J . Phys. Chem. 1989, 93, 2347-2358 trans-octatetraene and have reassigned it using ab initio S C F frequencies scaled by our technique. Although they do not correct for electron correlation as we do here, their scaled results are in good agreement with ours. Their proposed final assignment of the light molecule is essentially identical with ours. It is particularly pleasing that our prediction of a medium infrared band near 1580 cm-' has been borne out by the new experiment. Several other weak bands that were predicted by theory have also been found. Another, very recent paper on this subject has been published by Zerbetto et al. [ J . Chem. Phys. 1988, 89, 36811.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American
2347
Chemistry Society, for support of this research. The calculations were performed with a Celerity 1200 minicomputer, purchased with support of the National Science Foundation, under Grant NO. CH-8500487. Registry No.
all-trans-l,3,5,7-0ctatetraene,3725-31-3; all-cis-
1,3,5,7-octatetraene, 1871-50-7.
SuppBementary Material Available: Definition of the in-plane and out-of-plane internal coordinates (Tables VI11 and IX) and the scaled force constant matrices for all-trans- and all-cis-octatetraene (Table X-XIII) (6 pages). Ordering information is given on any current masthead page.
Twisted Internal Charge Transfer Molecules: Already Twisted in the Ground State C. Cazeau-Dubroca,* S. Ait Lyazidi, P. Cambou, A. Peirigua, Centre de Physique MolPculaire Optique et Hertzienne, UA 283, UniuersitP de Bordeaux I , 351 Cours de la LibPration, 33405 Talence. France
Ph. Cazeau, Laboratoire de Chimie Organique de I'Etain et du Silicium, UA 35, UniversitP de Bordeaux I , 351 C o w s de la LibPration, 33405 Talence, France
and M. Pesquer Laboratoire de Physico-chimie ThPorique, UA 503, UniuersitP de Bordeaux I, 351 Cours de la Liberation, 33405 Talence, France (Received: September 4, 1986; In Final Form: April 25, 1988) Starting with the experimental spectroscopicstudy of para-substituted N,N-dialkylanilines in solutions, we show the deforming effect of hydrogen bonds on the conformation of the amine in the ground state, which is planar when the molecule is isolated. The twisted conformation thus acquired causes spectroscopicanomalies: anomalous red shift (ARS) and dual fluorescence. We simulated by intermolecular interaction calculations the twisting influence of water molecules on the amine in the ground state. These simulationsconfirm our hypothesis. The importance of the twisting is seen to increase as more water molecules-up to three-cluster around the amine. We made a correlation between the relative intensity of the anomalous fluorescence and the probability of a twisted conformation of the molecule, as related to the quantity of water contained in the solvent. In general, we conclude that the molecule must have a twisted conformation when still in the ground state to allow the anomalous fluorescence to appear.
Introduction The anomalous dual fluorescence, discovered by Lippert' for p-(dimethy1amino)benzonitrile (DMABN) in solution, has provoked studies of the spectroscopy of stationary s t a t e ~ ' - ~as* well as of time-resolved s p e c t r o ~ c o p y . ' ~Several -~~ models have been proposed (successively) to account for the anomaly. Lippert et al.' suggested an inversion of states SI and S2due to the solvent; Mc Glynn et proposed the formation of an excimer, Kosower et ale3thought that a proton was transferred in the excited state, Chandross et aL4proposed a complexation with the solvent, Visser et suggested formation of exciplexes with free electron pairs of the solvent, and Rotkiewicz and Grabowski worked out the so-called twisted internal charge transfer (TICT) mode1.6T21 This model shows clearly that the abnormal fluorescence Fa, of greatest ~
~~~
~~
~
(1) Lippert, E.; Luder, W.; Boos, H. In Aduances in Molecular Spectroscopy; Mangini, A., Ed.; Pergamon: Oxford, 1962; p 443. (2) Khalil, 0.S.; Hofeldt, R. H.; Mc Glynn, S . P. Chem. Phys. Lett. 1972, 19, 479. Khalil, 0.S.;Hofeldt, R. H.; Mc Glynn, S. P.J. Lumin. 1973, 6, 229. Khalil, 0. S.; Meeks, J. L.; Mc Glynn, S. P. Chem. Phys. Lett. 1976, 39, 457. (3) Dodiuk, H.; Kosower, E. M. Chem. Phys. Lett. 1975, 34, 253. Kosower, E. M.; Dodiuk, H. J. Am. Chem. SOC.1976, 98, 924. (4) Chandrass, E. A. Exciplex; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975; p 187. (5) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z . R. Chem. Phys. Lett. 1973, 19, 315. (6) Rotkiewicz, K.; Grabowski, Z . R.; Krowczynski, A,; Kiinnle, W. J. Lumin. 1976, 12/13, 877.
0022-3654/89/2093-2347$01 S O / O
wavelength, is due to a state with a strong charge-transfer character. This character is brought about by a twisted conformation that the molecule acquires in the excited TICT state. The idea of TICT molecules has since been extended to other classes of molecules with anomalous double fluorescence. These molecules have either an aromatic structure with an acceptor and a donor group in para positions21cg22or two identical aromatic (7) Lipinski, J.; Chojnacki, H.; Grabowski, Z. R.; Rotkiewicz, K. Chem. Phvs. Lett. 1978. 58. 379. 18) Kirkow-Kaminska, E.; Rotkiewicz, K.; Grabowska, A. Chem. Phys. Lett. 1978, 58, 379. (9) Rettig, W.; Wermuth, G.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1979, 83. 692. (10) Rettig, W.; Bonavicic-Koutecky, V . Chem. Phys. Lett. 1979, 62, 115. Rettie. W.: Zander. M. Chem. Phvs. Lett. 1982. 87. 229. (1:) Nakashima; N.; Inoue, H.IMataga, N.; Yamanaka, C. Bull. Chem. Soc. Jpn. 1973, 46, 2288. (12) Nitsche, K. S.; Suppan, P. Chimia 1982, 36(9), 346. (13) Suman, P.; Guerrv-Buttv, E. J. Lumin. 1985, 33, 335. (14) Nikashima, N.; Mataga,-N.Bull. Chem. SOC.Jpn. 1973.46, 3016. (15) Struve, W. S.; Rentzepis, P. M.; Jortner, J. J . Chem. Phys. 1973, 59,
5014.
(16) Struve, W. S . ; Rentzepis, P. M. J. Mol. Sci. 1978, 47, 273. ( 1 7) (a) Struve, W. S.; Rentzepis, P. M. J. Chem. Phys. 1974,60(4), 1533. (b) Struve, W. S.; Rentzepis, P. M. Chem. Phys. Lett. 1974, 29(1), 23. (c) Huppert, D.; Rand, S. D.; Rentzepis, P. M.; Barbara, P. F.; Struve, W.S.; Grabowski, Z. R. J. Chem. Phys. 1981, 75(12), 5714. (d) Hilinski, E. F.; Rentzepis, P. M. Acc. Chem. Res. 1983, 16, 224.
(18) (a) Wang, Y.; Mc Auliffe, M.; Novak, F.; Eisenthal, K. B. J. Chem. Phys. 1981, 85, 3736. (b) Wang, Y.; Crawford, M. C.; Eisenthal, K. B. J. Am. Chem. SOC.1982, 104, 5874. (c) Wang, Y.; Eisenthal, K. B. J . Chem. Phys. 1982, 77(12), 6076.
