A,; Migirdicyan, E. Chem. Phys. Lett. 1989, 164, 443. (6) Platz, M. S. Ace. Chem. Res. 1988. 21, 236. (7) Savino. T. G.;Senthilnathan, V. P.: Platz, M. S. Tetrahedron 1986, 42. 2161.
0022-3654/90/2094-6632$02.50/0
2-NPC
In previous studies, Fujiwara et aI.* have shown by EPR that 1 - and 2-NPCs are ground-state triplet species. Maloney and Platz9 have found that 32-NPC exhibits conformational isomerism a t 77 K and that two well-resolved EPR spectra, due to the syn and anti forms, can be resolved at this temperature.
SYN
ANTI
The dynamics and reactivity patterns of 31- and 32-NPCs more closely resemble those of 3DPC than of 32-NC.9 However, the fluorescence spectra of 31- and 32-NPCs obtained at 77 K by Fujiwara et aL8 and of 32-NPC observed at ambient temperature by Scaiano and Weirlo are shifted by more than 100 nm to lower energy with respect to the emission of 3DPC. The data indicate that the first triplet-triplet transition of 1- and 2-NPCs is in the same energy range as that of 2-NC (the fluorescence origin band is at 567 nm), which reveals the predominant role played by the naphthalene chromophore in controlling the spectroscopic properties of these carbenes. Despite the extensive spectroscopic studies that have been reported, methylene and the halomethylenes are still the only carbenes whose gas-phase geometry is known in detail.” Some structural information has been deduced from ~~~~~~
~~
~
~~
~
~~
~
(8) Fujiwara, Y.; Sasaki, M.; Tanimoto, Y.; Itoh, M. Chem. Phys. Lett. 1988, 146, 133.
(9) (a) Maloney, V.; Platz, M. S . J . Phys. Org. Chem., in press. (b) Maloney. V . Ph.D. Thesis. The Ohio State University, Columbus, 1987. ( I O ) Scaiano. J . C : Weir, D. Can. J. Chem. 1988, 66, 491.
C)1990 American Chemical Society
Fluorescence and Spin Polarization of Carbenes
The Journal of Physical Chemistry, Vol. 94, No. 1 7 , 1990 6633
the analysis of ENDOR spectroscopic data obtained for diarylcarbenes immobilized in single crystals.I2 These studies have demonstrated that the electron-nuclear hyperfine coupling constants observed for the triplet carbenes, and ultimately the carbene geometry, depend on the structure of the host medium. In principle, very highly resolved fluorescence spectra can be analyzed to yield the precise geometric structure of a carbene or radi~a1.l~ I n this paper, the fluorescence spectra of 3 1 - and 32-NPCs are reported as a function of precursor concentration, host, and temperature. We have succeeded in finding conditions that produce sharp origin bands; however, the data do not as yet allow the deduction of the geometry of the carbenes under study. Previous studies have shown that the triplet-triplet fluorescence decays of m-xylylene biradicalsl, and of carbenes such as DPC and dibenzocycloheptadienyIidenelsare nonexponential and significantly modified by a weak magnetic field. This is also the case for 32-NPC trapped in an n-hexane Shpolskii matrix at 15 K. These observations indicate that spin polarization of carbenes and biradicals in their first excited triplet state is a general phenomenon.
