J. Phys. Chem. 1986, 90,2589-2596
2589
Excimer Emission in Protonated Pyridine Systems. 1. Fluorescence Spectroscopy of Protonated Pyridine and Its Methyl Derivatlves in Rigid Glass Solution at 77 K Takashi Handa, Yoshio Utena, Hirofumi Yajima,* Tadahiro Ishii, Department of Applied Chemistry, Science University of Tokyo, 1-3, Kagrurazaka, Shinjuku-ku, Tokyo 162, Japan
and Hiroshi Morita Department of Image Science and Technology, Faculty of Engineering, Chiba University, Chiba 260, Japan (Received: November 4, 1985; In Final Form: February 3, 1986)
The emission properties of pyridine and mone and dialkylpyridineshave been studied in solution in the presence of trifluoroacetic acid at room temperature and 77 K. At room temperature, mono- and dialkylpyridines exhibit a weak and broad fluorescence band with a peak at about 300 nm except for pyridine and 4-n-alkyl- and 2-methylpyridines. This fluorescence originates from a ( A T * ) state of protonated mono- and dialkylpyridines. However, they exhibit no excimer fluorescence even in a highly concentrated system. At 77 K, in the mixed solvent of tetrahydrofuran, methanol, and methyltetrahydrofuran (4:3: 1 by volume) in the presence of trifluoroacetic acid, mono- and dialkylpyridines exhibit a broad and structureless fluorescence band at about 325 nm, in addition to the normal fluorescence band. 4-n-Alkyl- and 2-methylpyridines apparently exhibit only a very weak fluorescence band at about 325 nm, but pyridine is nonfluorescent even at 77 K. It is concluded from the observations of absorption and fluorescence excitation spectra and the fluorescence characteristics that this broad and structureless band is ascribed to a particular excimer (termed dimerlike excimer fluorescence for convenience) which originates from the interaction between protonated monoalkylpyridines (or dialkylpyridines). The analysis of temperature and solvent dependence of fluorescence spectra and the phase transition of the mixed solvent show that the cage of the mixed solvent plays an important role in the dimerlike excimer formation. Further, on the basis of a four-electron ASMO approximation, the dimerlike excimer fluorescence is assigned to result from the in-plane twisted and plane parallel configuration of a compact pair of protonated pyridines.
Introduction Recently, we studied emission properties of poly(2-vinylpyridine)(PZVP) in solution in the presence of trifluoroacetic acid related to our previous studies of excimer formation in a series of polystyrenes.' The fluorescence spectrum of P2VP is found to be markedly different from that of polystyrene. The spectrum consists of three bands appearing at about 290,325, and 370 nm despite the fact that the absorption spectrum of P2VP is similar to the monomeric unit, 2-methylpyridine. The 290-nm fluorescence probably originates from a (m*)state of protonated pyridine chromophore, considering the correspondence between the position of this band and that of ethylbenzene. The lowest energy fluorescence is a broad and structureless band appearing ca. 6000 cm-' to the red of the normal fluorescence band and is not observed at 77 K. These fluorescence characteristicsz4 strongly suggest that the 370-nm fluorescence is ascribed to an excimer which comes from the intramolecular interaction between protonated pyridine chromophores. On the other hand, the 325-nm fluorescence band is also characteristically unstructured and is located at ca. 3400 cm-' to the red of the normal fluorescence band. The characteristic of this band is observed even a t 77 K. These results suggest that the 325-nm fluorescence is ascribed to a particular excimer formed between protonated pyridine chromophores in the same polymer. Until recently, only a few compounds such as poly(N-vinylc a r b a ~ o l e ) , ~dianthrylethane,'O -~ dianthrylpropane," and sub(1) Ishii, T.; Utena, Y.; Handa, T.; Mastushita, H. Rep. Prog. Polym.