0 1989 American Chemical Society
2348 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 structures that can bend about a single bound, such as b i a n t h r ~ l . ~ ~ In general, the appearance of TICT states seems to be favored by polar However, anomalous fluorescence has been observed in nonpolar environments: For instance, ethyl 4-(diethy1amino)benzoate (DEABEE) fluoresces abnormally in a nonpolar solvent.24 When this flexible molecule is isolated, its ground state is planar. Anomalous fluorescence is detected also from 4-cyano-N,N,2,6-tetramethylaniline(CTMA), another flexible molecule which is already twisted at a 60' angle in the ground state.25 In the above-mentioned studies (i.e., TICT), the authors have almost exclusively focused on the behavior of the excited state. In the simplest TICT theory, the molecule either completes its twist in the excited state before emitting (e.g., CTMA, which is partially twisted in the ground state) or makes the complete twist in the excited state (e.g., DMABN and other para-substituted N,N-dialkylanilines that are flat in the ground state). Recently, photoelectronic spectroscopy has given evidence that the latter are indeed planar in the ground states.26 However, some authors have suggested that there could be cases of very local complexation between the solvent and the molecule in the ground state, for instance, by hydrogen bonds between DMABN and butanol. Even more recently, Ottolenghi has stressed the importance of hydrogen bonding between DMABN in the ground state and silica gel.27 Various mechanisms have been suggested to account for the dual fluorescence of DMABN; however, two models are more generally invoked: (1) The TICT model, where the solvent is treated as a continuum using classical physics parameters (polarity, 6 , refractive index, ...).5-13 (2) The exciplex model, where the solvent plays an associative role and so must be described by a quantum mechanical model.20 Above all other arguments, the presence of dual fluorescence of the 3,5-dimethyl-4-(N,N-dimethylamino)b e n ~ o n i t r i l ein~ ~the gas phase is inconsistent with the exciplex mode120 (absence of specific solvent interaction). On the other ratio (abnormal and normal hand, the enhancement of the IF,/IFb fluorescence intensities) by the addition of traces of particular solvents is consistent with the exciplex model and disagrees with the TICT modeLZ' By contrast to these models, which pay attention to only the excited state, we have been particularly interested in the ground state. For non-parasubstituted N,N-dialkylanilines, we have shown an abnormal behavior of the absorption (collapse of the second absorption band as a function of temperature) at very low temperature and in a specific environment. We attributed this behavior to the acquisition of a twisted conformation.28a Moreover, (19) (a) Heisel, F.; Miehe, J. A. Chem. Phys. Left. 1983,100(2), 183. (b) Heisel, F.; Miehe, J. A.; Martinho, J. M. G. Chem. Phys. 1985, 98, 241. (c) Heisel, F.; Miehe, J. A. Chem. Phys. 1985, 98, 243. (20) (a) Visser, R. J.; Varma, C. A. G. 0.J . Chem. SOC.,Faraday Trans. 2 1980, 76, 453. (b) Visser, R. J.; Varma, C. A. G. 0.;Konijnenberg, J.; Bergwerf, P. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 347. (c) Visser, R. J.; Varma, C. A. G. 0.;Konijnenberg, J.; Weisenborn, P. C. M. J . Mol. Struct. 1984, 114, 105. (d) Visser, R. J.; Weisenborn, P. C. M.; Varma, C. A. G. 0.;Dehaas, M. P.; Warman, J. M. Chem. Phys. Lett. 1984, 104, 38. ( e ) Visser, R. J.; Weisenborn, P. C. M.; Varma, C. A. G . 0. Chem. Phys. Lett. 1985, 113, 330. (21) (a) Rotkiewicz, K.; Grellman, R. H.; Grabowski, Z. R. Chem. Phys. Lett. 1973,19, 315. (b) Rotkiewicz, K.; Grabowski, Z . R.;Krowczynski, A. J . Lumin. 1976, 12/13, 877. (c) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A,; Cowley, D. J.; Baumann, W. Nouo. J. Chim. 1979,3(7), 443. (d) Lipinski, J.; Chojnacki, H.; Grabowski, Z . R.; Rotkiewicz, K. Chem. Phys. Lett. 1980,70(3),449. (e) Grabowski, Z. R.; Dobkowski, J. Pure Appl. Chem. 1983, 55, 245. (22) (a) Rettig, W. J . Phys. Chem. 1982,86, 1970. (b) Lippert, E.; Rettig, W. J . Mol. Struct. 1979, 45, 373. (c) Rettig, W. J. Lumin. 1980, 26, 21. (d) Rettig, W.; Gleiter, R. J . Phys. Chem. 1985, 89, 4676. ( e ) Rettig, W.; Wermuth, G. J . Photochem. 1985, 28, 351. (23) (a) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1155. (b) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1970, 74, 624. (c) Beens, H.; Weller, H. Chem. Phys. Lett. 1969.3.666. (d) Rettig, W.; Zander, M. Ber. Bunsen-Ges.Phys. Chem. 1983,87, 1143. ( e ) Yamasaki, K.; Arita, K.; Kajimoto, 0.;Hara, K . Chem. Phys. Lett. 1986, 123(4), 277. (24) Ayuk, A. A.; Rettig, W.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 553. (25) Rotkiewicz, K.; Rubaszewska, W. J . Lumin. 1982, 27, 221. (26) Rettig, W. Nouu. J . Chim. 1983, 7(7), 425. (27) Levy, A.; Avnir, D.; Ottolenghi, M . Chem. Phys. Lett. 1985, 121, 233.
Cazeau-Dubroca et al. this abnormal absorption was strongly correlated with an emission abnormality of the intensity ratio, Iph@ph,,/Iflu,,, as a function of wavelength. This last abnormality is due to the fact that the Sz state acquires charge-transfer character because of the twisted conformation of the groundstate.28b*c Similar behavior of the emission has been reported for p - n i t r ~ a n i l i n e s . ~ ~ On the other hand, our recent shows that the anomalous fluorescence Fa of para-substituted N,N-dialkylanilines (flexible molecules that are flat when isolated) is due to the twisting of these molecules by hydrogen bonding when traces of water are present in the solvent. A conformational simulation of these complexed molecules seemed necessary. We did this by computing intermolecular interactions. These calculations are explained in the theoretical part of this article after a brief summary of our experimental results.
Experimental Details The experimental techniques and sources of the materials have been described earlier.29 In the case of water-controlled experiments, it is absolutely necessary to prepare the solutions and to measure the spectra under inert atmosphere (argon) to prevent any pollution by atmospheric moisture. Experimental Results and Discussion Origin of the Anomalous Red Shift (ARS) Absorption Spectra: Twist ofthe Ground State. Let us first recall experimental results that we already reported and some complementary results concerning the more or less twisted conformation of a TICT molecule in the ground state. Spectroscopic study of flexible TICT molecules which are flat when isolated (such as para-substituted N,N-dialkylanilines) has led us to conclude that their anomalous fluorescence in non-hydroxylic solvents is due to formation of complexes before excitation between the TICT molecules and the traces of water in the solvent.29 A consequence of this complexation by hydrogen bonding is that the TICT molecule acquires a twisted conformation, even though it is still in its ground state. This twisted conformation, causes an anomalous red shift (ARS) in the absorption spectrum; namely, the absorption shifts to longer wavelengths as the temperature is lowered (see Figure 1A). This ARS is observed in a very general way for all flexible molecules that are planar when isolated and in all the nonhydroxylic solvents studied, both polar and nonpolar (e.g., methylcyclohexane; a mixture of methylcyclopentane with methylcyclohexane; dipropyl ether; butyl chloride). This ARS is independent of the concentration in the large range of concentration (lod < C C lo4 M), and thus it cannot be explained by any self-aggregation formati01-1.~~ Origin of the Twist of the Ground State: Hydrogen-Bonded Complexes. Role of Traces of Water Present as Impurities in the Solvents. When the solvents are thoroughly dried, and the experiment is carried out under an inert atmosphere, the ARS disappears almost completely. Moreover, no ARS appears when the molecules are trapped in a perfectly waterproof matrix (polymer matrices such as poly(viny1 chloride), poly(methy1 methacrylate), or polyethylene) or hydrophobic solvents such as silicones. In these cases, we only observed the normal effect of lowering the temperature. In Figure 2A,B, we compare the effect of lowering the temperature on the absorption spectra of solutions of DMABN respectively in an ether (dipropyl ether) containing traces of water and in hydrophobic polyether (silicone) without any traces of water. In the ether, an ARS with Au N 2000 cm-l is observed, while in the silicone no abnormal shift is observed. It should be mentioned that for rigid, intrinsically plane ( a = 0') or intrinsically twisted ( a = 60') molecules in commercial solvents, that is, containing traces of water, no ARS was observed upon (28) (a) Dubroca, C. Chem. Phys. Lett. 1972, 15(2), 207. (b) Dubroca, C.; Lozano, P. Chem. Phys. Lett. 1974, 24( 1). 49. (c) Czaeau-Dubroca, C. Thbe d'Etat, Universitt de Bordeaux I, 1976. (d) Cazeau-Dubroca, C. J . Lumin. 1984, 29(3), 349. ( e ) Kadiri, A,; Martinaud, M.; Cazeau-Dubroca, C. Chem. Phys. Lett. 1979, 65(3), 484. (29) Cazeau-Dubroca, C.; Ait-Lyazidi, S.;Nouchi, G.; Peirigua, A,; Cazeau, Ph. N o w . J . Chim. 1986, 10, 337.
