6864
J. Phys. Chem. 1992,96,6864-6866
Twisted Internal Charge Transfer in (Aminophenyi)pyridinium Peter Fromberz' and Andrea Heilemann Abteilung Biophysik der Universitdt Ulm. D-7900 Ulm- Eselsberg, Germany (Received: May 15, 1992; I n Final Form: July 6, 1992)
The electron donor dimethylaniline and the electron acceptor methylpyridinium are coupled with a twistable bond as [(dimethylamino)phenyl]methylpyridinium and in the rigid frame of fluorene as (dimethy1amino)methylazafluorenium. The fluorescence of both chromophores is studied. The quantum yield of the rigid dye is around 50%. The quantum yield of the twistable homolog is low. It drops to about 0.05% in solvents of high polarity and fluidity. The difference is assigned to the formation of a TICT state (twisted internal charge transfer) with efficient radiationless deactivation.
latrduction
Derivatives of (aminostyry1)pyridinium are used as voltagesensitive probes in neuron membranes.Iv2 It is our goal to improve these dyes with respect to fluorescence intensity, voltage sensitivity, and bimmpatibility such that they can be used as reliable tools to elucidate signal processing within the arborization of a single neuron at a high spatio-temporal resol~tion.~ The basis for such a development is a rationalization of the physical mechanism of voltage sensitivity and more generally of the photophysics and photochemistry of these dyes.4" The fluorescence of (aminostyry1)pyridinium is competed with by photoisomerismand by a secondsolventdependent-process of radiationless decay.5 We assigned this process tentatively to rotamerism to a state of enhanced polarity with efficient internal conversion: Le., to twisted intemal charge transfer (TICT).' All features of this radiationless deactivation were reproduced in (aminopheny1)pyridiniumwhere any interference of photoisomerization is eliminated! We assigned the TICT process tentatively to a twist of the central bond between aniline and pyridine.6 To complete this line of reasoning, we suppress now the twist around the bond between aniline and pyridine in some analogy to the classical approach used for p(dimethylamino)bemnitrile.' We compare (dimethy1amino)methylazafluorenium tetrafluoroborate (dye I) with [(dimethylamino)phenyl]methylpyridinium tetrafluoroborate (dye 11).
Materials and Methuds Syntheses. 2-Methy 1- 7-(dimet hy1omino)-2-azafluorenium Tetrafluoroborate. 3-Mesitoyl-4-phenylpyridine was prepared in three stepss by a Friedel-Crafts reaction of nicotinyl chloride hydrochloride with mesitylene, by a Grignard reaction with bromobenzene and by dehydrogenation with chloranil. Cyclization to 2-azafluorenon was attained in polyphosphoric acid.8 7Amino-2-azafluorene was made in two steps9 by nitration with KN03/H2S04to 7-nitro-2-azafluorenoneand by reduction with zinc amalgame/HCl. Methyl groups were introduced with trimethyloxonium tetrafluoroborate.IO I-Methyl-4-[4-(dimethylamino)phenyl]pyridinium Tetrafluoroborate. N,N-Dimethylaniline and pyridine were coupled in the presence of A1Cl3 and trichloro-s-triazine." The 4-[4(dimethy1amino)phenyllpyridine was methylated with methyl 0022-3654/92/2096-6864S03.00/0
iodide.I2 Silver tetrafluoroborate was applied to exchange the counterion. All substances were identified by MS and IH NMR. Purity was checked by TLC. Spectrr. The solutions of the dyes in various solvents (1.68 pM for dye 1,3.32 pM for dye 11) were prepared from ethanolic stock solutions (0.25 mM for dye I, 1 mM for dye 11). They were not deaerated. Solvents of highest available purity were used (Merck/Darmstadt, Flulra/Neu-Ulm). Absorption spectra were measured at 25 OC in a Cary 219 photometer. Corrected fluorescence spectra were recorded at 25 OC by a Spex Fluorolog fluorimeter (excitation at 404 nm for dye I, 436 nm for dye 11, spectral width 0.85-4.25 nm). As standards for the quantum yield of fluorescence, we used coumarin Ii3 (Aldrich/Steinheim) in ethanol with a yield of 0.64 and acridine yellow (Aldrich) in ethanol with a yield of 0.47 for dyes I and 11, respectively.I3
Results .wl Discussion Quantum Yield of Fluorescence. We measured the quantum yield @F of dyes I and I1 in 18 solvents. The results are plotted in Figure 1 versus the logarithm of the dielectric constant t. The quantum yield of dye I is high, between 83% and 18%. There is a tendency that polar solvents lower the yield. The quantum yield of dye I1 is considerably lower. The effect of the environment is much more pronounced. In polar solvents the yield is only around 0.05%.14Comparing solvents of similar polarity, we observe a lower quantum yield in the solvent of lower viscosity, e.g., @F = 1.58%/0.6% in cyclopentanol (9.6 cP)/nbutanol (2.6 cP) or @F = 0.82%/0.1 l%/OW%in ethylene glyopl (16.8 cP)/dimethylformamide (0.8 cP)/acetonitrile (0.35 cP). The data may indicate that a process of radiationless deactivation is allowed in the twistable dye I1 that is promoted by polar and fluid environments. It is suggestive to postulate a twist of [(dimethylamino)pheny1]methylpyridinium in its excited state leading to a polar TICT state that is deactivated by internal conversion. However, to support such a TICT mechanism of radiatidw deactivation, we have to exclude first that the lowered yield of fluorescence is caused by solvent-dependent radiative decay. RadMve m y . We evaluate the rate constants kFof radiative We use decay from the spectra of absorption and fl~0rescence.l~ eq 1 with the maximal extinction coefficient tA,the wavenumber pA of maximal absorption, the half-width HWA of the absorption spectrum, the wavenumber vF of maximal fluorescence and the refractive index n: kF = 8m(ln 1 0 ) t ~ H W ~ n ~ e F ~ / s ~