Phys. Jpn. 1977, 20, 41 1. (2) Vala, Jr., M. T.; Haebig, J.; Rice, S. A. J. Chem. Phys. 1965, 43, 886. (3) Longworth, J. W. Biopolymers, 1966, 4 , 1131. (4) Yokoyama, M.; Tamamura. T.: Nakano. T.: Mikawa. H. Chem. Lett. 1972;499. ( 5 ) David, C.; Piens, M.; Geuskens, G. Eur. Polym. J. 1972, 8, 1291. (6) Johnson, G. E. J . Chem. Phys. 1975,62, 4697. (7) Itaya, A.; Okamoto, K.; Kusabayashi, S. Bull. Chem. SOC.Jpn. 1976, 49, 2082. (8) Hoyle, C. E.; Nemzek, T. L.; Mar, A.; Guillet, J. E. Macromolecules 1978, 11, 429. (9) Tagawa, S.; Washio, M.; Tabata, Y. Chem. Phys. Lett. 1919.68, 276. ~
0022-3654/86/2090-2589$01.50/0
stituted dinaphthy1pr0panes'~J~ were known to exhibit more than one type of excimers. It is very interesting to study emission properties of the 325-nm fluorescence in view of photophysical and photochemical studies. However, it is very difficult to study directly emission properties of the 325-nm fluorescence, because the polymer has an inherent complexity which originates from a large number of configurations and main chain conformations. As a first approach for assigning emission properties of the 325-nm fluorescence, it is significant to study emission properties of the simplest model compounds such as pyridine and mono- and dialkylpyridines. In the present paper, our attention is mainly concerned with emission properties of the 325-nm fluorescence of the abovementioned compounds. For this purpose, the absorption, fluorescence, fluorescence polarization, and phosphorescence spectra of pyridine and mono- and dialkylpyridines were measured in solution in the presence of trifluoroacetic acid at room temperature and low temperature. In addition, the assignment of the 325-nm fluorescence was carried out on the basis of a four-electron ASMO approximation by Azumi and M ~ G l y n n . ' ~ , ~ ~
Experimental Section Pyridine (I.R. grade), 2-methylpyridine (G.R. grade), 3methylpyridine (G.R. grade), 4-methylpyridine (G.R. grade), 2-ethylpyridine (E.P. grade), 3-ethylpyridine (E.P. grade), 4ethylpyridine (G.R. grade), 2-n-propylpyridine (G.R. grade), 4-n-propylpyridine (E.P. grade), 2,3-dimethylpyridine (E.P. grade), 2,4-dimethylpyridine (G.R. grade), 2,s-dimethylpyridine (G.R. grade), 2,6-dimethylpyridine (G.R. grade), 3,5-dimethylpyridine (10) Hayashi, T.; Suzuki, T.; Mataga, N.; Sakata, Y.; Misumi, S.Chem. Phys. Lett. 1976, 38, 599. (11) Ito, M.; Fuke, K.; Kobayashi, S. J. Chem. Phys. 1980, 72, 1417. (12) Itagaki, H.; Obukata, N.; Okamoto, A,; Horie, K.; Mita, I. Chem. Phys. Lett. 1981, 78, 143. (13) Itagaki, H.; Obukata, N.; Okamoto, A.; Horie, K.; Mita, I. J . A m . Chem. SOC.1982, 104, 4469. (14) Azumi, T.; Armstrong, A. T.; McGlynn, S . P. J . Chem. Phys. 1964, 41, 3839.
(15) Azumi, T.; Azumi, H. Bull. Chem. Soc. Jpn. 1966, 39, 1829.