TICT Molecules: Already Twisted in the Ground State
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2349
36363 35007 33390 32 706 31 746 30 769 29050 I
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Figure 1. (A) Absorption spectra of DMABN in n-butyl chloride at various temperatures; 10" < C < lo4 M. (B) Absorption spectra of p-cyanoN-ethylindoline in n-butyl chloride at various temperatures; 10" < C < lo4 M. (C) Absorption spectra of CTMA in n-butyl chloride at various M) at room temperature in commercial solvent temperatures; lod < C < lo4 M. (D) Excitation spectra of DMABN in n-butyl chloride (C E (bold line) in extra-dry solvent and under inert atmosphere (-),
cooling; see Figure 1B,C. Similarly, the ARS disappears in very dry solvent, as in the case of p-(dimethy1amino)benzaldehyde (DMABA) solutions in methylcyclohexane (MCH) (Figure 2C,D). We conclude therefore that a significant ARS can only be observed for flexible molecules that are flat when isolated (DMABN, etc.). These molecules require but little energy (about 8 kcal to twist around their C-N bond in the ground state. This ARS is consistent with the assumption that the molecule takes on a more and more twisted conformation as it acquires more hydrogen bonds (see next section, the CNDO simulated results). Indeed, the absorption spectrum of complexed molecules, stabilized at low temperatures, shows a maximum at about 310 nm. This value is similar to that observed for the twisted CTMA molecule, whose maximum does not vary with temperature, while the rigid, flat molecule p-cyano-N-ethylindoline absorbs most at 295 nm (see Figure IB,C). (30) (a) Cazeau-Dubroca, C.; Peirigua, A,; Ait-Lyazidi, S.; Nouchi, G. Chem. Phys. Left. 1983,98(5), 51 1. (b) Cazeau-Dubroca, C.; Peirigua, A,; Ait-Lyazidi, S.; Nouchi, G.; Cazeau, Ph.; Lapouyade, R. Chem. Phys. Lefr. 1986, 124(2), 110; Internafional Workshop on TICT Molecules; Swidno, Poland, 1984. (31) Mc Kenzie, R. K.; Mc Nicol, D. D. Chem. Commun. 1970, 1299.
Origin of Dual Fluorescence: Twisted Hydrogen-Bonded Complexes in the Ground State. 1. No Complex: No Anomalous Fluorescence, F,. For these molecules, flexible and flat in the free state, the twist acquired through hydrogen bonding is responsible for the anomalous fluorescence observed at room temperature. Their anomalous fluuorescence practically disappears in very dry solvents operating under inert atmosphere; see, for instance, the effect of drying on the emission of DMABN in acetonitrile (Figure 3). Thus, both anomalous dual fluorescence and the ARS disappear on thorough drying and do not appear in hydrophobic solvent (like silicone)-cf. Figure 3B. We also observed the effect of drying on the excitation spectra, and we made a correlation with the other spectra. Figure 1D represents the spectra of solutions of DMABN in commercial and thoroughly dried butyl chloride, respectively. In the excitation spectrum, the fluorescence at room temperature shows two obvious maxima, one at 290 nm and the other at 310 nm. After drying, the number of complexed molecules greatly diminishes and so we observe a decrease of the lower energy side-i.e., the 310-nm structure-of the spectrum. After drying, therefore, the excitation spectrum consists mostly of the contributions of "free" molecules, i.e., molecules free from hydrogen bonds, having a flat conformation in the ground state. At room temperature, the number
2350
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
rVC.:'
Cazeau-Dubroca et al.
38461 37037 35714 34183 33335 32258 31250 30303 29412 I
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of complexed molecules in the solution must be small, so that drying has only a very slight effect on the absorption spectrum, only a few nanometers hypsochromic shift.29 The contribution of these complexed molecules to the global absorption spectrum in a commercial solvent appears at the lower energy foot of the band (see Figure 1D). However, in the global excitation spectrum these few complexed twisted, molecules give rise to a very strong contribution (the maximum at 310 nm is as strong as the one at 290 nm; see Figure 1D) which is due to the great efficiency of twisted molecules for TICT fluorescence. This efficiency is well illustrated by the case of hydrogen-bonded polymer (such as poly(viny1 alcohol), polyamide, polyurethane) doped with TICT molecules for which the quantum yield of the anomalous fluorescence is greater than 50%30band therefore much greater than the quantum yield of the Fb fluorescence in s o l ~ t i o n . ~ 2. Exaltation of A R S and Dual Fluorescence in Rigid Hydrogen-Bonding Polymeric Matrices. In these materials, the complexed TICT molecules (in this case complexed with the hydrogen bond matrix) must be practically the only absorbing entities since, when these matrices are made very rigid, an ARS is observed in the absorption spectrum at room temperature (which shows the great stability of the complex). This anomaly is very similar to the one observed in the case of solutions, on lowering the t e m p e r a t ~ r e . ~ ~ , ~ ~ ~ Moreover, anomalies have been observed in neither the absorption (no ARS) nor the emission (no dual fluorescence) spectra for nontwisted molecules included in polymeric matrices where hydrogen bonding is i m p o ~ s i b l e . ~We ~ also noticed that, for matrices with hydrogen bonds, the absorption anomaly as well as the dual fluorescence is very sensitive to the rigidity of the
matrix. Both anomalies are strongest when the matrix is very rigid, which is when the hydrogen bonds are consolidated.30b Just as in solutions, the absorption ARS has a perfect correlation with the anomalous dual fluorescence, and both phenomena also depend on the stability of the complexes. In solutions, this stability is obtained by lowering the temperature while in polymeric matrices at room temperature, it is brought about by stiffening the solvent. In the case of these molecules, it is therefore absolutely essential that they form hydrogen bonds with the matrix in order to see the anomaly of dual fluorescence. Just as in the case of solutions, the observed ARS is, in agreement with a twisted conformation of the TICT molecule in the ground state, a twisting caused by its complexation. The exaltation of the dual fluorescence of these molecules that are flat when free, when they are included in hydrogen-bonded polymers, leads us to question the unfavorable effect viscosity is expected- to exert, as was suggested in the original TICT model.*' 3. Role of Addition of Water on Dual Fluorescence. Not only viscosity was supposed to have an unfavorable effect on the emergence of anomalous fluorescence; it was also thought that a nonpolar solvent was a very unfavorable condition.21d*eThough it is true that polarity has a great influence on the frequency of the anomalous fluorescence, its effect on the intensity is perhaps not as important as was thought. Indeed, when we make a comparison between the anomalous fluorescence, Fa, obtained in an almost nonpolar solvent such as glyme (p = 1.71 D; t = 7.2), which, by the way, is very hydrophilic, and that obtained in a very polar solvent such as acetonitrile ( w = 3.44; t = 3 7 . 9 , we find that the intensities are much the same (see Figure 4B). It appears that although glyme is very weakly polar, it contains almost as
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2351
TICT Molecules: Already Twisted in the Ground State
