J. Phys. Chem. 1994,98, 2278-2281
2278
Effect of Pressure on the Conformational Behavior of 4-[4-(Dimethy1amino)phenyllpyridine in the Ground and Fluorescent &-Excited States Dmitry S. Bulgarevich, Okitsugu Kajimoto, and Kimihiko Hara* Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan Received: November 15. 1993"
Both absorption and fluorescence spectra of 4- [4-(dimethy1amino)phenyll pyridine (CDAPP) were investigated in various solvents a t atmospheric pressure as well as a t high pressures (up to 500 MPa). The spectroscopic results are well explained by a steeper torsional potential curve and a more planar relaxed conformation in the &-excited state than in the ground state. It was found that the solvent viscosity retards this steric conformational change. The pressure effect on the dihedral angle at the ground state between dimethylaniline and pyridine groups was determined. It decreases with a rate of lo/10OMPa.
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Introduction Photoexcitation to a non-equilibrium Franck-Condon (FC) state can induce changes in the steric conformation of aromatic molecules by modifying the balance between electronic and steric interactions. It has been reported that biphenyl in solution, whose dihedral angle between the planes of two phenyl rings at the ground state has been determined as 3Oo-4O0 by IR and NMR measurements,Is2 is brought to a more planar conformation a t S1-excited state.34 4- [4-(Dimethylamino)phenyl] pyridine (4-DAPP), is likely to have a similar conformation to biphenyl in the ground state.We
.-3c 3
2. .2. v)
c 0) c
-c
Wave number I lo3 cm-' Figure 1. Absorption spectrum at 298 K in ethanol (-) and in n-hexane fluorescence spectrum (bXc = 313 nm) at 77 K in ethanol (-), excitation anisotropy (hobs = 392 nm) at 77 K in ethanol (- -), and fluorescence anisotropy (Lc= 313 nm) in ethanol at 77 K (- - -). (-e.-),
have considered two possible types of conformational changes that may occur during the course of the relaxation from the FC state. One possibility is that the molecule may relax to a more planar conformation between the heterocyclic and dimethylaniline moieties. In the other case, a "twisted intramolecular chargetransfer" (TICT) state's may beformed. Previously the formation of the TICT state has been suggested,g where the stabilization of the charge-transfer (CT) state is accompanied by the enhanced twisting of the electron donor dimethylaniline group relative to the electron acceptor pyridine group at the SI-excited state. The present paper describes the pressure effect of both absorption and fluorescence spectra in various solvents with varying polarity and viscosity with respect to the conformational changesin theground and fluorescent S1 excited states. It should be pointed out that the application of high pressure provides a convenient means of making substantial and continuous changes in solvent viscosity while only changing the solvent shell structure a small amount. In addition, the measurements of both absorption and fluorescence as a function of pressure are helpful in characterizing electronic states of both ground and excited states. Experimental Section 4-DAPP obtained commercially was purified by recrystallization and sublimation in vacuo before use. Spectroscopic-grade solvents of n-hexane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), ethanol, and acetonitrile were used without further purification. All solutions for fluorescence measurements were degassed by several freeze-pumpthaw cycles prior to measurements. The absorption and emission of impurities
* To whom correspondence should @
be addressed. Abstract published in Advance ACS Absrracrs, February 1, 1994.