Experimental Section Materials. 1 -Naphthylphenyldiazomethane (1) and 2naphthylphenyldiazomethane (2) were prepared by conversion of 1- and 2-naphthyl phenyl ketones to their corresponding hydrazones by refluxing the ketones with hydrazine in ethanol. The hydrazones were oxidized to the corresponding diazo compounds by stirring with a suspension of yellow mercuric oxide in ether containing a catalytic amount of potassium hydroxide in ethanol. The diazo compounds were purified by chromatography using Grade V alumina and using pentane as the eluant. I-Naphthylphenylhydrazone:94% yield, white solid, mp 75-80 OC; IR (CH2C12)3590, 3433 (N-H), 3000, 1600,840 cm-’; MS m / e calc for C17H,2N2244.1000, obsd 244.1027, diff 0.0027. I-Naphthylphenyldiazomethane: 52% yield, red solid, mp 82.5-84.5 OC; IR (KBr) 3060,2980,2040, 1600,800,780 cm-l; MS m/e calc for Cl,H12N2244.1000, obsd 244.1027, diff 0.0027. 2-Naphthylphenylhydrazone:28% yield, yellow powder, mp 141-142.5 OC; IR (Nujol mull) 3400,3300, 1170, 1130,910,870, 825, 780, 755, 715; H N M R (90 MHz) 6 5.45 (br, 2 H), 7.2-8.1 (12 H); MS m/e calcd for CI7H,,N2 (M’) 246.1157, found 246.1 163. 2-Naphthylphenyldiazomethane:83% yield, purple solid, mp 79-80 OC; IR (CCI,) 3060, 2040, 1625, 1490, 1285, 1270 cm-I; H N M R (60 MHz, CDC13) 6 7.23-8.0 (m, 12 H). 3-Methylpentane (Aldrich) was purified by stirring with an equal volume of concentrated sulfuric acid, followed by distillation over potassium hydroxide and storage over molecular sieves. n-Hexane and n-heptane (Merck Uvasol) were used without further purificatio, Experimental Setup. Ambient temperature fluorescence spectra were recorded by decomposing the diazo precursors 1 and 2 and exciting the nascent carbene with a single pulse of a Lumonics Model TE-861 M-4 excimer laser (KrF line, 249 nm; 80 mJ/pulse; IO-ns pulse width). The laser-induced fluorescence (LIF) from the sample was collected with a PARC Model 1460 ( I I ) For reviews of methylene see: (a) Schaefer, H. F., 111. Science 1986, 231, 1100. (b) Shavitt, I . Tetrahedron 1985.41. 1531. (c) Bunker, P. R. In Comparisons of Ab Initio Quantum Chemistry with Experiment; Barlett, R. J.; Ed.; Reidel: Dordrecht, 1985. For reviews of the halomethylenes see: (d) Murray, K. K.; Leopold, D. G.; Miller, T. M.; Lineberger, W. C. J . Cfiem. Pfiys. 1988, 89, 5442 and references therein. (e) Powell, F. X.; Lide, D. R., Jr. J . Cfiem. Pfiys. 1966, 45, 1067. ( I 2) (a) Doetschmann, D. C.; Hutchison, C. A., Jr. J . Cfiem. Pfiys. 1972, 56, 3964. (b) Closs, G. L.; Hutchison, C. A,, Jr.; Kohler, B. E. J . Cfiem.Pfiys. 1966,44,413. (c) Hutchison, C. A., Jr.; Kohler, B. E. J . Chem. Pfiys. 1969, 51, 3327. (d) Anderson, R. J. M.; Kohler, B. E. J . Cfiem. Pfiys. 1976, 65, 2451. ( I 3) For a review see: Fosters, S.C.; Liu, X.;Miller, T. A. J . Phys. Cfiem 1989, 93, 5986. (14) (a) Lejeune, V.; Desprts, A,; Fourmann, B.; Benoist D’Azy, 0.; Migirdicyan, E. J . Pfiys. Cfiem. 1987, 91, 6620. (b) Despres, A,; Lejeune, V.; Migirdicyan, E.; Siebrand, W. J . Pfiys. Chem. 1988, 92, 6914. ( I 5 ) Haider, K. W.; Platz, M. S.; Desprts, A.; Lejeune, V.; Migirdicyan, E. J . Pfiys. Chem. 1990, 94, 142.
I
500
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600
700
I
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-800
500
600
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Figure 1. Fluorescence spectra of I-NPC (curves A and B) and 2-NPC (curves C and D) in 3-methylpentane a t 77 K (curves A and C) and a t ambient temperature (curves B and D). The spectra are excited with the excimer laser KrF line a t 249 nm.