M solution, and under inert atmosphere, this same initial water concentration is about 100 times less, so the additional effect will be more sensitive. In the same way, the water effect of additional traces of water on the ZF,/ZFb ratio is strongest at the beginning (Figure 3C). Similarly Wang43reported the absorption spectrum of DMABN changes significantly upon the addition of primary amines (contrary to tertiary amines), suggesting hydrogen-bonding interactions. As expected, a controlled addition of water to the solution of DMABN in glyme results in a distinct enhancement of the anomalous fluorescence, Fa, relative to the normal fluorescence, Fb. As the percentage of water in the solution is increased continuously, the relative intensity of Fa grows to a maximum, then decreases, and finally collapses for large amounts of water. See Figure 4B,E. A similar phenomenon has been described by Visser et al. for the addition of trifluoroethanol to solutions of DMABN in d i ~ x a n e . * ~ ~ , ~ * In our experiment on the addition of water to the glyme solution, we show a correlation between the energy shifts in the absorption spectrum and variations in intensity of the anomalous fluorescence Fa (see Figure 4A). In the first part of the experiment, adding water increases the relative intensity of Fa, and the maximum of the absorption spectrum shows a bathochrome shift. This shift becomes bigger, passes through a maximum, and decreases to zero. This point corresponds to the collapse of the relative intensity of Fa. The shift eventually becomes hypsochromic, when very large amounts of water are added.49 We also measured excitation spectra during this experiment. Adding water brings about a change in the excitation spectrum of the normal fluorescence, Fb (see Figure 4 c ) , while that of the anomalous fluorescence, Fa, stays much the same (see Figure 4D). Figure 4D shows in fact the excitation spectrum of the DMABN water complex. This is a twisted (or partially twisted, as explained in the theoretical part below) absorbing entity with an absorption maximum around 310 nm. When it is impossible to form a complex and the molecule is compelled to stay flat, there is no anomalous fluorescence, Fa, and therefore no contribution to the excitation spectrum. Discussion. However, the normal Fbfluorescence is due to both noncomplexed molecules (whose absorption maximum is at 280 nm) and molecules that were partially twisted in the ground state and that, after having been excited, relax toward the plane conformation. The following diagram illustrates relaxation in the excited state, starting either with a partially twisted molecule (e, 305 nm) or with a nontwisted molecule (e, = 280 nm) in the ground state.
A
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M , at room temperature: in commercial solvent (-); in extra-dry solvent (---). X = 3100 A. (B) Fluorescence of DMABEE in diethyl ether, and s i b cone-4 and -5 a t room temperature; C = M. (-) silicone-4; (-- - ) silicone-5; (-.-) commercial diethyl ether, C(H20) = 7%; (--) diethyl ether of spectroscopic grade, C(H20) < 0.1%; (.-) diethyl ether for "partially" dried by molecular sieves (see the work of Burfield et the drying efficiency of molecular sieves). (C) Effect of additional traces of water on the dual fluorescence of D M A B N in acetonitrile, C = M, at rmm temperature (intensities calibrated on the Fb maximum). (1) acetonitrile "partially" dried by molecular sieves;s2 (2) acetonitrile of spectroscopic grade; (3) acetonitrile with M water added; (4) acetonitrile with water concentration >1 M .
much water as a very polar solvent such as acetonitrile. The quantity of water in the solvent, rather than its polarity, must be taken into account. Contrary to the case of dilute acetonitrile solutions (10" M) where the absorption spectrum of DMABN is not much modified by additional traces of water (cf., for instance, ref 29), with more concentrated solutions, the additional effect is more obvious. With M solutions, the initial water impurity concentration is about 10 orders of magnitude of solute concentration instead of the
Fb
where SNT*represents nontwisted molecules that have been excited, or
1
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Fb
Fa
where ST*represents twisted molecules that have been excited. b and a are respectively the relative number of molecules that will relax toward the plane and the twisted conformations. This scheme explains why nontwisted molecules contribute more efficiently to the normal fluorescence than do twisted molecules when the concentration of water is greater: Nontwisted molecules give rise to normal fluorescence only, while twisted molecules have two possible radiative relaxation channels, Fa and Fb. Although it is clear that hydrogen bonds as such play a deforming (Le., twisting) role, hydrogen bonds with water seem to be much more efficient than those with alcohols. Indeed, the anomalous fluorescence, Fa, as compared to Fb,of DMABN is much smaller in pure ethyl alcohol (completely free of water) than it is in commercial ethyl alcohol (containing 5% water); see Figure
2352
Cazeau-Dubroca et al.
The Journal of Physical Chemistry, Vol. 93, No. 6,1989
If
I
_-_-
i 360
40440
cs
000000
CS
......
410
460
510
560
'0
B
Cl
-
610 A ( n m )
- - c4
I
270
1
I
I
I
I
l
280
290
300
310
320
330
cs
e (nm)
E
1
10
20
30
40
[ H ~ o ] en*/.
Figure 4. (A) Absorption spectra of DMABN in glyme, a t room temperature, C = lo-' M, for various concentrations of water: Co= 0% water added, C , = 2.6%, C, = lo%, Cp = 18%. (B) Fluorescence spectra of DMABN in glyme at room temperature, C = IO-, M , for various concentrations of added water: Co = 0%, C , = 1.3%, C, = 2.6%, C, = 10% C, = 15%, C, = 18%, c6 = 40%. (with the intensity calibrated on the Fbmaximum). (C) Excitation spectra of fluorescence Fb for the same various concentrations of added water with the intensity calibrated on the 285-nm maximum. (D) Excitation spectra of fluorescence Fa for C, = 0% and c6 = 40% added water. (E) Variation of the ratio IFa/IFb for various concentrations of added water in the case of DMABN solutions in glyme (1,2-dimethoxyethane).
5 . The shift observed between the maxima of the Fa fluorescence in these two solutions (Av = 3600 cm-I) can be explained by the fact that the DMABN molecules in the commercial alcohol are much more completely surrounded by water molecules. This brings about a locally stronger value for the dielectric constant
6 which in turn affects the TICT emitting state. It has been demonstrated earlier that the TICT emitting state is very sensitive to the polarity of the environment. The greater twisting efficiency of water, compared to alcohol, can be explained partially by the smaller size of the molecule,
TICT Molecules: Already Twisted in the Ground State
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2353 based on parameters determined from previous ab initio calculations. Fraga successfully used Clementi’s parameters to do more specific calculations on b i o m o l e ~ u l e s . ~Our ~ work is based on Fraga’s program. Clementi et al. were able to compute the interaction energy between a biomolecule B and a water molecule W by a method different from the classical SCF-LCAO method. Their result can be expressed as AE(B-W) =
(
C iEBjEW
1
1
I
I
I
I
350
400
450
500
550
I
*
600h(nm)
Figure 5. Fluorescence of DMABN, at room temperature in absolute ethanol (-) and in commercial ethanol (bold line), C = M.