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could not be detected within the wavelength region studied. All measurements were carried out in M solutions at room temperature. The high-pressure cell and the apparatus used to measure emission and absorption have been described previously.IGI2 The absorption measurements were carried out at high pressures up to 400 MPa, while the fluorescence measurements were up to 500 MPa except those with acetonitrile. Fluorescence polarization was measured at 77 K in ethanol at normal pressure with a standard method of L-format.l3 Results and Discussion Assignment of Absorption Spectra. The absorption spectrum of 4-DAPP in ethanol and in n-hexane is shown in Figure 1. The absorption maximum at 29 900 cm-l has a large extinction coefficient (e > lo4) and is shifted toward lower energy with increasing solvent polarity. The excitation anisotropy observed at the fluorescence peak maximum (25510 cm-l) has a positive and almost constant value of ca. +0.2 within the energy range of this absorption band. These are the typical features for an lL,-lA transition.14J5 In nonpolar solvents, it is possible to observe an additional low-intensity absorption band in the 37 000-40 000-cm-1 region, as shown for n-hexane solvent in Figure 1. This band can be assigned to an IL&A transition. A decrease of the excitation anisotropy excited at higher energy is observed and may reflect the contribution of this band. Absorption assigned to the lL&A transition has been observed for benzene,l4 pyridine,l4 biphenyl,l4 and 4-phenylpyridine16 at the same energy region as 4-DAPP. For biphenyl and 4-phenylpyridine (4-PP), the lLa-lA transition shows a larger
0022-365419412098-2278%04.50/0 0 1994 American Chemical Society
Pressure Effects on the Conformation of 4-DAPP
Wave number I lo3 cm-'
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 2219
Wave namber I I O 3 cm-'
Wave number I lo3 cm-' Wave namber I lo3 cm-l Figure 2. Fluorescence spectra (& = 3 13 nm) of 4-DAPP at 0.1 MPa (-) and at 500 MPa (- -) at 303 K (a) in n-hexane, (b) in HMN, (c) in acetonitrile (at 400 MPa), and (d) in ethanol.
lower-energy shift as compared with those of benzene and pyridine. This increased shift in biphenyl and 4-PP is caused by the resonance conjugation between two phenyl rings in biphenyl or between pyridine and phenyl rings in 4-PP. Thus the absorption peak of the 'Lb-'A transition for biphenyl and 4-PP is located too close to be observed due to the absorption tail of the lL,-lA transition. Similarly, it is possible to suppose for 4-DAPP that the intensity of the 1Lb-lA transition at around 37 000-40 000 cm-I is quite low or even apparently vanishes in polar solvents. As a result, we can conclude for 4-DAPP that the strong band a t 29 000 cm-I is assigned to the IL,-lA transition, and the weak one at 37 000-40 000 cm-I, to the ILb-IA transition. These assignments of absorption bands are reasonably explained by the increased conjugation between pyridine and aniline groups. The increased conjugation induces the larger lower-energy shift in the 'La transition and produces a large separation of the two bands. In addition, by comparing the spectrum depictedin Figure 1 to the absorption spectrum of 2,6-diphenylpyridine,17we can assign the band around 42000-44000 cm-I to the IBb-IA transition. FluorescenceSpectra and ConformationalChange in the Excited State. The fluorescence spectra of 4-DAPP at 303 Kin n-hexane, HMN, acetonitrile, and ethanol are shown in Figure 2. We can observe a large shift toward the lower energy with increasing solvent polarity. In addition, the fluorescence anisotropy excited at the absorption maximum of the IL,-lA transition, which is also included in Figure 1, is parallel to the excitation anisotropy and holds a positive and almost constant value of ca. +O. 16. From these results we can conclude that the fluorescence band is mostly originated fEom the IL,-type state. It is interesting to note that the absorption spectrum has no structure but the fluorescence spectrum exhibits a vibrational structure especially in nonpolar solvents. A reasonable explanation for this behavior is described below. If we suppose a torsional potential curve for the ground state with a broad minimum against the dihedral angle between phenyl and pyridine rings, then the observed absorption spectrum will become an envelope of the various absorptions which stem from various dihedral angles. This fact may widen the absorption band resulting in the absence of vibronic structure. Thus, the presence of a vibronic structure in the fluorescence spectrum indicates that the potential curve of the fluorescent state is not as broad as that of the ground state. A similar discussion has been made for bi~henyl.~" Figure 3 shows fluorescence spectra obtained a t 77 K in solid ethanolusing three different excitation energies. When decreasing the excitation energy, thevibronic bands shift toward lower energy and change their relative intensity. The dependence of the excitation wavelength in highly viscous condition is explained by the emission from the nonrelaxed SIstate. When the relaxation is accompanied by some conformational change, it will be significantly affected by the viscous drag. The two possible types of conformational change in the course of the relaxation in the &-excited state are described by the Scheme 1. The one possibility is the formalion of the TICT state. Although dual fluorescence is a typical feature of TICT molecules in polar s0lvents,~~8 the fluorescence spectrum of
Wave number / lo3 cm-' Figure 3. Fluorescence spectra in ethanol at 77 K with different wavelengths of excitation.