optical multichannel analyzer (OMA). The OMA was activated 5 ns after the laser pulse for a 100-ns time window. LIF at 77 K was recorded in a similar fashion after immersing the sample cell in liquid N, prior to inserting it into the precooled sample compartment. The solutions were prepared such that their optical density was between 0.5 and 1.0 at 249 nm. For experiments performed in Shpolskii matrices at 5-77 K, 1- and 2-NPCs were generated in situ by photolysis of the corresponding diazo precursors dispersed in frozen n-hexane or nheptane with 370-390-nm radiation (isolated from a 150-W Osram high-pressure xenon lamp and a silica prism Jobin-Yvon monochromator). Fluorescence spectra were analyzed with a THR I500 Jobin-Yvon spectrometer. Those spectra recorded at 77 K, or at 4.2-15 K prior to annealing, correspond to quickly frozen samples. The rapid cooling of the samples is expected to isolate the diazo precursor and consequently 1- and 2-NPCs in substitutional sites of the matrix. Samples were annealed by warming slowly from 4.2-15 K to approximately 90 K overnight and subsequent cooling back down to 4.2-1 5 K. Fluorescence decays were excited with 337-nm radiation of a nitrogen laser and measured by using equipment and techniques as previously described.’, The pulse width at half maximum was 6 ns.
Results and Discussion I. Fluorescence Spectra. A. LIF Spectra a t 77 K and at Ambient Temperature. Laser flash photolysis (249 nm) of 1 and 2 in 3-methylpentane at 77 K (curves A and C, respectively) and at ambient temperature (curves B and D, respectively) produces the fluorescence spectra shown in Figure I . The better resolved spectra obtained at 77 K present origin bands Am at 596 nm for 1-NPC and at 583 nm for 2-NPC, in agreement within 1 or 2 nm with Am observed by Fujiwara et aI.* Our spectra present vibronic bands at 646 nm for I-NPC and at 600 and 634 nm for 2-NPC. In the latter case, the two resolved vibronic bands are separated by 490 and 1380 cm-I from the origin. These frequencies are very close to the ground-state frequencies of 510 and 1380 cm-l corresponding respectively to a C-C-C planar bending mode and a C-C stretching motion of the central link of the naphthalene molecule.I6 The broader LIF spectra with origin bands around 600 nm for 1-NPC and around 585 nm for 2-NPC obtained at ambient temperature have the same vibronic structures as those observed at 77 K and are therefore attributed to the same species. The fluorescence origin X,that we observe for 2-NPC is in agreement within 3 nm with that obtained by Scaiano and Weir.Io Interestingly, in the spectra measured at 77 K, the shift of the origin (16) Mc Clure, D. S . J . Cfiem. Pfiys. 1956, 24, I .
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Desprts et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, I990
jB
j
*/"In
Figure 2. Concentration dependence of the fluorescence spectra of I-NPC (A) and 2-NPC (B) in n-heptane at 77 K. The spectra of 1-NPC and 2-NPC are respectively excited at 370 and 385 nm.
560
band is only 4 nm for 31-NPC and 2 nm for 32-NPC relative to the position of the origin band observed in liquid solution, whereas it is 23 nm for diphenylcarbene studied under the same condition~.~~ B. Concentration Dependence of the Fluorescence Spectra. The fluorescence spectra of I - and 2-NPCs generated at 77 K in n-heptane from solutions of 1 and 2 at various concentrations (c) are presented in Figure 2, parts A and B, respectively. The spectra were recorded under constant excitation with 370-nm radiation for I-NPC and with 385-nm radiation for 2-NPC. I n Figure 2A, the fluorescence spectrum of 1-NPC obtained with c = IC3M is composed of both a broad and a sharp spectrum with origin bands at 594 and 61 8 nm, respectively. The relative intensities of the two origin bands change significantly under excitation with 370- and 387-nm radiation, indicating that the broad and the sharp spectra correspond to two distinct 1-NPC species. This observation is very similar to that found for 3DPC in the same matrix.Is As the concentration decreases, the relative intensities of the two origin bands at 594 and 618 nm change M, only the drastically. At the lowest concentration, c = broad spectrum is observed. This spectrum (A, = 594 nm) is very close to the LIF spectrum of 1-NPC in 3-methylpentane obtained at 77 K (A, = 596 nm) and at ambient temperature (A, = 600 nm) with very dilute samples. Figure 2B shows a similar study of the concentration effect on the spectra of 2-NPC measured in the early stages of the excitation (vide infra). For c = 3 X IOp3 M, the fluorescence origin band presents a sharp maximum at 600 nm. As the concentration of diazo precursor decreases, a broad origin band with A,, = 583 nm appears and increases in intensity. At the lowest concentration, c = 3 X 10" M, the broad spectrum predominates with vibronic bands at 490 and 1380 cm-I, as in curve C, Figure I . The broad and sharp fluorescence spectra presented in Figure 2 are attributed to different N P C species. The different 1- and 2-NPC species are due to carbenes that are trapped in different matrix sites or to carbenes that are generated and frozen in different geometries in the matrix, due to the rigidity of the host medium. The changes in the relative intensities of the broad and sharp spectra observed for I - or 2-NPC as a function of concentration are attributed to energy transfer between the two different NPC species, the efficiency of which increases with carbene concentration. Energy transfer between species having triplet-triplet transitions is expected to be very efficient since it is allowed by spin selection rules by a dipole-dipole interaction or Forster type mechanism. C. Light-induced Transformation of 32-NPC Fluorescence Spectra. Figure 3 presents the changes observed in the fluores-
600
640 X/nm
Figure 3. Light-induced transformations of the fluorescence spectra of 2-NPC in n-heptane at 77 K under excitation with 385-nm radiation.