which allows a stronger interaction (there is less steric hindrance), and partially by the fact that one water molecule offers the possibility of two hydrogen bonds (see below, the computation results and discussion). From all these experimental results we conclude that when no traces of water are present in a solution, there can be neither ARS nor Fa for a flexible molecule that is flat when free. The presence of water as an impurity in the solvents is really the cause of the observed ARS and Fa. The addition or small quantities of water to the solutions enhances the ARS (which becomes observable at room temperature) as well as the intensity of the Fa. This means that the complexing of the TICT molecules with water is responsible for the emergence of ARS and F. Since our theoretical simulations (see next section) have shown that the effect of this complexation with water is precisely to twist the amino group of the TICT molecule, and this the more so as more water molecules are added (up to a certain limit), we are very much inclined to believe that it is this twisted conformation of the ground state which is a prerequisite to the anomalous Fa and the ARS. While DMABN and similar molecules need hydrogen bonds to twist them, intrinsically twisted molecules can do without to produce an anomalous fluorescence. This latter case is illustrated by the CTMA molecule, which has an intrinsic twist of 60°,26for which anomalous fluorescence has been reported in the vapor phase.25 Another case is that of PIPBN @-pyrrolidinobenzonitrile) with an intrinsic twist of 30°,26for which has been observed obvious abnormal fluorescence in poly(methy1 m e t h a ~ r y l a t e ) . ~ ~ Furthermore, the fact that both ARS and Fa disappear when very large quantities of water are added seems to show that the conformation of the complexed molecule (TICT:nH20) is very sensitive to the number n of water molecules in the complex. We therefore decided to confront our model of more or less twisted TICT molecules in the ground state, due to complexing with water molecules, with a precise mathematical model based on intermolecular interaction calculations.
Theoretical Method Nowadays, it is much too expensive for a group of theoreticians to perform quantum ab initio calculations on molecular complexes. It is therefore worth looking at the method used by Clementi et al.33 Their method is novel and allows simplified calculations (32) Al-Hassan, K. A.; Rettig, W. Chem. Phys. Lett. 1986, 126(3,4), 273. (33) (a) Clementi, E.; Cavallone, F.; Scordamaglia, R. J . Am. Chem. Soc. 1977, 99(17), 5531. (b) Bolis, G.; Clementi, E. J. Am. Chem. SOC.1977, 99(17), 5550. (c) Corongiu, G.: Clementi, E.; Preach, E.; Simon, W. J. Chem. Phys. 1979, 70(3), 1266. (d) Ragazzi, M.; Ferro, D. R.; Clementi, E. J . Chem. Phys. 1979, 70(2), 1040. (e) Carrozo, L.; Corongili, G.; Petrongolo, C.; Clementi, E. J . Chem. Phys. 1978, 68(3), 787. (f‘) Scordamaglia, R.; Cavallone, F.; Clementi, E. J . Am. Chem. SOC.1977, 99(17), 5545. (9) Lie, G. C.; Clementi, E. J . Chem. Phys. 1974, 60(4), 1275. (h) Clementi, E.
Computotional Aspects for Large Chemical Systems; Springer Verlag: Berlin, 1980. (i) Clementi, E. J . Chem. Phys. 1967, 46, 3842. (j) Clementi, E.; Routh, A. Int. J . Quantum Chem. 1972, 6, 525. (k) Popkie, H.; Clementi, E. J . Chem. Phys. 1972, 57, 4870.
~
i
Rip
j
Bij
):
Ri,12
where R, is the distance between the ith atom of the biomolecule and the j t h atom of the water molecule. This form of AE is well-known since the summation shows the electrostatic, the dispersion, and the repulsion energies between atoms i and j. A,, B,, and C, are coefficients characteristic of the ( i j ) pair. They are independent of the internuclear distances. In order to determine these coefficients, Clementi et al. carried out a great number of calculations corresponding to different positions of molecules B and W. Then, using a fitting technique akin to the least-squares method, they were able to draw up tables of the pair potentials A,], B,, and C,. Fraga’s idea has been to use these results to set up a simple model of molecular mechanics that would yield information about the interactions between molecules. The results he obtained by this method are very satisfactory, particularly when applied to the behavior of organic molecules in solutions. The simplicity of the method and the quality of the results made us decide to use it in order to investigate the conformation of the TICT:nH20 molecular complex in the ground state. The total interaction energy between two molecules can be written as
that is, the total energy is a sum of the interactions between all possible pairs of atoms of the complex. For each potential, we may write a development in 1/R, and the coefficients of these developments have been parametrized by Fraga using Clementi’s very complete computation results. For a pair potential, we can write
where E t , Er‘, E?, and ET’ represent the electrostatic, the polarization, the dispersion, and the repulsion terms between atoms i and j . Calculations yield the values
E: = 1389.4qiq,/RI1
&;P
= 4.18cicj/Rij12
These numerical coefficients give results in units of kilojoules per mole (kJ mol-’) when the distances Rij are expressed in A, the polarizabilities ai and aj in A3,and the charges qi in electrons. The effective charge on atom i is ni while cI’sare coefficients that Fraga determined from Clementi’s results. In order to characterize the most frequently encountered atoms of organic chemistry (C, N, 0, H), about 30 different classes34were established. We shall be particularly interested in the characterization of the hydrogen (34) (a) Fraga, S. Comput. Phys. Commun. 1983.29, 351. (b) Fraga, S. J . Comput. Chem. 1982, 3(3), 329. (c) Fraga, S. Molecular associations and Reactions. In Current Aspects of Quantum Chemistry; Carbo, R., Ed.; Elsevier: Amsterdam, 1982; p 313. (d) Fraga, S. Can. J . Phys. 1983,62,680. ( e ) Nilar, S. H. M.; Fraga, S. J . Comput. Chem. 1984, 5(3), 261. (f‘) Fraga, S. J . Mol. Strucf. 1983, 94, 251.
2354
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
TABLE I: Interaction Energies (kJ mol-') of 1:n Complexes for Different Twistioe Andes. a a,deg 1:l 1 :2 1:3 1 :4 15 -210 0 -170 -246 -351 -338 30 -168 -266 -300 -340 -355 60 -158 -262 -290 -370 -331 90 -145 -236 -287 -346 -340
and the oxygen atoms since they make up the water molecule. In order to obtain the minimum energy configuration of a TICT:nH20 complex, we start by giving it an initial geometry. Then, we calculate all the interaction energies, after which we try to minimize the total energy by using a gradient method. In each case, different initial geometries have been tried out so as to optimize the results. This formalism is a semiempirical method to obtain the interaction energies. Its very simplicity allows us to carry out simple and cheap calculations. Moreover, it makes it possible to understand many important phenomena of physical chemistry. This last aspect will be discussed more fully below. Among all the possible TICT molecules that can be twisted by the action of hydrogen bonds, we choose the DMABA molecule @-(dimethy1amino)benzaldehyde) to present our theoretical study on the interaction between a TICT molecule and water. The absence of crystallographic data leads us to adopt the geometry of similar, better known molecules as a model (for example, that of p(ethy1amino)benzaldehyde hydrobromides). We used the pyramidic conformation of the nitrogen atom. When calculating the interaction energy between the DMABA and the surrounding water molecules, we considered several possible geometric conformations for the DMABA molecule and we varied the twisting angle a about the C,,-N bond. As we already mentioned, the main purpose of this article is to study the behavior of molecules like DMABA in the presence of water or some other polar solvent. The questions we tried to answer are the following: Which site on the molecule (Le., the aromatic center, the amine, ,.., etc.) is most likely to attract a water molecule, and is the ensuing hydrogen bond a strong one? Is the twisting of the amino group favorable to the fixing of water molecules, and does it contribute to stabilizing the complexes? How many water molecules do the complexes include, and what is their structure? Table I shows most of the results about the interaction energies between one DMABA molecule and n water molecules (abbreviated 1:n complex) as a function of the twisting angle CY which varies from 0" to 90'. These energies correspond to the absolute minimum (the minimum minimorum) for each complex. In all cases, the water molecules are found to be close to the nitrogen lone pair. There exist other minima on the interaction energy surface, and some of these will be discussed below. Note. The DMABA molecule as an isolated entity has been studied with the help of CNDO. The results of this study are 8 = 0'
8 = 40"
8 = 40'
CY
CY CY
= 0'
E = -102.281 586 au
= 0"
E = -102.284987 au
= 90"
E = -102.283392 au
From this we may conclude two things: ( 1 ) The pyramid (8 = 40°, a = 0") is the most stable form of the molecule since there is a difference of 8.9 kJ mol-' between it and the 8 = 0", a = 0" configuration. (2) When 8 = 40",the most stable conformation of the molecule is the plane, with a = O", even though the energetic difference with the completely twisted, LY = 9O0, form amounts to only 4.2 kJ mol-'. The 1: I Complex. We did our calculations considering successively three different parts of the DAMBA molecule-the aldehyde group, the aromatic ring, and the amino group-as possible fixing points for the water molecule. In each case, the search for the energetic minima was optimized by a gradient method. We shall comment on the structure of the complex corresponding to some of these minima.