(1)
The results are plotted in Figure 2. The rate constants kF are rather similar for both dyes and independent of the solvent. We find for dye I an average value kF = 0.15 ns-' and for dye 11 an average value kF = 0.17 ns-' . These results are confirmed by an evaluation of the rate constants from the quantum yield and the lifetime of fluorescence as kF = @F/:F.'~ The data indicate that the electronic structure of both dyes is similar, and it proves that the low, solvent-dependent quantum yield of fluorescence for dye 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 6865
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Figure 1. Logarithm of the quantum yield of fluorescence 9Fversus logarithm of dielectric constant c for dye I (dots) and dye I1 (circles). The solvents (c) are chloroform (4.72), decanol (8.1), dichloromethane (8.93), octanol (9.33, benzyl alcohol (13.1), hexanol (13.3), pcntanol (13.9), cyclohexanol (15.0),butanol (17.51), cyclopentanol (18.0), propanol (20.33), acetone (20.7), ethanol (24.55), methanol (32.7), acetonitrile (36.7), dimethylformamide (37.0), ethylene glycol (37.7), water (78.4).
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3. Wavenumbers v., and % of the maxima of absorption (squares) and fluoracence (circles) versus logarithm of dielectric constant c for dye I (full symbols) and dye I1 (open symbols). F ' i i
package MOPAC.~-'*~'~ Dye I is found to be almost planar. In the ground state 74%of the charge is in the pyridinium moiety. In the excited state 62% of charge is in the aniline moiety. By excitation the center of the charge distribution is shifted by 0.17 nm along the axis of the central bond. The excitation energy is 24 600 cm-'. Dye I1 is twisted by 49' with respect to the two planes of the aromatic rings; 84% of charge is in the pyridinium moiety. In the excited Franck-Condon state (at 49') 69%of charge is in the aniline moiety. By excitation the center of charge distribution is shifted by 0.24 nm. The excitation energy is 22 100 an-';90% of charge becomes localized in the aniline moiety by an enhanced twist of 90' in the excited state. The computation confirms the similar electronic structure of dyes I and I1 in the presumed light-absorbingand light-emitting conformations,and it indicates the possibility of twisted internal charge transfer of dye I1 from the conformation attained by Franck-Condon excitation. General Remarks. The results show that the low, solvent-dependent yield of fluorescence in [ (dimethy1amino)phenyllmethylpyridinium is caused by a nonfluorescent TICT state, twisted around the anilinepyridine bond. Let us summarize the arguments: (i) The methylene bridge of dye I has no major effect besides the suppression of rotamerism. The interaction of the solvent with [(dimethylamino)phenyl]pyridinium and with (dimethy1amino)methylazafluorenium is similar. (ii) The rate constant of radiationless deactivation of [(dimethylamino) phenyllmethylpyridinium is enhanced by a factor of lo00 in fluid, polar solvents as compared to (dimethy1amino)methylazafluorenium. (iii) Enhanced twist around the anilinepyridine bond in the excited state gives rise to an almost complete accumulation of electrical charge in the aniline moiety. That is, a TICT state is formed that is stabilized in a polar environment. The features of solvent-dependent fluorescence were found to be almost identical for homologous derivatives of biphenyl and of stilbene-(aminopheny1)pyridinium dyes and (aminostyry1)pyridinium It is likely that the mechanism of radiationless deactivation is identical. The present observations confirm the suggestion that a nonfluorescent TICT state is formed in the stilbene by rotamerism around one of the phenyl-ethylene bonds.s These features of (aminostyry1)pyridinium differ from a closely related derivative of stilbene where benzonitrile is the electron acceptor: Immobilization of both phenyl-ethylene bonds in (dimethy1amino)cyanostilbene lowers the yield of fluorescence, an effect which is assigned to the suppressed formation of a fluorescent TICT state.20 Finally we may note that the profile of fluorescence yield for (dimethy1amino)methylazafluorenium (Figure 1)-though weak-exhibits some similarities to the profile for [(dimethylamino)phenyl]methylpyridinium. Whether another TICT transition is involved-with a twist of the dimethylamino groupcannot be decided.