cence spectra of 32-NPC in n-heptane at 77 K ( c = 3 X M) as a function of time, under constant excitation with 385-nm radiation. In the early stages of the excitation, the fluorescence consists of two spectra starting at 583 and 600 nm with their respective vibronic bands displaced by about 500 and 1380 cm-l from the origin. These spectra are similar to those observed in Figure 2B M. As the excitation progresses, the sharp origin for c = 3 X band at 600 nm decreases in intensity and is replaced by a new band at 589 nm, while the intensity of the broad origin band at 583 nm remains unchanged. These results show that in an n-heptane matrix at 77 K, there are two types of 32-NPC species: a light-insensitive species I such as that emitting at 583 nm and the light-sensitive species I1 and I11 that fluoresce at 600 and 589 nm. Previously we have mentioned that Maloney and Platz9 have reported that 32-NPC exists in different rotomeric forms at 77 K. The syn and anti forms of 32-NPC present different EPR spectra at this temperature. It is possible that the syn and anti forms of the carbene have distinct emission spectra, hence the large number of origin bands depicted in Figures 3 and 6 (vide infra). Perhaps the interconversion of the 600-nm band to the 589-nm band is due to conformational isomerization. In this regard, both diphenylcarbene and dibenzocycloheptadienylidene, where the possibility of conformational isomerism does not exist, present only a single sharp origin band in n-heptane at 10-77 K (see Figures 2 and 5 of ref IS). However, we cannot determine whether the different species we have observed by fluorescence spectroscopy are due in part to conformational isomerism or entirely to a single conformer in different sites of the matrix. The light-induced transformation of species I1 into species I11 that is observed in a n-heptane matrix at 77 K is considerably reduced in rate in a n-hexane matrix at 77 K. Such a light-induced transformation has not been detected for 1 -NPC in Shpolskii matrices at 77 K . D. Fluorescence Spectra Measured at 4.2-15 K . The fluorescence spectrum of 32-NPC in n-hexane (c = 6 X lo4 M) at 15 K is depicted in Figure 4, curve A. It consists of three prominent sharp origin bands at 582.5, 588.7, and 600.1 nm in addition to a broad origin band centered near 581 nm. The broad and the sharp origin bands are followed by vibronic bands at 510 and 1375 cm-I, frequencies that correspond to the totally symmetric planar bending mode and the stretching motion of the central link of the naphthalene chromophore, respectively. Upon
Fluorescence and Spin Polarization of Carbenes
The Journal of Physical Chemistry, Vol. 94, No. 1 7 , 1990 6635 I
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.
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600 lnm
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100 200 300 MAGNETIC FIELD INTENSITY /GAUSS
Figure 5. Effect of a weak magnetic field on the two sharp fluorescence origins of 2-NPC at 588.7 and 600.1 nm. The spectra measured at 8 K are excited with 370-nm radiation.