0 1986 American Chemical Society
2590
The Journal of Physical Chemistry, Vol. 90, No. 12. 1986
(G.R. grade), and 5-ethyl-2-methylpyridine (E.P. grade) were obtained from Tokyo Kasei Industries. Pyridine was refluxed over CaH2 for 10 h and then carefully distilled 4 times in a nitrogen atmosphere under reduced pressure. Mono- and dialkylpyridines were purified by the same method as that of pyridine. 1,2-Dichloroethane (DCE), tetrahydrofuran (THF), methanol (MeOH), 2-methyltetrahydrofuran (MTHF), isopentane, ethanol, ethyl ether, and methylcyclohexane (Dotite Spectrosol) were purified by the usual methods. Glycerin (non-fluorescein grade) and ethylene glycol (chromatographic grade), obtained from Nakarai Chemical Industries, were used without further purification. Trifluoroacetic acid (TFA), obtained from Tokyo Kasei Industries, was refluxed with P2O5 for 7 h and then distilled over P2O5 under atmospheric pressure. Mixtures of ethyl ether, isopentane, and ethanol (5:5:2 by volume) (EPA), THF, MeOH, and M T H F (4:3: 1 by volume) (THF-MeOH-MTHF), methylcyclohexane and isopentane (1 :6 by volume) (MP), ethylene glycol and H 2 0 (1:l by volume) (EGW), and glycerin and H 2 0 (4:l by volume) (GW) were used as solvents at 77 K. Absorption spectra were measured at room temperature with a Hitachi EPS-3T spectrophotometer. Absorption spectra were obtained at 77 K with a Cary 14 spectrophotometer, and measurements were made on the sample in a 5-mm quartz cell in the Dewar with quartz windows. Emission and excitation spectra were recorded at room temperature and 77 K with a Hitachi MPF-4 type spectrofluorometer. Emission and excitation spectra were measured at 77 K with a quartz tube cell of 5 mm diameter, immersed in liquid nitrogen in a quartz Dewar. Excitation spectra were measured in the true recording mode. Emission spectra were measured at the excitation wavelength of 255nm. Relative quantum yields of samples at room temperature were obtained by comparison with the fluorescence spectrum of anthracene. A quantum yield of 0.23 was used for anthracene in ethanol.16 The concentrations of samples at room temperature and 77 K were and 2.0 X M, respectively. The solutions of 1.0 X samples were not degassed because no effect were observed on the spectra or lifetimes. The degree of polarization, P, was determined by using the and l I , are the inequation P = (Z,l - ZL)/(Zl, + Zl), where Il tensities of the emitted light polarized parallel and perpendicular, respectively, to the excitation light. The P value was corrected in the usual manner for the selective transmission of the emission monochromater and the selective reflection of the Dewar and sample tubes. l 7 Fluorescence lifetimes were measured by the single-photoncounting method using an Ortec nanosecond lifetime apparatus. Hydrogen gas at about 0.5 atm pressure was used as the light source and its half-width was about 3.4 ns. Fluorescence decay curves were analyzed by using a nonlinear least-squares curve method.18J9 The quality of agreement between the observed decay and the trial decay function is evaluated by using the reduced value, x?, which is defined by19 1N N,=I
xv2 = -C(K
- FJ2/Y1
where Y, and Fi are the values of the experimental data and trial decay function corresponding to the time channel and N is the number of experimental points. Measurements of temperature effect on the fluorescence were made by a constant flow of cold nitrogen gas. The flow of nitrogen gas was controlled by heating liquid nitrogen, and the temperature of a sample solution was measured with a thermocouple. Thermal analyses by differential scanning calorimetry (DSC) of the phase transition of the mixed solvents were made with a Daini Seikosha Type SSC-56OU. The DSC measurements were carried out at a heating rate of 4 "C/min. (16) Parker, C . A. Anal. Chem. 1962, 34, 502. (17) Azumi, T.; McGlynn, S. P. J . Chem. Phys. 1962, 37, 2413. (18) Grinvald, A.; Steinberg, I. Z . Anal. Biochem. 1974, 59, 5 8 3 . (19) Roberts, A. J.; Phillips, D. J . Chem. Soc., Faraday Trans. 1 1981, 77, 2725.
Handa et al.
280 n m
260
240 lo3 CH3
ux
(cm-' I
Figure 1. Absorption spectra of 4-MP in DCE in the presence of TFA at room temperature. TFA: (1) 0 M; (2) 8.6 X lo-' M.
280
240 I
1
4.2
I
I
I
I
3.0
320 I
I
,
,
,
3.4
,
360 ,
3.0
,
,
,
400 n m ,
,
,
,
,
,
2.6
Y x 1oV4(cm-')
Figure 2. Absorption (Ab), fluorescence (Fl),and fluorescence excitation (Ex) spectra of 2-EP in DCE in the presence of TFA at room temperature. Ab: TFA (1) 0 M; (2) 8.6 X lo-' M. F1:TFA (3) 8.6 X lo-' M; (4) concentrated system, 2-EP 0.2 M and TFA 0.2 M. Ex (- - -): TFA ( 5 ) 8.6 X M; emission 310 nm. The fluorescence spectrum of the
concentrated system was measured by using a triangular cell.
