3228
J. Phys. Chem. 1987, 91. 3228-3233
the sign of JPF must be negative. Since SF is positive, it is the low-frequency line in the fluorine spectrum which is broad. In summary, we have shown that over a wide range of correlation times (lo-" 2 7,2 lo-'), the experimental and theoretical values of the transverse relaxation times for both the fluorine- 19 and the phosphorus-3 1 relaxation times in PFOj2- solution are in excellent agreement. We have shown unequivocally that the is negative, in absolute sign of the spin-coupling constant, JPF, particular JpF= -870.0 f 0.05 Hz, at room temperature in an ethylene glycol-water solution. In favorable cases such as PF0,2(or HI3C-X3, where X is nonmagnetic), relatively straightforward measurements of the transverse relaxation time as a function of temperature should be able to provide accurate measurements of the CSA for both nuclei, the internuclear distance and the correlation time. One can expect in coupled spin systems to see such differential line broadening under the following conditions: (a) When 100 2 2 0.01. Therefore nuclei such as phosphorus-3 1, carbon-1 3, nitrogen-15 , fluorine-19, silicon-29, selenium-77, platinum-195, and so on are likely candidates for DLB. (b) When the magnetic field strength is equal to or greater than 4.7 T, or until the point is reached where 6, >> p . (c) When the molecular motions are relatively slow. DLB becomes most pronounced when W ~ = T 1. ~ Therefore spin systems in macromolecules," or absorbed on high surface area materials,I0 or in other environments where their molecular motions will be slow, are most likely to exhibit DLB. (d) When the two relaxation processes (in the present case, dipolar and CSA) have a common correlation time. (e) When intramolecular relaxation is the dominant relaxation. That is, intermolecular relaxation, which usually involves an in-
dependent correlation time, will tend to mask the differential broadening. A simple but, we believe, very significant consequence of the present work is the positive identification of CSA as an important NMR relaxation mechanism in coupled spin systems when working in high magnetic fields (Bo2 4.7 T). Since the relaxation of both the individual and the total magnetizations in a multiplet can be strongly affected by the relative orientation of the chemical shift and dipolar tensors,l8 care should be taken to include this parameter in any calculations. For a coupled spin system the relaxation is complex but can be completely analyzed by using density matrix theory. For experiments in which one of the spins is decoupled it is not yet clear how to interpret the relaxation results; care should be taken in attempts to measure molecular parameters, such as correlation times, in such experiments at high fields. In the present case it is also possible that the indirect spin coupling tensor (J)has a significant anisotropy. In this event the dipolar parameter, p , defined in eq 10 should be modified to include the anisotropic J term. Finally, we should note that quite recently DLB has also been observed for proton spectra.Ig
Acknowledgment. We acknowledge the help of Marvin Kontney with the rf electronics and Ilene Locker for many helpful discussions, suggestions, and comments. We gratefully acknowledge the fellowship support of the DuPont Co., the Venezuelan Conicit program, and the National Science Foundation Grant No. CHE-8306696. (18) Farrar, T. C.; Adams, B. R.; Grey, G. C.; Quintero-Acaya, R. A,; Zuo, Q. J . Am. Chem. SOC.1986, 108, 8190. (19) Anet, F. A. L. J. A m . Chem. SOC.1986, 108, 7102.
Dual Fluorescence of Isobacteriochlorins Fritz A. Burkhalter, Erich C. Meister, and Urs P. Wild* Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH- Zentrum, CH-8092 Zurich, Switzerland (Received: December 15, 1986)
Room-temperature solutions of two synthetic isobacteriochlorins are shown to have dual fluorescence with 0-0 bands near 635 and 585 nm. The full two-dimensional fluorescence intensity as a function of the excitation and emission wavelength is presented. It is concluded that the dual fluorescence results from two different tautomeric forms which involve a diagonal and an adjacent arrangement of the two free base hydrogens. The long-wavelength fluorescence has a lifetime of about 3 ns, and the short-wavelength emission has a lifetime of about 8 ns. Semiempirical calculations of the QCFF/PI and the CNDO/CI type were performed in order to discuss the absorption spectra of the different tautomers.