T-4.ZK
Figure 4. Fluorescence spectra of 2-NPC in n-hexane at 15 K before (curve A) and after (curve 8 ) the annealing treatment.
annealing the sample, we obtain a spectrum, presented in Figure 4, curve B, that is significantly modified: the broad band at 58 1 nm and the sharp band at 582.5 nm have disappeared, whereas the two other sharp bands remain unchanged. The disappearance of the broad band at 581 nm depicted in Figure 4, curve B, is reminiscent of that observed with the broad origin band of DPC at 483 nm,I5 when both samples were annealed by the same procedure. It is remarkable that the ratio of the well-separated vibronic band of 2-NPC, displaced by 1375 cm-' from the origin band, to the origin band itself, is larger in the broad spectrum than in the sharp spectra starting at 588.7 or 600.1 nm. This can be interpreted as a consequence of changes in geometry between the ground state and the emitting excited state, which are more pronounced for the species responsible for the broad emission starting at 58 1 nm. A similar interpretation involving geometrical changes upon excitation has been used by Anderson et a1.I' to explain the exceptionally broad phonon side bands associated with weak zero-phonon lines observed in the fluorescence spectrum of DPC in benzophenone at 2-12 K . This structure has been attributed to a strong excitation-phonon coupling. It is possible that for the carbenes such as DPCIS and 2-NPC, the broad fluorescence spectra observed in Shpolskii matrices at 5-1 5 K are exclusively composed of phonon bands with no detectable zerophonon lines under these conditions. Such spectra may correspond to species trapped in specific crystalline sites where the geometrical changes in the carbene upon excitation create a great distortion in the surrounding lattice, as a result of the strong excitationphonon coupling. In contrast, the sharp spectra starting at 588.7 and 600.1 nm may correspond to 2-NPC in such an environment that little geometrical change is allowed upon excitation. This corresponds to the situation of a rigid molecule with limited Franck-Condon overlap between the ground and excited states. Upon annealing the sample, the geometry of the carbene in the ground state may be modified such that the excitation-phonon coupling gets weaker. Upon cooling back to 15 K, the carbenes are then trapped in new matrix sites and present sharp spectra. Alternatively, the carbenes exhibiting broad spectra may undergo an irreversible chemical reaction upon annealing. In addition, we have found that the effect of a weak magnetic field is very different on the sharp bands at 588.7 and 600.1 nm, under continuous excitation with 370-nm radiation, as shown in Figure 5. Whereas the intensity of the band at 600.1 nm decreases (17) Anderson, R. J . M.; Kohler, B. E.; Stevenson, J. M. J . Chem. Phys. 1979, 71, 1559.
Figure 6. Fluorescence spectra of 2-NPC in n-heptane at 4.2 K, under excitation with 385-nm radiation.
substantially when the field increases up to 320 G, the intensity change observed on the band at 588.7 nm under the same conditions is at the limit of detectability. This field effect is related to the changes that are observed on the fluorescence decays of these bands (vide infra). Finally, when the matrix is n-heptane at 4.2 K, the fluorescence of 32-NPC excited with 385-nm radiation is composed of seven sharp spectra with origin bands distributed from 574 to 600.4 nm, as shown in Figure 6. The annealing treatment modifies the relative intensities of these origin bands, but none of the bands disappear completely during this treatment. Both the number of sharp origin bands and their dispersion over 25 nm are remarkable and in contrast with the fluorescence spectra of diphenylcarbene and dibenzo~ycloheptadienylidene.~~ 11. Fluorescence Decays. A. Results. The fluorescence decays of '2-NPC in n-hexane at 7 K excited with the 337-nm N, laser radiation and measured on the two sharp fluorescence origin bands at 600.1 and 588.7 nm are very similar, within the accuracy of our experiments. The decay curve measured at 600.1 nm is presented as curve A in Figure 7. In order to evaluate the upper limit in time resolution of our system, a dummy sample (a solution of rhodamine in ethanol) has been excited at room temperature with the N, laser and measured at 600.1 and 588.7 nm. The impulse response of this dummy, which is the same at these two observation wavelengths, is recorded as curve B in Figure 7. Comparison of curves A and B indicates that the corresponding decays are in the same time range, and consequently, the determination from curve A of the decay characteristics of 32-NPC requires a convolution taking into account curve B. The best fit for curve A is obtained by the convolution of curve B with a single exponential corresponding to 18 ns. However, this fit is only in rough agreement with experimental curve A, suggesting that the
6636 The Journal of Physical Chemistry, Vol 94, N o 17, 1990 -
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.
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.