SCHEME 1 8-40'
so
'La (planar)
TICT
4-DAPP in ethanol as well as in acetonitrile consists only of a single band with vibronic structure assigned to the emission from the IL,-type state. In addition, the study of the fluorescence a t low temperatures and at high pressures (uide infra) showed no indication of the dual fluorescence a t all. In Figure 1 the fluorescence spectrum in ethanol at 77 K is also included. From thisviewpoint the possibility of TICT-state formation is eliminated for the 4-DAPP molecule. The other possibility is the formation of the hyperconjugated structure with planar conformation of the conjugated system as depicted in Scheme I. On the basis of the above discussion about the potentials, this f6rmation isadopted more reasonably. A large peak shift in polar solvents can also be caused by the larger dipole moment in I at the excited state. However, as for the l-methy1-4-[4-(dimethylamino)phetlyl]pyridinium cation, there has been suggested a possibility of nonfluorescent TICT-state formation with fast radiationless deactivations by rotation around the central single bond.'* Pressure Effects on the Ground-State Conformation. Figure 4 shows the absorption spectrum in n-hexane a t various pressures. It is found that the molar extinction coefficient (e) increases with pressure. The large change in e can be brought about by the change in the steric conformation as suggested below. In the case of electronic transitions between a nonpfanarground state and a near-planar excited state, the magnitude of the transition moment will be a function of 8,-6,, where 8, and Be are the dihedral angles of the ground and the excited states,
2280 The Journal of Physical Chemistry, Vol. 98, No. 9, 1994
Bulgarevich et al.
40
I
\
m
36
100
300
Pressure / MPa
d
Figure 5. Pressure effect on the dihedral angle (e&.
t
36
32
2a
Wave number / 1O3 cm-' Figure 4. Long-wavelengthabsorption at 303 K at different pressures: (a) 0.1, (b) 100, (c) 200, (d) 300, and (e) 400 MPa.
respectively. A simple relation has been proposed for such a mode119 Be
9g Dihedral angle
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where eo is the extinction coefficient a t 0, = Be. By making an additional assumption that Be Oo, eq 1 reduces to
By applying eq 2, we can estimate the pressure effect on the change of Os for 4-DAPP in n-hexane. Since the correct value of eo has not been determined for 4-DAPP, 30-40' was adopted for the 8,value at atmospheric pressure (0.1 MPa). On the basis of the repulsion between orthohydrogens as in the case of biphenyl, this magnitude is considered to be reasonable. As a result, it is found that the 0, value decreases with pressure with a rate of 0.9-1,4°/100 MPa. Figure 5 shows the result for 0, = 40'. For biphenyl -2O/100 MPa has been reported.20 This indicates that the intrinsic molecular volume of 4-DAPP decreases by internal rotation toward the more planar conformation, which is caused by an increased overlapping of van der Waals spheres between orthohydrogen atoms in a more planar form for 4-DAPP molecule. Form of Torsional Potential Curve a t High Pressures. The fluorescence spectra of 4-DAPP a t atmospheric pressure (0.1 MPa) and at 500 MPa in n-hexane, HMN, ethanol, and acetonitrile (at 400 MPa) are shown in Figure 2. Spectra of 4-DAPP in n-hexane and in H M N were deconvoluted into three Gaussian peaks by means of least-squares fitting. The two stronger vibrational peaks exhibited similar peak shifts with pressure. The initial peak shifts determined by the slope at 0.1 MPa for n-hexane and H M N are given in Table 1. Unfortunately it was impossible to carry out an accurate fitting for ethanol and acetonitrile because of the poor separation in the original bands. Although it is difficult to discuss accurately, the difference in the
Figure 6. Schematic torsional potential curve of 4-DAPP at atmospheric pressure (-) and at high pressure (- -).