Cazeau-Dubroca et al. SCHEME I: Conformation of Untwisted Supermolecule 1:l (DMABA:H,O)" A
\\9
0
"(A) Hydrogen bonding involving one water hydrogen. (B) Hydrogen bonding involving both water hydrogens.
When the water molecule is close to the aromatic ring or to the aldehyde group, the interaction is nearly independent of the orientation of the amino group. When the water molecule points one of its hydrogen atoms toward the aromatic ring, the calculated minimum is at -95 kJ mol-', with a corresponding distance between the ring and the hydrogen atom of 1.7 A. When the water molecule is near the aldehyde group, we obtain a minimum of -125 kJ mol-'. It is only when the water molecule nears the amino group that we can differentiate the twisted from the nontwisted conformation. In this case, if the DMABA molecule is twisted at 9O0, the interaction energy minimum is at -145 kJ mol-' and the distance between the nitrogen atom of DMABA and the oxygen atom of water is 2.25 A. However, when the DMABA molecule is flat, we find two very close minima, one at -170 kJ mol-' and one at -161 kJ mol-'. The first corresponds to the absolute minimum of the 1:l complex which then has a structure such that both hydrogen atoms are bonded to the DMABA molecule (see Scheme IA): one is bonded to the nitrogen atom (with the distance OH-N being 1.73 %.)while the other points toward the aromatic ring. In the case of the second minimum, just one hydrogen atom points at the nitrogen atom. Notwithstanding the complexity of the energy surface, our calculations do show a certain number of favorable positions for a water molecule presented to different parts of the DMABA molecule. We note that for the 1:l complex, the plane conformation of the DMABA molecule remains the most stable one. The interactions induced by one water molecule are not strong enough to uncouple the free electron pair of the nitrogen from the T cloud of the benzene ring. The 1 :2 Complex. We now introduce a second water molecule into the 1:l complex. We paid special attention to three conformations in the case of a flat DMABA molecule. The most stable of these is such that both water molecules are near the nitrogen lone pair and both are on the same side of the plane of the benzene ring (see Scheme IIA). The interaction energy is then equal to -210 kJ mol-'. If the two water molecules, though still both near the nitrogen, are each on a different side of the benzene plane (see Scheme IIB), the complex is less stable and the interaction energy amounts to -166 kJ mol-'. In this case, the water molecule which was introduced in the second place is closest to the nitrogen atom, with a distance N - - 0 = 2.28 A.
TICT Molecules: Already Twisted in the Ground State SCHEME II: Conformation of Untwisted Supermolecule 1:2 (DMABA:2H,O)'
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2355 SCHEME III: Conformation of Untwisted Supermolecule 1:3 (DMABADHzO)'
01 A
A
\\9 ,
I
9
I
0
B
\9
1
'(A) Two water molecules interacting both together and with amino group. (B) Two water molecules not interacting with each other. (C) Two water molecules interacting together and with the T system. (D) Conformation of twisted supermolecule 1:2 (DMABA:ZHzO). When the DMABA molecule is twisted, the absolute value of the interaction energy is greater for all 1:2 complexes and this whatever the value of the angle a. The most stable structure of a twisted DMABA molecule surrounded by two water molecules occurs when the twisting angle is between 30' and 60°.50 However, the most stable of all positions for this complex (see Table I ) is a much more interesting one. It corresponds to both water molecules being close to the lone pair of the nitrogen while the DMABA is twisted (see Scheme 2D). The absolute value of this energy (-266 kJ mol-') is much greater than that for any minimum for a 1:2 complex with a flat conformation. Thus we conclude that the complex DMABA-(H20)2, is more stable when the DMABA is twisted even though DMABA by itself and DMABA-H20 are more stable when the DMABA is flat. We can say that two water molecules together are able to uncouple the electron lone pair of the nitrogen from the 7 cloud of the benzene ring. The 1:3 Complex. For systems containing three water molecules, the twisted conformation is very clearly favorable to the stabilization of the complex. We shall first analyze two minima with the DMABA in a flat conformation. They correspond to the structures shown in Scheme IIIA,B. The complex in Scheme IIIA has an interaction energy of -246 kJ mol-' and is the most stable one. For the complex in Scheme IIIB, the interaction energy
D
'(A) Water molecules interacting together and with the K system and the amino group. (B) Water molecule interacting together and with the amino group. (C) Conformation of twisted supermolecule 1:3 (DMABA:3H20). is equal to -239 kJ mol-', the third water molecule being rather far away from the nitrogen lone pair. In the latter case, the distance N-0, is close to 4 A while the other two distances, N-O1 and N.-O2, are equal to 2.68 and 3 A, respectively. There exist other minima for the flat DMABA, when the third water molecule is close to either the aldehyde group or other region far removed from the nitrogen lone pair. However, since their stabilizing energies are not comparable to those of the situations analyzed in the preceding paragraph, we did not think it necessary to discuss them in this article.
Cazeau-Dubroca et al.
2356 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 When the DMABA molecule is twisted at 90°, the most stable configuration is represented on Scheme IIIC, and it has an interaction energy of -287 kJ mol-'. Because of the twist, the three water molecules can gather about the nitrogen lone pair without being hindered by the methyl groups attached to the nitrogen. This means that they can come quite close to the nitrogen since the distances N-0,, N--02, and N-03 are equal to 2.25, 2.76, and 3.33 A, respectively. In this complex again-and even more obviously than in the previous c a s e t h e twisted conformation of the DMABA molecule greatly favors the stability of the complex as a whole. 1:4 and 1:5 Complexes. When four or five water molecules are present in the complex, the energetic minima for twisted and nontwisted DMABA molecules are very close together. If we look at the corresponding structures, we see that the fourth and fifth water molecules are very far away from the nitrogen lone pair and therefore the interaction they can have with the DMABA molecule is necessarily screened by the other three molecules of water. This has as a consequence that the interaction energies calculated for the 1:4 and the 1 :5 complexes come mostly from the interaction between the extra molecule(s) and the three inner water molecules. It becomes clear that up to three water molecules contribute to stabilize the DMABA-(H20), complexes but that any number of extra water molecules are not going to have a very interesting or profound influence on the structure of the DMABA molecule. In the next section we are going to try to compare our theoretical model with the experimental results. Correlation of the Theory with the Experimental Results. In brief, our theoretical simulations on TICT:n(H20) complexes give the following results: the probability of having a more and more twisted conformation for the TICT molecule grows with n until n = 3. When n is greater than 3, the water molecules start building a second layer around the TICT molecule and the twisting decreases again. For very large n, the twisted conformation is just as probable as the nontwisted one since all extra water molecules, from the second layer on, tend to interact mostly among themselves. It is therefore interesting to compare the experimental behavior of the luminescence spectra, as the quantity of water in the solvent changes, with the theoretical twisting of the TICT molecules as the number of water molecules in the complex changes. 1 . Case of Hydrophobic Solvents (Silicones): Uncomplexed TICT Molecules. In hydrophobic solvents such as silicones, the only fluorescence observed is the normal fluorescence. The same thing is true for hydrophobic matrices doped with TICT molecules. The total absence of water in these solvents excludes the possibility of forming a complex. The normal fluorescence, Fb, therefore arises from the nontwisted form of the TICT molecule. 2. Case of Neutral Solvents (MCP or MCH): 1:l Complex. The fluorescence spectrum of DMABEE in MCP or in MCH (or that of DMABN in MCP) shows the anomaly of dual fluorescence, but with a relative intensity R = IFa/ZFb R4 > 1 P,(twisted) > P4(twisted) 3 P4(nontwisted) >4' P,(twisted) E P,(nontwisted) Ob
P,, I (twisted) < P,( twisted)
'TICT:nH20complexes. Solutions containingjust traces of water, as an impurity. [ZOH] increases from n = 0 to n = 4. cSolutions containing large amounts of water, or hydroxylic or water solutions. [ Z O H ] > some %.