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Figure 7. Fluorescence decay of '2-NPC in n-hexane at 7 K in the absence (curve A ) and in the presence (curve C) of a 2 2 0 4 magnetic field. Fluorescence decay of a dye measured at 600.1 nm at room temperature (curve B). The decays are excited with the N, laser line at 337
nm.
fluorescence decay of '2-NPC should be nonexponential, as is the case for the sharp fluorescence of DPC.15 Upon application of a 220-G magnetic field, the fluorescence decay of '2-NPC measured at 588.7 nm remains unchanged, while the decay measured at 600.1 nm is significantly modified, as indicated by curve C in Figure 7. The observations of the fluorescence decays excited with the N 2 laser agree with the results presented in Figure 5 on the intensities of the fluorescence bands continuously excited at 370 nm. In both cases, the weak magnetic field alters the fluorescence measured at 600.1 nm but has no effect on that measured at 588.7 nm. B . Interpretation. Nonexponential decays have already been found for the triplet-triplet fluorescence of m-xylylene biradicals14 and of aromatic carbenes such as DPC and dibenzocycloheptadienylidene.I5 The results were attributed to the fluorescence from independent spin sublevels at a rate that is faster than the rate of equilibration by spin-lattice relaxation. This interpretation was supported by the drastic modification of the decay curves upon application of a magnetic field that mixes the wave functions of the triplet sublevels. Such a model involving spin polarization can also be used to explain the magnetic field sensitive decay of '2-NPC measured at 600.1 nm. Maloney and Platz9 have measured the zero field splittings (ZFS) between the spin sublevels in the ground triplet state To of the syn and anti forms of 2-NPC in various organic matrices at low temperature. The D and E values thus determined in a 2-methyltetrahydrofuran glass are (Dl/hc = 0.4044 cm-' and (E(/hc= 0.0168 cm-I for the syn form and (DJ/hc= 0.3898 cm-' and (EJ/hc= 0.0195 cm-' for the anti form. The large ID(/hc values result from spin-spin interaction between the unpaired electrons in the u and a orbitals centered on the carbene carbon. The ZFS parameters have not been determined in the first excited triplet state T , , which is the emitting state. By analogy with the m-xylylene and DPC model, the decay of 32-NPC measured at 600.1 nm is attributed to the fluorescence from independent TI sublevels at a rate that is faster than the rate of equilibration by spin-lattice relaxation. Although the rate of spin-lattice relaxation is not known for carbenes, it is reasonable to assume that this requirement is satisfied for the short T, To fluorescence of '2-NPC, as it was for the sharp fluorescence of DPC.I5 The magnetic field effect observed at 7 K for the 600.1-nm fluorescence decay of j2-NPC implies that it is nonexponential. I t is possible that this decay contains a fast component with a
-
Desprts et al. lifetime shorter than 10 ns that cannot be detected in our system. This explains why the convolution of curve B with an exponential corresponding to 18 ns did not fit exactly experimental curve A in Figure 7. Fujiwara et aLs have obtained a fluorescence lifetime of 17 ns for 32-NPC in 2-methyltetrahydrofuran glass at 77 K. This lifetime is in good agreement with our average component lifetime of 18 ns. However, Fujiwara et aL8 do not mention any magnetic field effect on the fluorescence decay of 32-NPC and they do not interpret their results utilizing model involving spin polarization. A 220-G magnetic field is expected to mix two states separated by a few hundreths of a wavenumber. The EPR data obtained by Maloney and Platz9 indicate the presence, in the ground state To, of two sublevels separated by such a small energy (2(E/hc( = 0.0336 cm-l for the syn form and 0.039 cm-' for the anti form). If the ZFS parameters are of the same order of magnitude in the first excited triplet state T , 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 observed on the fluorescence band intensity and the decay measured at 600.1 nm. The interpretation involving spin polarization applies only if the T I sublevels decay with different rates. However under purely Coulombic interactions, transition moments are spin independent. In the absence of spin-orbit coupling (SOC),the fluorescence from the T I state is therefore expected to be exponential. However, the nonexponential decay measured at 600.1 nm suggests that SOC has to be taken into account. According to Salem and Rowland,I8 SOC should be substantial in carbenes because the two singly occupied atomic orbitals are perpendicular in the triplet state. The out-of-plane polarized phosphorescence of aromatic hydrocarbons indicates that SOC mixes t h e m * triplet state with higher-lying UT singlet states. In these molecules, the amount of singlet character in the phosphorescent triplet state is small because the energy gap