TABLE 1: Pressure Effect on the Initial Peak Shift of Absorption @a)and Fluorescence @"), the Dihedral Angle (OR), and the Ratio of Force Constants (W./W.) -(dk/dP) -(din/@) -(de,/ 'd% solvent (cm-l/(lOO MPa)) (cm-l/(lOO MPa)) dP) cq 3 cq 4 n-hexane 320 110 0.9-1.4 1.18 1.15 HMN 230 240 1.03 1.11 peak shift with pressure does not seem large between polar and nonpolar solvents. The notable features in the pressure effect on the fluorescence spectrum of 4-DAPP are (i) the change in relative intensity of vibrational peaks, (ii) the considerably greater shift toward lower energy as compared with an ordinary A+* transition,21and (iii) the decrease in the Stokes shift, which was determined in n-hexane and in HMN. The change in relative intensity of the vibrational peaks and the decrease in the Stokes shift with pressure are well correlated to the change in 0, as discussed above. Namely, this fact is represented by a horizontal displacement of the potentials of ground and excited states with respect to each other. The relative change of the torsional potential with pressure is schematically represented in Figure 6. The large red shift is verified by the larger dipole moment in the SIstate and also by the horizontal displacement of potentials. In Figure la,b, the peak shifts of absorption and fluorescence with pressure that occur in n-hexane and H M N solvents are compared. These results can be explained by a solvent viscosity effect. The viscosity of n-hexane changes from 0.296 mPa-s at
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994 2281
Pressure Effects on the Conformation of 4-DAPP
that of the SOstate. No indication of the TICT state formation was detected. (2) The torsional potential curve is steeper in the excited state than in the ground state. This conclusion is clearly supported by the value of (oc/og)> 1. (3) Pressure affects the dihedral angle of the ground state (eK), resulting in the decrease of Br with a rate similar to that of biphenyl at -lo/lOO MPa. (4) A significant solvent viscosity effect against the proposed steric conformational change during the relaxation in the S1state was clearly observed in the high-viscosity region. The application of the high-pressure technique is a quite useful means of revealing such dynamic solvent effects.
1
0
200
400
Pressure / MPa
0
200
400
Pressure / MPa
Figure 7. Pressure effect on the peak shift of 'L,-lA transition a t 303 K in n-hexane (-) and in HMN (- -): (a) absorption; (b) fluorescence (bXc = 313 nm).
0.1 MPa to 3.7 mPa-s at 500 MPa,22whereasin thesame pressure range the viscosity of H M N changes from 2.6 mPa-s to 7410 m P a . ~ . The ~ ~ considerable saturation of the fluorescence peak shift in HNM in the higher pressure region corresponding to a viscosity higher than ca. 100-200 mPa.s is likely to be caused by the emission from the nonrelaxed state. This may reflect the effect of viscous drag against the steric conformational change in the course of the relaxation in the S1 state, while the absorption peak is monotonically shifted toward the lower energy with increasing viscosity. The data of both absorption and fluorescence peaks as a function of pressure are very helpful in characterizing electronic states. By supposing a single configuration coordinate and harmonic potential wells about this problem, the following two equations have been derived for the pressure effect on peak shift (;I - ;2) and on half-width ( 6 E 1 / 2 ) . ~ ~