solubility of water is greater than in the nonpolar solvents of the preceding paragraph. Higher order complexes must therefore occur more frequently. From the values of the interaction energies calculated in the theoretical section, we may conclude that the most probable conformations of the TICT molecules are those with a = 30" and a = 60°,without excluding the possibility of other forms. These partially twisted conformations must be responsible for the anomalous fluorescence Fa with a greater efficiency than in the preceding case. 4 . Case of Very Hydrophilic Solvents (Glyme, Acetonitrile): 1 :3 Complexes. All TICT molecules dissolved in very hydrophilic solvents such as glyme and acetonitrile-in both of which water is infinitely solubleshow very exalted anomalous f l ~ o r e s c e n c e . ~ , ~ ~ (Figure 4). In these very hydrophilic solvents, complexes of even higher order become probable. We suppose that in these solvents the 1:3 complex is the most probable form for the TICT molecule to be in. Following our calculations of the interaction energy this means that we have a predominance of strongly twisted conformations ( a = 60" and a = 90'). This would account very well for the observed exaltation of the anomalous fluorescence, Fa. We should like to recall that on adding small quantities of water grows while at the same time to these solutions, the ratio IFa/IFb the maximum of the absorption spectrum is shifted to lower energies. Adding water of course increases the number of higher order complexes and therefore the number of very twisted molecules. This favors the anomalous fluorescence, FaZoa(Figure 4). 5. Case of Hydrophilic Solutions to Which Large Amounts of Water Have Been Added: 1 :4 and I :5 Complexes. We saw that when small quantities of water were added to solutions of DMABN in glyme, the ratio ZF,/ZFb grew. When more water is added until a concentration of about 15% is reached, this ratio attains a maximum. The ratio starts to decrease slowly as the concentration of water is increased thereafter. At high concentrations of water, the probability of having 1:4 and 1:5 complexes becomes very high. For these complexes, the fourth and fifth molecules no longer interact directly with the TICT molecule and the calculated interaction energies only result from their interaction with the other water molecules, which seems to weaken the twisting action of the latter. In any case, the calculated interaction energies differ little for the various angular conformations; in particular those with a = 0" and those with a = 90" differ little energetically. The calculated energy differences are very small, and are in fact insignificant if we take into account the intramolecular energies. Consequently, even though the more twisted forms were probable for 1:3 complexes, we find that for 1:4 and 1:5 complexes the twisted and non-twisted conformations become energetically equivalent. Both forms must therefore coexist in equal proportions when the concentration of water is high. This is confirmed by the decrease of IF,/IFb when the amount of water added exceeds a certain threshold. Table I1 shows a summary of the principal conclusions as described in this part. Generally, the water contained as an impurity in the organic solvent is correlated with its polarity, except very special solvents, either very hydrophilic ones like dioxane or dimethoxyethane
TICT Molecules: Already Twisted in the Ground State (glyme) (C (water) is about 1 M; cf. ref 39), or very hydrophobic ones like silicone. In all cases, the minimum addition of water to change the ratio IF,/IFb is in the same range as the water concentration already present and the evolution of this ratio with added water is very similar in all the cases (traces or a few percent water added): at the beginning a very rapid increase, then a maximum (corresponding to a saturation51), followed by a rapid decrease, and finally a much less rapid decrease (see Figure 4E in the glyme case and ref 44 in the acetonitrile case). Moreover, the change of slope of the decrease of the ratio is strictly correlated with that of the shift Av(max) of the maximum intensity.44 The rapid increase and decrease are relevant to specific interactions (one shell of water around the nitrogen). In the following slow decrease the solvent can be considered like a continuum.
Conclusion Our calculations on p-(N,N-dimethy1amino)benzaldehyde (DMABA) have yielded the following results: the most hydrophilic site on the TICT molecule (the dialkylamino group); the interaction energies leading to the relative probabilities of observing the various stoichiometric (1 :n)complexes; the spatial organization of the water molecules around the TICT molecule; for each 1:n complex, the most probable conformation as a function of the twisting angle. In order to compare these theoretical results with experiment, we observed the behavior of TICT molecules in a series of increasingly hydrophilic solvents: totally hydrophobic solvents (silicones, or polymeric matrices, PE, ...), where we found R = ZF,/IFb = 0; solvents containing only traces of water (nonpolar solvents such as MCP, MCH), where we found R 1. Since these four types of solvents are classified according to growing water content, we made a correspondence with the 1:n complexes, attributing to each class of solvents a 1:n complex, with n growing as the water content-grows. This 1:n complex would be the mast probable kind of complex formed in the corresponding class of solvents. Each 1:n complex may have one or several stable configurations since the surfaces of the interaction energies are rather flat and do not always allow the choice of one particular conformation. However, from our calculations does emerge a general law, which is that as n grows, up to 4, the TICT molecule in the 1:n complex tends to take on a more and more twisted conformation. From this we may infer that in our four classes of solvents the probability of containing many very twisted TICT molecules grows with increasing hydrophilic character (and increasing amount of traces of water in the solutions). Our experimental luminescence results show that anomalous fluorescence, Fa, increases with the concentration of water in the solvent (up to a certain limit). This agrees very well with our hypothesis that Fa is caused by TICT molecules that are already twisted before being excited. We have also shown experimentally that, whereas the addition of small quantities of water (or alcohol) exalts the Fa, when greater quantities of water are added (of the order of some %) the effect is reversed and the ratio R = ZF,/ZFb decreases again. It is obvious that the solutions containing a lot of water, and presenting a diminished ratio R, contain mostly high ordered complexes, with n = 4, 5 , .... Now our simulations show that for these high ordered complexes ( n 3 4), the first three molecules screen off the next ones and nontwisted TICT molecules tend to become just as probable as twisted or partially twisted ones. The assumption that the twisted forms of the TICT molecules are exclusively responsible for the anomalous fluorescence, Fa, is in agreement with the following: (1) the inversion of the ratio R = ZFa/ZFb when large amounts of water (or of alcohol) are added to the solution; (2) a ratio R < 1 observed in hydroxylic solvents and in water-a situation equivalent to the preceding one. Given the very close correlation between the ratio R = ZF,/ZFb and the quantity of water contained in the solution, TICT molecule can be used as a very sensitive probe to determine the quantity of water contained in an aprotic environment. We would like to make a methodological conclusion as well. Although the study of DMABA and similar molecules by classical methods of theoretical chemistry leads to a flat nontwisted structure in the ground state, we have shown that this is no longer the most stable structure as soon as a certain number of water molecules are present in the surroundings. This should make us aware of the limits of theoretical methods applied to isolated species when explaining such complicated systems as those studied in this article. The presence of hydrogen-bonding complexes in the groundstate origin of the dual fluorescence of TICT molecules (for the flexible but initially planar bare TICT molecules) involves the following equilibrium: DMABN
+ nHzO
planar conformations
1
DMABN/nH20 some twisted conformations
This equilibrium is shifted towards the complexed molecule by decreasing temperature and by increasing the concentration of one the complexing species. In this case, it is worth noting that the water concentration generally follows the solvent polarity. At the beginning of the shift (reaction 1) of the equilibrium, when n is still low, we have a statistically greater amount of the twisted conformations because the n > 3 complexes have a lower concentration than the n < 3 complexes. The outcome is a preferentially planar conformation. A saturation effect is observed thereafter. This “solvent” ground-state perturbation may be compared with the conclusions reported by Eisenthal et aLWabout the ”apparent” decrease of the energy barrier in the excited state either by in-
J . Phys. Chem. 1989,93, 2358-2362
2358
creasing the polarity of the solvent or by decreasing the temperature. This statistical enhancement of the twisted conformations may also explain why the similarity of the transient absorption S1* S,* spectra of DMABN and CTMA (60' ground-state twisted c ~ n f o r m a t i o nreported )~~ by Mataga et al. is more striking as the solvent polarity increases. Contrary to the results for DMABN, where there is a statistic of conformations with different twist angle in the ground state, with an hindered molecule like HMABN (2,3,5,6-tetramethyl-4-(N,N-dimethylamino)benzonitrile) presenting in the ground state a 60' twisted conformation only, no time-dependent change was observed in the 1-pentanol and no polarity-dependent spectral change was observed with solvents of different polarity.42 The presence of large numbers of conformations pretwisted in the ground state by hydrogen bonding may explain the very close values found for the Franck-Condon- and TICT-state dipole ~~ by Baumann. By exmoments b(FC)* = M ( T I C T )reported
-
citation, the adequate pretwisted conformations directly reach a charge-transfer state. The spectroscopy by jet-cooled technique of DMABN/ROH complexes$' where the alcohol cannot be considered as a continuum, supports the formation of the a*(TICT) state only when the alcohol molecules are sufficiently numerous. Similarly, no a* state was found by jet-cooled technique45 in the case of the bare DMABN molecule.