between the two interacting x x * triplet and U T * (or u * x ) singlet states is large. In carbenes, this energy gap should be much smaller than in aromatic hydrocarbons because the UT* (or x u * ) and x x * states are expected to be close, as a result of the smaller energy gap between the nonbonding u and x molecular 0rbita1s.l~ Consequently, some triplet sublevels in carbenes should have more singlet character than the triplet states of aromatic hydrocarbons. SOC mixes UT singlets with x x * triplet sublevels having the same symmetry. This mixing destroys the equality of the three spin-allowed transition moments between the triplet sublevels of To and T I states. Alternatively, the T, sublevels can decay with different rates as a result of intersystem crossing induced by SOC. As the rate of radiationless transitions is very rapid in molecules of large size, intersystem crossing in competition with the radiative decay will efficiently depopulate the triplet sublevels that have the largest amount of singlet character, thus leading to a nonexponential fluorescence decay. Finally, there remains a need to explain the absence of a magnetic field effect on the fluorescence decay measured at 588.7 nm. The fluorescence spectra with origins at 588.7 and 600.1 nm have the same vibronic structure (see Figure 4). Furthermore, the decays measured on these two origins are similar, within the accuracy of our experiments. This suggests that both spectra belong to 32-NPC. Tentatively, the emissions starting at 588.7 and 600.1 nm are attributed to the syn and anti conformers of I2-NPC detected by EPR.9 This attribution is similar to the one made above to interpret the light-induced interconversion of the fluorescence bands at 600 and 589 nm observed at 77 K (see Figure 3). The magnetic field effect on fluorescence decays is observable when the field mixes two triplet sublevels that have different amounts of singlet character, entailing different total decay rates. This seems to be the case for the conformer that emits at 600.1 nm. However, it is possible that the conformer that emits at 588.7 (18) Salem, L.; Rowland, C. Angew. Chem., Inr. E d . Engl. 1972, 2, 92. (19) Kohler. B. E. Ph.D. Thesis, University of Illinois, Chicago. 1967.
J. Phys. Chem. 1990, 94, 6637-6641 nm has a geometry such that the two sublevels separated by 2E are contaminated with equal amounts of singlet character and therefore they both have the same total decay rates. For that conformer, the mixing of these two sublevels under a weak magnetic field will not alter the fluorescence decay curve.
Conclusion The fluorescence spectra of 31- and '2-NPCs dispersed in liquid and solid solutions have been studied as a function of temperature, concentration, host and sample history by laser flash photolysis and conventional spectroscopy. The data show that the spectroscopic properties of these carbenes are mainly controlled by the naphthalene chromophore. In particular, the high-resolution fluorescence spectra of 32-NPC in Shpolskii matrices at 15 K can be analyzed by using the modes and ground-state frequencies of the naphthalene molecule. This is another application of the method of isodynamic molecules, first established for benzyl type radicals20 and then used to analyze the fluorescence spectra of m-xylylene biradicals and jDPC. As for 3DPC in Shpolskii matrices, the fluorescence spectra of '1- and '2-NPCs in the same host present broad and sharp spectra. The broad spectra are (20) Grajcar, L.; Leach, S. J . Chim. Phys. Phys.-Chim. Biol. 1964, 61, 1523.
6637
assumed to be due to carbenes located in particular matrix sites where their geometry may change upon excitation. Such spectra appear when the carbene contains two aromatic groups that are mobile with respect to each other. They are absent in the fluorescence spectra of dibenzocycloheptadienylidene where the two benzene rings are linked by an ethano bridge. The sharp bands may correspond to carbenes trapped in sites in which little geometric change is possible upon excitation. The decays of 32-NPC measured on the two sharp fluorescence origins at 588.7 and 600.1 nm are similar. In the presence of a 220-G magnetic field, however, the decay measured at 600.1 nm is altered while that measured at 588.7 nm remains unchanged. In both cases, the decays are attributed to the fluorescence from independent TI sublevels at a rate faster than the rate of spinlattice relaxation. The emissions starting at 588.7 and 600.1 nm are tentatively assigned to the syn and anti conformers of 2-NPC. In the conformer that fluorescences at 600.1 nm, the decay is altered because the weak magnetic field mixes the two sublevels, separated by 2E, which decay with different total rates. In the conformer that emits at 588.7 nm, the decay remains unchanged because the field mixes two sublevels that probably decay with the same total rate. Registry NO. 1, 841-86-1; 2, 1029-73-8; I-NPC, 1151 10-18-4; 2-NPC, 54031-13-9.