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Figure 2. Rate constants kF of radiative decay versus logarithm of dielectric constant t for dye I (dots) and dye I1 (circles). The rate constants are computed from the absorption and emission spectra (Strickler-Berg formula).
I1 cannot be due to reduced and solvent dependent radiative decay. We may evaluate the rate constant kNRfor nonradiative decay from the quantum yield aFand the rate constant kF according to eq 2. Combining the data of Figures 1 and 2, we find that nonradative decay is enhanced in polar solvents by a factor up to about lo00 for dye I1 as compared to dye I. It is likely that the enhanced radiationless deactivation is due to the twist in dye I1 toward a nonfluorescent TICT state. However, to support this mechanism, we have to exclude a kind of specific interaction of the solvents with the excited state of dye I1 that is not effective for dye I and that could cause fast nonradiative decay. spectra of Absorptioa ~F IU b"I e We obtain independent information on the interaction of the excited dyes with the solvent from the absorption and emission spectra. The wavenumbers pA and pF of the maxima of absorption and fluorescence are plotted in Figure 3 versus the logarithm of the dielectric constant of 18 solvents. From pA and gF we may evaluate the 00 energy goo = (vA iiF/2 which reflects the energy difference of the solvated excited state and of the solvated ground state. Considering Figure 3, we observe that iiw is rather similar for dyes I and I1 and depends little on the polarity of the solvent. The result confirms that the electronic structure of both dyes is similar. There is no indication for a spectacular change of solvation of the light-emitting state that may cause the fast solvent-dependent nonradiative deactivation of dye 11. MNDO compubtiaa. We evaluated the distribution of charge of dye I and dye I1 by the MNDO model using the program
+
J. Phys. Chem. 1992, 96, 6866-6869
6866
Conclusions A TICT state with fast radiationless deactivation is formed by rotamerism around the central single bond in [(dimethylamino)phenyl]methylpyridinium. Suppression of rotamerism enhances the yield of fluorescence by orders of magnitude. Similar nonfluorescent TICT states are formed in homologs with a double bond between aniline and pyridine or with derivatives of naphthalene as donor and acceptor.ss21 These dyes are used as voltage-sensitive fluorescence probes in neuron membranes.2 By suppression of rotamerism we may expect to obtain voltagesensitive probes with a high yield of fluorescence, i.e., an enhanced signal-to-noise ratio at given conditions of staining and illumination. Acknowledgment. We thank Dr. C. Riicker for measuring fluorescence lifetimes and Uwe Theilen for technical assistance. The project was supported by the Deutsche Forschungsgemeinschaft (Grant Fr 349/5)and by the Fonds der Chemischen Industrie. References and Notes (1) Cohen, L. B.; Lesher, S. In Optical Merhods in Cell Physiology; de Weer, P., Salzberg, B. M., Eds.; Wiley: New York, 1986; p 71. (2) Fromherz. P.: Dambacher. K. H.: Eohardt. H.: Lambacher. A.: Miiller. C. 0.; Neigl, R.;'Schaden, H.; Ghenk, 0.: Vetter, T: Ber. Bunsen-Ges. Phys: Chem. 1991, 95, 1333. (3) Fromherz, P.; Vetter, T. Proc. Narl. Acad. Sci. U.S.A. 1992,89, 2041. (4) Fromherz, P.; Lambacher, A. Biochim. Biophys. Acta 1991,1068, 149.