1. Introduction The chromophore of siroheme, closely related to a naturally occurring intermediate in the vitamin B,2 biosynthetic pathway and a prosthetic group in redox enzymes, has been shown to be of the isobacteriochlorin type.I Several substituted model comand pounds have been synthesized by Chang,* Battersby et Eschenmoser et al. 4,5 A significant change of the vibrational structure in the visible and UV absorption spectra of several isobacteriochlorins was observed by Chang2 as a function of temperature. It was suggested that, at least, two tautomeric forms are in rapid equilibrium at (1) Murphy, M. J.; Siegel, L. M. J. Bioi. Chem. 1973, 248, 691 I . (2) Chang, C. K. Biochemistry 1980, 19, 1971. (3) Battersby, A. R.; Jones, K.; McDonald, E.; Robinson, J. A,; Morris, H. R. Tetrahedron Lett. 1977, 2213. (4) Montforts, F. P.; Ofner, S.; Rasetti, V.; Eschenmoser, A,; Woggon, W.-D.; Jones, K.; Battersby, A. R. Angew. Chem., Int. Ed. Engl. 1979, 18, 675. (5) Naab, P.; Lattmann, R.; Angst, C.; Eschenmoser, A. Angew. Chem., Int. Ed. Engl. 1980, 19, 143.
0022-3654/87/2091-3228$01.50/0
room temperature. The two strongest absorption bands near 590 and 370 nm were tentatively assigned to the principal form, with two diagonally located free base hydrogens in the center, whereas the long-wavelength band at 635 nm and the shoulder at 400 nm, which disappear upon cooling of 2-methyltetrahydrofuran solutions to 77 K, were assigned to a form with the two free base hydrogens bound to adjacent pyrrolic nitrogens (Figure 2 ) . The study of the light-induced shifts of the free base hydrogens in porphyrins and related ring systems has lead to a series of exciting result^.^-^ Using hole burning at low temperatures, we could follow the photoconversion even in porphine, where the two forms are equivalent from the molecular symmetry standpoint and the small difference in the transition energies results only from (6) Gorokhovskii,A. A.; Rebane, R. K.; Rebane, L. A. JETPLett. (Engl. Transl.) 1974, 20, 216. (7) Voelker, S . ; Macfarlane, R. M. IBM J . Res. Deo. 1979, 23, 547. (8) Burkhalter, F. A.; Suter, G. W.; Wild, U. P.; Samoiienko, V. D.; Rasumova, N. V.; Personov, R. I. Chem. Phys. Lett. 1983, 94, 483. (9) Wild, U. P.; Bucher, S. E.; Burkhalter, F. A. Appl. Opt. 1985, 24, 1526.
0 1987 American Chemical Society
Dual Fluorescence of Isobacteriochlorins
The Journal of Physical Chemistry, Vol. 91. No. 12, 1987 3229
CN I
0 80
300 K
0 60 W 0
a a m
OLO
51 a m
a
0 20
sm,
LOO
303
mo
600
0 LO
300 K
.
2
0 30
W
u z
2K 51 m
N-
/
/
/
020
6
0 10
-2
300
LOO
500
600
700
WAVELENGTH [ NM 1
WAVELENGTH INMI
Figure 1. Absorption spectra of synthetic isobacteriochlorins in 2-MTHF at 300 and 77 K. 1: 5-cyano-2,2,8,8,12,13,17,18-octamethylisobacterioch~orin (c = 7 X 10" M). 2: 2,2,7,7,12,13,17,18-octamethylsiobacteriochlorin ( c = 5 X lod M). The concentrations refer to the room-temperature values; no correction for the contraction a t low temperature has been applied. Cell thickness 8 mm.
35
30
25
M k K
6
35
30
25
2OkK
6
35
30
25
2 O k K t 5
Figure 2. Three tautomeric forms of the isobacteriochlorin chromophore and their calculated absorption spectra.
an asymmetry in the solvent environment. In chlorin7+ a single tautomeric form is stable at room temperature. The phototautomer which can be formed at low temperature is easily identified by a blue shift in the fluorescence spectrum. A more complex behavior must be expected for the isobacteriochlorins, where at least two tautomers are already present a t room temperature.2 The low-temperature photochemistry of the isobacteriochlorins in Shpol'skii matrices has been studied in detail by Burkhalter.Io An interesting study of the Stark effect of isobacteriochlorin in monocrystalline n-heptane has been reported recently by Johnson et al." They found an unusually large dipole moment difference between the SIand So states. The hole-burning pattern of the 580-nm 0-0 bands was in agreement with the Chang* hypothesis of a tautomer with hydrogens at opposite nitrogens. In the isobacteriochlorins it is not immediately obvious how the absorption spectra relate to the different tautomers. We therefore performed a series of semiempirical calculations on the three tautomeric chromophores which differ in the position of the ETH No. 7492, Zurich, 1984. (11) Johnson, L.W.; Murphy, M. D.; Pope.,C.; Foresti, M.; Lombardi, J. R., to be published. (10) Burkhalter, F. A Diss.