Ab Initio Heats of Formation of Medium-Sized Hydrocarbons. 12. 6-316" Studies of the Benzenoid Aromatics Rosalie C. Peck, Jerome M. Schulman,* and Raymond L. Disch Department of Chemistry, Queens College, City University of New York, Flushing, New York 11367 (Received: January 8, 1990; In Final Form: March 15, 1990)
The geometries and energies of 16 aromatic hydrocarbons are obtained from ab initio molecular orbital calculations at the 6-31G* SCF level. The energies are used to derive group equivalents that enable calculation of accurate heats of formation. Several applications of the group equivalents are described.
Ab initio molecular orbital theory provides an important and practicable framework for the study of molecular thermochemistry. Recent work has shown the feasibility of extending this method to the benzenoid aromatics in order to provide accurate heats of formation using the ab initio total energies and two types of newly derived aromatic group equivalents.' The previous study was based upon geometries optimized in the minimal STO-3G basis set.* In this paper we present optimized 6-31G*2 S C F geometries and energies of 16 aromatic hydrocarbons. New aromatic group equivalents are obtained for the 6-31G* basis set and several new applications of the method are described.
Geometries The geometries of the molecules shown in Chart I were optimized at the 6-31G* S C F level in the symmetries described in our earlier study.' The 6-3 lG*CC and CH bond lengths are given in Table I. The rms deviation from experiment for 101 CC bond lengths is 0.016 A, compared with 0.019 8, at the STO-3G level. Cartesian coordinates for the molecules are given in the supplementary material. All of the molecules except 3,4-benzophenanthrene and corannulene are planar. Values for the seven independent dihedral angles of benzophenanthrene in C2symmetry (Figure 1) are given ( I ) Schulman, J . M.; Peck, R. C.; Disch, R. L. J . Am. Chem. SOC.1989,
in Table 11. The experimental angles, obtained by X-ray diff r a ~ t i o n show , ~ significant distortions of the structure from C2 symmetry, due perhaps to thermal effects or packing forces. The 6-31G* angles differ from the averages of their counterpartsvalues that should be equal in C2 symmetry-by less than 2'. It is interesting that even with its nonplanar structure, benzophenanthrene has a AHf' only 7 kcal/mol greater than that of chrysene. The bond lengths and angles of corannulene in its bowl-shaped C,, form (Figure 2) are in good agreement with the experimental X-ray values4 The angles A, B, C, and D (indicated in Figure 2) describing the deviation of its carbon skeleton from planarity are calculated (and measured4) to be 10.0 (10.4)', 25.5 (26.8)', 21.1 (22.4)', and 12.0 (11.6)'. In optimizing the geometry of acenaphthene, we assumed a C , structure. This assumption is supported by S C F calculation of structures slightly distorted from C , symmetry according to the lowest frequency (a2)coordinate, which is predominantly twisting of the methylene groups. At the STO-3G level, there is a small increase in energy; at the 3-21G2 level, there is no significant change. Thus the energetic consequences of the twisting of the methylene carbons out of plane are insignificant. A C , structure was also found in a recent neutron diffraction study in the solid (3) Hirshfeld, F. L.; Sandler, S.; Schmidt, G. M. J. J . Chem. SOC.1963,
111, 5675.
2108.
(2) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A . A6 lnirio Molecular Orbird Theory; John Wiley and Sons: New York, 1986.
(4) (a) Barth, W. E.; Lawton, R.G. J . Am. Chem. SOC.1971, 93, 1730. (b) Hanson, J. C.; Nordman, C. E. Acra Crysrallogr. 1976, B32, 1147.
0022-3654190J 2094-6631$02.50/0
0 1990 American Chemical Society