(5) Ephardt, H.; Fromherz, P. J . Phys. Chem. 1989, 93, 7717. (6) Ephardt, H.; Fromherz, P. J . Phys. Chem. 1991, 95, 6792. (7) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A,; Cowley, J.; Baumann, W. Nouv.J. Chim. 1979, 3, 343. (8) Fuson, R. C.; Miller, J. A. J . Am. Chem. SOC.1957, 79, 3477. (9) PCron-Roussel, 0.; Jacquignon, M. P. C . R. Acad. Sci. Paris C 1974, 278, 219. (10) Reichardt, C.; Kaufmann, N. Chem. Ber. 1985, 118, 3424. (1 1) Migachev, G. I.; Stepanov, B. I. J . Gen. Chem. USSR 1968,38,1320. (12) Bergmann, E. D.; Crane, F. E.; Fuoss, R. M. J . Am. Chem. Soc. 1952, 74, 5979. (13) Olmsted 111, J. J. Phys. Chem. 1979, 83, 2581. (14) The fluorescence yield in chloroform (*F = 1.5% at c = 4.72) is by a factor of 10 lower than expected from the general trend in Figure 1. The
solubility of the dye in chloroform is low. Selective solvation by ethanolpresent in all solutions at a volume fraction of 0.3%-may play a significant role. (15) Strickler, S. J.; Berg, R. A. J . Chem. Phys. 1962, 37, 814. (16) Lifetime and quantum yield of fluorescence for dye I in methanol are T F = 2.0 ns and +F = 0.28. We obtain k~ = *F/TF = 0.14 ns-'. For an analog of dye I1 [(dibutylamino)phenyI]pyridinium butylsulfonate) in methanol, we obtained from T~ = 150 ps and OF = 0.01 a value kF = + F / T F = 0.16 ns-1.6 (17) The spectra of dye I1 are identical to those of [(dimethylamino)phenyl]pyridinium butylsulfonate and very similar to those of [(dibutylamino)phenyl]pyridinium butylsulfonate.6 The solvatochromism is independent on the counterion. It is a sole property of the charged chromophore itself. (18) Dewar, M. J. S.; Thiel, W. J. J . Am. Chem. SOC.1977, 99, 4899. (19) Stewart, J. J. P. MOPAC Manual, 2nd ed.; University of Texas: Austin, TX; QCPE No. 455. (20) Rettig, W.; Majenz, W.; Lapouyade, R.; Haucke, G. J . Phorochem. Phorobiol. A: Chem. 1992, 62, 415. (21) Ephardt, H.; Fromherz, P., manuscript in preparation.
Fullerene Formation in Sputtering and Electron Beam Evaporation Processes Rointan F. Bunshah,* Shyankay Jou, Shiva Prakash, Hans J. Doerr, Department of Material Sciences and Engineering, University of California, LQS Angeles, California 90024- 1595
Lyle Isaacs, Arno Wehrsig, Chahan Yeretzian, Hyunchae Cynn, Department of Chemistry and Biochemistry, University of California, LQs Angeles, California 90024- 1569
and Franqois Diederich* Laboratorium fur Organische Chemie, ETH- Zentrum, Universitatsstrasse 16, CH-8092 Zurich, Switzerland (Received: May 24, 1992; In Final Form: June 30, 1992)
We report the formation of fullerenes from graphite by sputtering and electron beam evaporation. Under conditions that differ dramatically from those in the previously known fullerene production processes, the new methods preferentially yield the soluble higher fullerenes C70, C76, C78rand CS4in addition to minor amounts of CWonly. Upon passage of carbon prticles formed by electron beam evaporation through an electrostatic field, fullerenes are mainly isolated from the cathode, not from the anode, which supports the formation of cationic intermediates in fullerene growth mechanism. The variables thought to be important for fullerene production can be controlled efficiently in the new processes.
Introduction The availability of gram quantities of Ca by resistive',* or arc heating3 of graphite has led to a period of intense ongoing research into the chemical, physical, and materials properties of this first molecular allotrope of arbo on.^,^ In addition to c 6 0 (=60-70%), the toluene-extractable fraction of the carbon soot formed in both the resistive and arc heating processes contains C70(20-30%) and approximately 5% of higher fullerenes in the range between c76 and C96.6 The isolation of the higher fullerenes, e.g., D&6, C2u-C78r and D3-C78,from this mixture i s tedious and yields only limited amounts of pure material, which explains why the properties of these carbon spheres remain almost entirely unexplored. Since first investigations now show that the chemistry of the higher fullerenes promises to be diverse and distinctively different from the chemistry of buckminsterfullerene and C70: a search for new fullerene preparations yielding preferentially the higher derivatives
represents a worthwhile research target. Besides the commonly used resistive and arc heating processes, fullerene preparations by laser vaporization of a rotating graphite target in a tube furnace8 and by inductive heating of graphite powder9 have been reported. In both processes, Ca is by far the predominant product. Different C@/C70ratios have been isolated from the soot produced in oxidizing benzene flames and, depending on the conditions, variation of the c 7 0 / c 6 0 ratio over the range 0.26-5.7 was achieved.1° We now report that fullerenes are formed by vaporization of graphite in sputtering and electron beam evaporation processes and that both methods consistently yield preferentially C7,, and the higher fullerenes with only minor amounts of c60. Experimental Section The experimental setup for fullerene growth in sputtering processes is shown in Figure 1. A 7.6-cm-diameter graphite target
0022-365419212096-6866%03.00/0 0 1992 American Chemical Society