inner hydrogen atoms using the QCFF/PI and the CNDO/CI method in order to facilitate the interpretation of the spectra (Figure 2). The dual fluorescence of the model compounds is best illustrated by the two-dimensional spectra (TDS) (Figures 3 and 4) which represent the fluorescence intensity as a function of both the excitation and emission frequencies (Figure 5 ) . The technique of total luminescence spectroscopy (TLS) is particularly suited to the investigation of multiple emission^.^^-^^ Furthermore, by studying the time-resolved fluorescence spectra (Figure 6 ) , we determined the lifetimes and spectra of the components.
2. Semiempirical Calculations The electronic absorption spectra of porphyrins and related tetrapyrroles have been analyzed by applying a wide range of semiempirical methods. A thorough review relating the optical (12) Suter, G. W.; Kallir, A. J.; Wild, U. P. Chimia 1983, 37, 413. (13) Suter, G. W.; Wild, U. P. Chem. Phys. 1982, 73, 421. (14) Kallir, A. J.; Suter, G. W.; Wild, U. P. J . Phys. Chem. 1B5.89, 1996. (15) Kallir, A. J.; Suter, G. W.; Bucher, S.E.; Meister, E.; Liiond, M.; Wild, U. P. Acta Phys. Pol., in press.
3230 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 670
670
520
520
440
440
Burkhalter et al.
x
t
f: c M
V
W x
760
nm
380
380
340
340
0
630
EHISSION
740 (NH)
3
630
EHISSION
740 (NH)
Figure 3. Contour maps of the two-dimensional fluorescence spectra of 1 and 2 in n-hexane at 300 K (1, c = 2 X M; 2, c = 3 X M).
460
660
nm
460
Figure 5. Excitation and emission spectra of the isobacteriochlorins 1 and 2 in n-hexane solutions a t 300 K. The excitation spectra are corrected for the wavelength dependence of the apparatus; the emission spectra are not. At about 585 nm, the excitation spectrum of the yellow emitting tautomers can be obtained by experiment. (top) In the cyanoisobacteriochlorin 1, the red emitting tautomer predominates. Its excitation spectrum can be obtained quite purely. (i, ii) Emission spectra, excited a t the wavelengths indicated by arrows. (iii, iv) Excitation spectra, monitored a t the wavelengths indicated by arrows. (bottom) In the isobacteriochlorin 2, the emission of the red emitting tautomer is overlapped quite strongly by vibronic bands of the emission of the other tautomer. Its excitation spectrum is estimated by taking several linear combinations (vii-x) of the experimental spectra iii and v.
Figure 4. Total luminescence spectra of 1 and 2 (see also Figure 3).
spectra and the electronic structure was given by Gouterman.16 Many of the main features of the absorption spectrum of conjugated ring systems can already be obtained from a simple free electron c a l ~ u l a t i o n but , ~ ~ ab initio calculations using a floating sphere Gaussian basis set have also been reported.'* In semiempirical models the gap between the long-wavelength Q bands and the strong Soret band near 380 nm is generally overestimated. A good description of the absorption spectrum of porphine was obtained by introducing screening p0tentia1s.l~ Rawlings et al?O used a M I N D 0 / 3 geometry optimization and the INDO/S/CI method for the calculation of the transition energies and oscillator strengths of the isobacteriochlorin chromophore. They investigated the tautomers BD and CD (see Figure 2) and showed that they have very similar ground-state energies. In agreement with the hypothesis of Chang, the CD tautomer has a red-shifted absorption spectrum. It was surprising, however, that a similar approach in the parent compound porphine lead to calculated absorption spectra which were almost indistinguishable for the BD and the CD forms. In this work we discuss the absorption spectra of all three tautomers of isobacteriochlorin which can be postulated using (16) Gouterman, M. In The Phorphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 111, pp 1-165. (17) Simpson, W. T. J. Chem. Phys. 1949, 27, 1218. (1 8) Spangler, D. Maggiora, G. M.; Shipman, L. L.;Christoffersen, R. E. J. Am. Chem. Soc. 1977, 99, 7470. (19) Sekino, H.; Kobayashi, H. J. Chem. Phyi. 1981, 75, 3477. (20) Rawlings, D. C.; Davidson, E. R.; Gouterman, M. Theor. Chim. Acta 1982, 61, 227. . .