Acknowledgment. We express our gratitude to Professor S. Fraga, who kindly permitted us to use his program. We also thank the Centre National Universitaire de Calcul de Montpellier (CNUSC), where we were able to carry out our calculations. We are indebted to Dr. K. Rotkiewicz for kindly providing some CTMA. Registry No. DMABA, 100-10-7; DMABN, 1197-19-9; C T M A , 60082-00-0; H M A B N , 106681-10-1; H 2 0 , 7732-18-5.
Radiolytic Studies of the Redox Reactions of Ruthenium Porphyrins S. Mosseri, P. Neta,* Chemical Kinetics Division, National Bureau of Standards, Gaithersburg, Maryland 20899
and P. Hambright Department of Chemistry, Howard University, Washington, D.C. 20059 (Received: March 22, 1988; In Final Form: September 19, 1988)
Oxidation of Ru" porphyrins to the Ru"' and Ru" states and ligand-exchangereactions of the various states have been studied by radiolytic methods. Ru" porphyrins, stabilized with a CO ligand, undergo one-electron oxidation on the porphyrin ring to form the ?r-radical cation. When Ru"(P)(CO) (P = octaethylporphyrinor tetraphenylporphyrin) is oxidized by irradiation in CH2Cl2,the initial radical cation combines with C1-, formed by radiolysis of the solvent or added beforehand, with a rate constant of about 1 X lo5 M-' s-l to yield Ru"(P'+)(CO)(CI-). Irradiation of the same porphyrins in acetonitrile results not in oxidation but rather in uptake of the CN- produced by the radiolysis to form Ru"P(CO)(CN-). When this product is oxidized by irradiation in acetonitrile/CC14 solutions, oxidation occurs first on the ligand to form Ru"(P'+)(CO)(CN), which is unstable and transforms into Ru"'P(CN-), as the final product, with a rate constant of 4.5 X IO3 s-l. The Ru" state exists mainly as the pox0 dimer, which was produced by radiolysis of Ru"P(C0) in CH2C12saturated with KOH. The product, [(HO)RU'~P]~O, undergoes a gradual exchange of the OH groups with one and then two chlorides upon radiolysis in CH2CI2,as was demonstrated by the spectral changes. Chemical redox reactions were carried out to complement the radiolysis results.
Introduction Ruthenium porphyrins have been studied as models for hemes because of the high affinity of Ru"P (P = porphyrin) to O2and CO,'-* the variety of oxidation states available in such porphyrins, and the general similarity between ruthenium and iron. The relative stability of the various oxidation states of these metalloporphyrins, however, are somewhat different. The ruthenium porphyrins, unlike their iron analogues, are generally more stable in the Ru'" state in the form of a pox0 dimer.5,9 The other stable ( 1 ) Hopf, F. R.; OBrien, T.P.; Scheidt, W. R.; Whitten, D. G. J . Am. Chem. SOC.1975, 97, 277. (2) Farrell, N.; Dolphin, D. H.; James, B. R. J. Am. Chem. Soc. 1978,100, 324. (3) Paulson, D. R.; Addison, A. W.; Dolphin, D.; James, B. R. J . Biol. Chem. 1979, 254, 7002. (4) Paulson, D. R.; Hwang, D. S . Inorg. Chim. Acta 1983,80, L59. (5) Collman, J. P.; Barnes, C. E.; Brothers, P. J.; Collins, T.J.; Ozawa, T.; Gallucci, J. C.; Ibers, J. A. J . Am. Chem. SOC.1984, 106, 5151. ( 6 ) Barringer, L. F., Jr.; Rillema, D. P.; Ham, J. H., IV J . Inora. Biochem. 1984, 21, 195. (7) James, B. R.; Mikkelsen, S . R.; Leung, T. W.; Williams, G. M.; Wong, R. Inorg. Chim. Acta 1984, 85, 209. (8) Corsini, A,; Chan, A,; Mehdi, H. Talanta 1984, 31, 33.
state, Ru", exists in the form of Ru"P(CO)(L) or Ru"P(L)2,134-5J&'2 where L is a ligand other than CO, most commonly ROH, RCN, R3P, or pyridine. The latter form is generally less stable, and when the ligands are somewhat labile, it undergoes slow autoxidation to the RuIVdimer.1~2~5~6*7 The Ru111,5913-19 Ru'" (9) Collman, J. P.; Barnes, C. E.; Collins, T. J.; Brothers, P. J.; Gallucci, J.; Ibers, J. A. J . Am. Chem. SOC.1981, 103, 7030. (10) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. J . Am. Chem. SOC.1978, 100, 3015. (11) Holloway, C. E.; Stynes, D. V.; Vuik, C. P. J. J . Chem. SOC.,Dalton Trans. 1982, 95. (12) Domazetis, G.; James, B. R.; Dolphin, D. Inorg. Chim. Acta 1981, 54, L47. (13) Brown, G. M.; Hopf, F. R.; Ferguson, J. A.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. Soc. 1973, 95, 5939. (14) Brown, G. M.; Hopf, F. R.; Meyer, T.J.; Whitten, D. G. J . Am. Chem. SOC.1975, 97, 5385. (15) Smith, P. D.; Dolphin, D.; James, B. R. J . Organomet. Chem. 1981, 208, 239. ( 1 6 ) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. J . Chem. SOC.,Chem. Commun. 1981, 982. (17) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. Can. J . Chem. 1983, 61, 2389.
0022-3654/89/2093-2358$01.50/00 1989 American Chemical Society