(3) (4)
where subscripts 1 and 2 refer to two different pressures. wg and o,represent force constants of potential wells of ground and excited states, respectively. Averaged ratiosof w,/wK calculated by using both eq 3 and 4 in n-hexane and in H M N are listed in Table 1. All values thus obtained from the slope at 0.1 MPa are (w,/w,) > 1. This is additional evidence of the above conclusion that the potential curve of the SI-excited state is steeper in comparison with that of the ground state. Concluding Remarks On the basis of the discussion of both absorption and emission behavior of 4-DAPP in solution at atmospheric as well as at high pressures, the following conclusions were deduced: (1) The relaxed conformation in the &-excited state is a planar hyperconjugated structure whose dipole moment is greater than
Acknowledgment. This research was supported in part by a Grant-in-Aid for Scientific Research No. 04640442 from the Ministry of Education, Science and Culture. We would like to thank Prof. Y.Taniguchiof Ritsumeikan University for permitting the use of the high-pressure absorption apparatus. We thank also Prof. M. I. Knyazhanskyof Rostov University for stimulating and helpful discussions. References and Notes (1) Mobius, K. 2.Naturforsch. A 1965, 20, 11 17. (2) Akiyama, M.; Watanabe, T.; Kakihama, M. J. Phys. Chem. 1986, 90, 1752. (3) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971. (4) Hughes, E., Jr.; Wharton, J. H.; Nauman, R. V. J. Phys. Chem. 1971, 75, 3097. (5) Naqvi, K. R.; Donatsch, J.; Wild, U. P. Chem. Phys. Lett. 1975,31, 285. (6) Fuju, T.; Komatsu, S.;Suzuki, S.Bull. Chem. Soc. Jpn. 1982, 55, 2516. (7) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, J.; Baumann, W. Nouv. J. Chim. 1979, 3, 449. (8) Rettig, W. Angew. Chem. 1986, 3, 969. (9) Bulgarevich, D. S.; Dmitruk, S.L.; Druzhinin, S.I.; Knyazhansky,
M. I.; Olekhnovich, E. P.; Uzhinov, B. M.; Kharlanov, V. A. Khim. Geterorsikl. Swdin. 1992, 5, 625. (10) Hara, K.; Arase, T.; Osugi, J. J. Am. Chem. Soc. 1984,106, 1968. (11) Hara, K.;Morishima, I. Rev. Sci. Instrum. 1988, 59, 2397. (12) Sawamura, S.; Taniguchi, Y.; Suzuki, K. Chem. Lerr. 1977, 823. (13) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983. (14) Jaffer, H. H.; Orchin, M. Theory and Applications of Ultraviolet Spectroscopy; Wiley: New York, 1962. (15) Seliskar. C. J.: Khalil. 0.S.:McGlvnn. S.P. In ExcitedStares: Lim. E. C.,'Ed.;Academic Press: New York, London, 1974; Vol. I, p 231. (16) Sobczyk, L. Bull. Acud. Pol. Sci. 1961, 9, 237. (17) Chakravorti, S.;Sarkar, S.K.; Mallick, P. Chem. Phys. Lett. 1991, 187, 93. (18) Fromberz, P.; Heiltmann, A. J . Phys. Chem. 1992, 96, 6864.
Lyapustina, S. A.; Matelista, A. V.; Bulgarevich, D. S.;Alexccv, Y. E.; Knyazhansky, M. I. J. Photochem. Photobiol. A 1993, 75, 119. (19) Braude, E. A.; Sondheimer, F. J . Chem. Soc. 1955, 3754. (20) Kato, M.;Higashi, M.;Taniguchi, Y. J . Chem. Phys. 1988,89,5417. (21) Hara, K.; Rettig, W. J . Phys. Chem. 1992, 96, 8307. (22) Thomas, M. M.; Drickamer, H. G. J. Chem. Phys. 1981, 74,3198. (23) Bridgman, P. W. Collected Experimental Papers;HarvardUniversity Press: Cambridge, MA, 1964; Vol. IV. (24) Drickamer, H. G.; Frank, C. W.; Slichter, C. P.Proc. Nar. Acad. Sei. 1972, 69, 933. Drickamer, H. G.; Frank, C. W. Electronic Transitions and the High Pressure Chemistry and Physics of Solids; Chapman and Hall: London, 1973.