classical formulas (Figure 2). In order to calculate the three spectra on equal assumptions, a two-step procedure was employed. The first step involved the calculation of a self-consistentgeometry using the QCFF/PI method as described by Warshel and Levitt.21 In a test run on porphine, where the bond lengths are accurately known from X-ray studies, it was shown that the calculated C-C bond lengths overestimated the experimental values by 0.025 A. After this correction was applied, the remaining differences were smaller than 0.007 A. We therefore expect that this procedure, which takes a correlation between the r bond orders and the bond lengths into account, will lead to molecular geometries which allow a well-founded prediction of the tautomer spectra. The geometries obtained were subsequently used in a second step to calculate the absorption spectra by using the CNDO method with configuration interaction. An updated version of the program of Baumann22was employed, and all singly excited configurations within 10-eV excitation energy were considered. The resulting spectra are shown in Figure 2. The following conclusions can be drawn from the calculations: (i) The most stable tautomer BD is planar and has a diagonal arrangement of the two free base hydrogens in the center. (ii) The ground-state energies of the tautomers with the hydrogens in adjacent positions (CD and BC) are about 20 kiomol-l higher. In these tautomers the hydrogens are located 0.3 A above and below the molecular plane. The displacement of the corresponding nitrogens is about 0.15 A. (21) Warshel, A.; Levitt, M. QCFF/PI: QCPE Program 247, Indiana University. (22) Baumann, H. CNDUV99: QCPE Program 333, Indiana University.
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3231
Dual Fluorescence of Isobacteriochlorins
3 0
Figure 6. Time-resolved fluorescence spectra of 1 and 2 in 2-MTHF a t M; 2, c = 2 X lo-' M) excited at 300 nm. 300 K (1, c = 5 X
(iii) Of all three tautomers, only the C D form is expected to absorb in the 625-nm region. The tautomers BD and BC each contain a pyrrole ring with a pyridinic nitrogen (Le., not binding a hydrogen). The calculated bond order between the two 0 carbons in these rings is high (0.85), indicating that this double bond is quite short and almost isolated from the rest of the T system. The effective conjugation path is thus decreased, and it is not suprising that the PPP as well as the C N D O calculations predict a blueSo) transition. shifted (S, (iv) The absorption spectra of the tautomers BD and BC are almost identical, having a long-wavelength absorption band near 500 nm. A distinction of these two tautomers on the basis of their visible spectra is hardly feasible. (v) All three tautomers have two strong absorption bands in the Soret region which are almost (BD and BC) or exactly (CD) polarized perpendicular to each other. (vi) From the study of a series of reduced porphyrins such as chlorin, bacteriochlorin, and isobacteriochlorin the spectral behavior can be rationalized in terms of the following structural characteristics: (a) In order to satisfy the octet rule, at least one of the inner hydrogens must be bound to a fully conjugated pyrrole ring. The second hydrogen can be bound to a pyrrole or a reduced pyrrole ring. (b) If the second hydrogen is bound to a fully conjugated pyrrole ring, the lowest transition is shifted to about 620 nm. If it is bound to a reduced pyrrole ring, the longwavelength absorption occurs around 500 nm.
-
3. Experimental Section Absorption Spectra. Absorption spectra were recorded on a Perkin-Elmer Lambda-9 UV/VIS/NIR spectrometer. Lowtemperature measurements of glassy samples were performed with an Oxford DN-704 liquid nitrogen cryostat. Total Luminescence Spectra and Data Handling. All fluorescence spectra were measured on a high-resolution spectrometer equipped with photon counting and digital data processing (DEC LSI 11/23). The excitation source consisted of an Osram XBO 2.5-kW high-pressure xenon lamp combined with a Spex 1402 double monochromator. The emission light was passed through a second Spex 1402 double monochromator and
detected by a cooled RCA C31034 photomultiplier with excellent red sensitivity and negligible dark count rate. The two-dimensional spectra (TDS) were obtained by sequentially recording a series of emission spectra with different excitation frequencies. The intensity matrices used in this work consist of 128 X 64 values, corresponding to about 1 point per 1 13 cm-I in emission and 73 cm-' in excitation, and required a measuring time of about 6 h. All luminescence measurements utilized a front face geometry. This allowed the use of samples with a relatively high optical density. To monitor the excitation intensity, the spectral characteristics of the excitation subsystem were measured simultaneously with the luminescence intensity by using a calibrated photodiode. This analog signal was converted to a digital pulse train with a 10-MHz voltage-to-frequency converter. The spectra were corrected by software to a constant photon flux in the excitation intensity and for the wavelength dependence of the monochromator-photomultiplier combination. The data analysis was carried out on a DEC PDP 11/34 minicomputer. The most convenient and interpretable representation of a TDS is a contour plot. The data points of equal intensity are connected by a contour line, and the resulting representation can be interpreted as if it were a conventional map. All the TDS presented in this paper have 12 logarithmic contours spaced by a factor of 0.6, thus covering a signal ratio of about 300: 1. A full description of the apparatus and the software employed is given elsewhere.23 Time-Resolved Fluorescence Spectra. Time-resolved measurements were performed on a time-correlated single photon counting (TCSPC) system described recently.24 The excitation source consists of a mode-locked Coherent Innova I- 15 argon ion laser and a synchronously pumped extended rhodamine 6G or styryl9 dye laser equipped with a Coherent 7200 cavity dumper. The output was a regular train of 8-ps-fwhm pulses with a separation of 13.22 ns. The pulses were frequency-doubled to 300 or 400 nm means of angle-tuned nonlinear L i I 0 3 crystals. Emission from the sample was analyzed through a Spex 1400 double monochromator which was modified for subtractive dispersion and imaged on the photocathode of a Hamamatsu R 928 photomultiplier. The timing reference signal was taken from a Spectra Physics 403B fast photodiode. Two-dimensional time-resolved spectra were obtained by sequentially measuring fluorescence decay curves at different monochromator settings. The detection system is fully computer controlled by a DEC LSI 11/73. Fluorescence decay deconvolution was performed in Fourier space with the method of Wild et aLZ5 Chemicals. 2-Methyltetrahydrofuran (2-MTHF, Fluka, puriss.) was distilled over sodium. 3-Methylpentane (3-MP, Fluka, purum) was filtered through Celite/sulfuric acid. Both solvents were filtered through basic aluminum oxide immediately before preparing the solutions. n-Hexane (Fluka, spectroscopic grade) was used as supplied.
4. Results and Discussion Absorption Spectra. The room- and low-temperature spectra of the two isobacteriochlorins 1 and 2 investigated are given in Figure 1. Cooling the solution of compound 2 in 2-MTHF from 300 to 77 K resulted in the simultaneous disappearance of the two absorption bands at 400 and 635 nm. This confirms the experimental findings of ChangZand allows an assignment of these bands to the thermodynamically unstable tautomer. The reaction enthalpy of the tautomer interconversion is expected to be very small, since in 3-MP no significant change of the tautomer composition was observed upon cooling from 300 to 77 K. The absorption spectra of compounds 1 and 2 differ significantly in the intensity of the two absorption bands at 400 and 635 nm. It must be concluded that the tautomeric form, which corresponds to the thermodynamically unstable tautomer of 2, predominates (23) Kallir, A. .I. Diss. ETH No. 7960, Zurich, 1986. (24) Canonica, S.; Wild, U. P. Anal. Insfrum. (N.Y.) 1985, 14, 331. (25) Wild, U. P.; Holzwarth, A. R.; Good, H. P.Reu. Sci. Insfrum. 1977, 48, 1621.
3232 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987
Burkhalter et al.
TABLE I: Fluorescence Lifetimes of Isobacteriochlorin 1 in 2-MTHF at 300 and 77 K
T, K 300 300 300 17 77
I1
excitation wavelength,
emission wavelength,
nm
nm 585 635 635 585 635 635
300 300 400 300 300 400
amplitude
decay time
amplitude
a1
rl,ns 1.4 f 0.2 2.6 f 0.2 3.1 f 0.1 7.8 f 0.1
a 2
r2, ns
0.77 f 0.02
8.2 f 0.2 7.1 f 1.0
0.23 f 0.01 0.75 f 0.08 1 .oo I .oo 1.oo 1.oo
0.25 f 0.08
decay time
7.8 f 0.1 6.9 f 0.1
TABLE II: Fluorescence Lifetimes of Isobacteriochlorin 2 in 2-MTHF at 300 and 77 K
T, K 300 300 300
I1 17 77
excitation wavelength,
emission wavelength,
amplitude
decay time
nm 300 300 400 300 300 400
nm
ffI
585 635 635 585 635 635
0.13 f 0.01 0.44 f 0.03 0.79 f 0.06 1 .oo 1.oo 1 .oo
r I , ns 1.3 f 0.3 2.1 f 0.2 2.2 f 0.2 9.7 f 0.1 9.3 f 0.1 9.7 f 0.1
in compound 1. Only small changes in the absorption band structure were observed (Figure 1) upon cooling 2-MTHF or 3-MP solutions of 1. Total Luminescence Spectra. Dual fluorescence was observed in both isobacteriochlorins, as shown in Figures 3 and 4. The long-wavelength emission occurs at 635 nm (15 750 cm-', red band); the shorter wavelength emission is at 585 nm (17 100 cm-l, yellow band). Excitation into the long-wavelength absorption band at 635 nm lead to a pure emission spectrum of the red form. The corresponding fluorescence spectra for compounds 1 and 2 are very similar. The red emission from the isobacteriochlorin 1, which also has a more intensive long-wavelength absorption band, is however much stronger. On the one hand, the TDS given in Figures 3 and 4 may be analyzed by discussing the excitation spectra for the two forms emitting at 585 and 635 nm. The excitation spectrum monitored at 585 nm corresponds to a vertical section through Figure 3. It contains peaks at 585, 540, and 375 nm and can be assigned to the short-wavelength emitting form. The excitation spectrum monitored at 635 nm is more complicated. First, it must be noted that all the bands which were assigned to the short-wavelength form are also present. In addition, there are peaks at 635, 5 10, 480, and 395 nm which are assigned to the long-wavelength emitting form. On the other hand, the analysis of the TDS spectra given in Figures 3 and 4 shows that the fluorescence spectrum excited at 635 nm, which corresponds to a horizontal section through Figure 3, may be assigned to the pure long-wavelength emitting form. Here, the fluorescence spectrum excited at 585 nm is more complicated and results from a superposition of the fluorescence spectra of the different tautomeric forms. The emission and excitation spectra of the pure components can be obtained by taking properly weighted linear combinations of the experimental spectra. This procedure is shown in Figure 5. It is evident that the excitation as well as the emission spectra of the two tautomeric forms are markedly different. The differences in the spectra of the corresponding tautomers of the compounds 1 and 2 are, however, quite small. It may therefore be concluded that the two investigated isobacteriochlorins show a very similar behavior with respect to the luminescence properties of the chromophore. The position of the tautomer equilibrium depends, however, strongly on the substituents and on the solvent used. Time-Resolved Fluorescence Spectra. The TDS spectra (Figures 3 and 4) show that excitation at 585 nm results in emission maxima at 585 and 635 nm. The question arises whether the emission at 635 nm is the 0-0 band of the red emitting form or a vibrational band of the yellow emitting form. In the first case the following three pathways which could lead to a red fluorescence must be distinguished: (i) direct excitation of the
amplitude ffz 0.87 0.02 0.56 f 0.03 0.21 f 0.07
*
decay time rz, ns 8.6 f 0.1 10.6 f 0.4 9.8 f 1.8
red emitting form; (ii) excitation of the yellow emitting form, which is followed by an adiabatic reaction on the excited-state surface, leading to the red emitting tautomer; and (iii) energy transfer. Since the temporal behavior for these types of fluorescence is expected to be different, we performed time-resolved measurements of both 1 and 2 at roomtemperature and at 77 K at different excitation wavelengths. The time-resolved spectra given in Figure 6 reveal that the 585and 635-nmemissions have a different time dependence. The red emission band is shorter lived than the yellow band. At an excitation wavelength of 300 nm all tautomers are expected to absorb with comparable extinction coefficients. Quantitative values for the lifetimes were obtained from curve-fitting p r o c e d ~ r e s .At ~~ room temperature excellent fits could be obtained with two exponential functions in all cases (Tables I and 11). The red emitting form has an unusually sharp absorption band near 400 nm. Irradiation into this band should almost exclusively excite the red emitting form. Indeed, excitation at 400 nm with the frequency-doubled output of a styryl9 dye laser resulted in a very strong enhancement of the short-lived emission. In this case a satisfactory fit could even be obtained with a single ex1 )3.1 ns, Tl(2) = 2.2 ns). Almost the same ponential ( ~ ~ ( = fluorescence lifetimes ( ~ ( 1 )= 3.0 ns, T(2) = 2.6 ns) were also obtained when the red form was directly excited via its 0-0 band at 635 nm. Our experiments did not show any evidence of a growth in the red emission; thus, energy-transfer reactions and adiabatic photoreactions which link the yellow emitting with the red emitting form can be ruled out. The fluorescence decay curves monitored at 635 nm and excited at 300 and 585 nm are biexponential. The shorter lifetime is in agreement with the lifetime determined from the monoexponential decays discussed above and is assigned to the red emitting form. 1 )8.2 ns, ~ ~ ( =2 8.6 ) ns) must be The longer lifetimes ( ~ ~ ( = assigned to the independently emitting yellow form. Thus, we expect that monitoring the emission at 585 nm would show only the yellow emitting form with a lifetime in the order of 8 ns. This is indeed the main component. However, a careful analysis of the decay requires a biexponential fit. The additional ) ns, ~ ~ ( =2 1.3 ) ns) with an short-lived component ( ~ ~ ( =1 1.4 integrated intensity of about 5% is tentatively assigned to the third tautomeric form BC postulated in section 2. The contribution of this form to the total emission is truly weak, and no evidence for its existance was found in the static luminescence studies. This is, however, not surprising, since the semiempirical calculations suggest that the BD and the BC tautomers are virtually indistinguishable on the basis of the optical absorption spectra alone. Alternatively, the presence of a small amount of impurities emitting at 585 nm cannot be rigorously excluded, although it is
J . Phys. Chem. 1987, 91, 3233-3237 unlikely that both compounds 1 and 2 contain an impurity having similar emission properties.
5. Conclusions The room-temperature fluorescence behavior of the isobacteriochlorins can be rationalized on the basis of the three postulated tautomers: (i) The main thermodynamically stable tautomer BD emits at 585 nm (0-0 band) and at 635 nm (vibrational band) with a decay time of about 8 ns. (ii) The tautomer C D emits in the red region (635 nm) with a decay time of 2-3 ns. (iii) The tautomer BC with hydrogens at adjacent nitrogens had the shortest decay time of about 1.5 ns. According to the
3233
semiempirical calculations, its emission shows a strong overlap with the main tautomer emission. The contribution of this form to the total emission is small. At 77 K-as is known from the absorption measurements-only the tautomer BD is present in 2-MTHF solutions. Indeed, at 77 K the lifetimes of the 585- and 635-nm emissions are equal within experimental error.
Acknowledgment. We thank Prof. Eschenmoser for providing us with the two synthetic isobacteriochlorins. Financial support by the Swiss National Science Foundation is gratefully acknowledged.
Dissociation Energies of the Benzene Dimer and Dimer Cation J. R. Grover,* Chemistry Department, Brookhaven National Laboratory, Upton, New York 1 1 973
E. A. Waiters,* and E. T. Huit Chemistry Department, University of New Mexico, Albuquerque, New Mexico 871 31 (Received: December 29, 1986)
The dissociation energy of the benzene dimer was measured indirectly via a precision measurement of its ionization potential, using tunable synchrotron radiation and an argon-benzene nozzle beam optimized for (C6H6)2. This ionization potential, 8.690 i 0.023 eV (1427 i 4 A), was combined with other data from the literature to obtain Do([C6H6]2) = 2.4 i 0.4 kcal mol-' and Do([C6H6]2+) = 15.3 i 0.9 kcal mol-'. The corresponding heats of formation are AHfo,,([C6H6],) = 30.4 f 0.4 kcal mol-' and AH,oo([C6H6]2+)= 230.8 i 0.9 kcal mol-'. The threshold portion of the photoionization efficiency function of (C6H6)2 shows no sign of autoionizing Rydberg structure, so direct ionization is the process that determines the threshold function shape. This shape also indicates that the geometry of (C&)2 is significantly different from that of (C6H6)2+,which is thought to have a sandwich structure, but is still sufficiently similar to permit the threshold to be distinctly observed.
Introduction The gas-phase neutral dimer of benzene is important as the prototypical aromatic homodimer and as such has been intensively studied, both experimentally'-" and theoretically,'2-20 with the immediate objectives of elucidating its thermochemistry, structure, and spectroscopy, and the more far-reaching goals of understanding its chemical reactivity and the relationship of its properties to nucleation phenomena and colligative phases. Its study presents difficult problems, however. The most useful synthesis of benzene dimers is via the jet expansion of He or Ar mixed with a few percent of benzene, which results in dimers cooled to near 0 K. By this method dimers are necessarily part of a mixture containing benzene monomer and larger clusters, which include heteroclusters, especially when expanded with argon. Analysis for beam composition is not routine since convenient and reliable methods are just being developed. The presence of the larger clusters can seriously complicate the interpretation of some kinds of data. In addition, it is not clear whether the benzene dimers are present as more than one isomer. As a result of these and other problems relating to the interpretation of spectra no experimental value of the dissociation energy has been reported, and several quite different geometries have been proposed for the structure of the ground state. Dissociation Energy. Although there is apparently no reported experimental value for the dissociation energy, the result of the time-of-flight infrared photodissociation experiment of Nishiyama and Hanazaki3 provides an excellent upper bound (see Appendix). 'Participant in the National Synchrotron Light Source/High Flux Beam Reactor (NSLS/HFBR) Faculty-Student Support Program at Brookhaven National Laboratory.
0022-3654/87/2091-3233$01 S O / O
They observed a mean center-of-mass translational energy of benzene monomer fragments of 0.14 kcal mol-' when benzene ( I ) Bornsen, K. 0.;Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1986,85, 1726-1132. (2) Riihl, E.; Biding, P. G. F.; Brutschy, B.; Baumgartel, H. Chem. Phys. Lett. 1986, 126, 232-231. (3) Nishiyama, I.; Hanazaki, I. Chem. Phys. Lett. 1985, 117, 99-102. (4) Law, K. S.; Schauer, M., Bernstein, E. R. J . Chem. Phys. 1984, 81, 4871-4882. (5) Bornsen, K. 0.;Selzle, H. L.; Schlag, E. W. Z . Narurforsch. A 1984, 3 9 4 1255-1258. (6) Fung, K. H.; Selzle, H. L.; Schlag, E. W. J . Phys. Chem. 1983, 87, 51 13-51 16. (1) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981,85, 3139-3 142. (8) Langridge-Smith, P. R. R.; Brumbaugh, D. V.; Haynam, C. A,; Levy, D. H. J . Phvs. Chem. 1981. 85. 3142-3146. (9) Vernbn, M . F.; Lisy,'J. M.;Kwok, H. S.; Krajnovich, D. J.; Tramer, A.; Shen, Y. R.; Lee, Y. T. J . Chem. Phys. 1981,85, 3321-3333. (10) Steed, J. M.; Dixon, T. A,; Klemperer, W. J . Chew?.Phys. 1979, 70, 4940-4946. (11) Janda, K. C.; Hemminger, J. C.; Winn, J. S.; Novick, S. E.; Harris, S. J.; Klemperer, W. J . Chem. Phys. 1975, 63, 1419-1421. (12) van de Waal, B. W. Chem. Phys. Lett. 1986, 123, 69-72. (13) Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1985,82, 3722-3727. (14) Pawliszyn, J.; Szczesniak, M. M.; Scheiner, S. J . Phys. Chem. 1984, 88. 1726-17m - -- (15) Karlstrom, G.; Linse, P.; Wallqvist, A.; Jonsson, B. J . Am. Chem. Soc. 1983. 105, 3111-3182. (16) Fraga, S. J . Comput. Chem. 1982, 3, 329-334. (17) Odutola, .I. A,; Alvis, D. L.; Curtis, C. W.; Dyke, T. R. Mol. Phys. 1981, 42, 267-282. (18) MacRury, T. B.; Steele, W. A.; Berne, B. J. J . Chem. Phys. 1976,64, 1288-1 299. (19) Evans, D. J.; Watts, R. 0. Mol. Phys. 1976, 31, 83-96.
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0 1987 American Chemical Society