Results Absorption and Fluorescence Spectra at Room Temperature. The absorption spectra of 4-methyl- and 2-ethylpyridines (4-MP and 2-EP) in DCE in the presence of TFA are shown in Figures 1 and 2 as typical cases. The absorption spectrum of 4-MP consists of two bands at about 260 and 287 nm, which are assigned to the S2(71.7r*) -So and S,(n?r*) Sotransitions, The S2(71.n*) So band exhibits a vibronic structure with a main peak at 257.5 nm. On an addition of a saturated TFA concentration M, the maximum extinction coefficient increases of 8.62 X from 1.7 X lo3 to 4.6 X lo3,and the main peak wavelength is shifted to the blue by about 3 nm. In the Sl(nr*) So band region (-287 nm), the absorption intensity of 4-MP in DCE in the presence of TFA decreases compared with that of 4-MP in DCE. This result is due to the blue shift of the S l ( n r * ) So band by the p r o t o n a t i ~ n . ~ ' - ~ ~ The effect of TFA concentration on the absorption spectrum of 2-EP resembles that of 4-MP except for the following main
-
+-
-
-
(20) Rush, J. H.; Spner, H. J . Chem. Phys. 1952, 20, 1847. (21) Stephenson, H.P. J . Chem. Phys. 1954, 22, 1077. (22) Kasha, M. Discuss. Faraday SOC.1950, 9, 14. (23) Andon, R. J. L.; Cox, J. D.; Herington, E. F. G. Trans. Faraday Soc. 1954, 50, 918. (24) Barrow, G. B. J . Am. Chem. SOC.1956, 78, 5802. (25) Odinokov, S. E.; Mashovsky, A. A,; Glazunov, V. P.; Iogansen, A. V.; Rassadin, B. V. Spectrochim. Acta 1976, 32, 1355.
Protonated Pyridine Systems 240 I
280 I
I
I
Ex
I
4.2
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2591
I
320 I
1
1
I
I
360 1
1
1
1
~
FI
3.4
3.O
u x lo-~(cm-')
-
differences: The main peak wavelength is shifted to the red by about 3 nm. In the low-energy region (-287 nm), where the Sl(na*) So band is considered to be located,20 the spectral behavior is the inverse of that of 4-MP-TFA system. This result presumably results from the marked overlapping between the weak S,(n?r*) So and strong Sz(n?r*) So bands in free 2-EP. Consequently, the blue shift of the Sl(n?r*) So band by the protonation is not clearly resolved. The intensity enhancement of the Sz(.rr~*) So transition for pyridine and mono- and dialkylpyridines due to the protonation has been n0ted.~22~362'In particular, this phenomenon for pyridine has been studied theoretically.2e29 The spectral data of other molecules are given in Table I. The fluorescence and fluorescence excitation spectra of 2-EP in DCE in the presence of TFA are shown in Figure 2. 2-EP exhibits no observable fluorescence in DCE in the absence of TFA. The fluorescence spectrum of 2-EP solution containing a satM exhibits a fairly weak urated TFA concentration of 8.6 X and broad band with a peak at 295.0 nm, the tail reaching around 400 nm. This fluorescence spectrum shows approximately mirror-image relation with the absorption spectrum. The fluorescence excitation spectrum obtained by monitoring the fluorescence wavelength of 310 nm is similar to the absorption spectrum. The fluorescence lifetime is found to be shorter than 0.5 ns (instrumental limitation), and the quantum yield is on the order of Further, as shown in the figure, even for a highly concentrated system of 2-EP (0.2 M)-TFA (0.2 M), the fluorescence spectrum exhibits only a single band with a peak at about 300 nm, and its spectral shape is similar to that of 2-EP (2.0 X lo4 M)-TFA (8.6 X M) system. The change of the peak wavelength from 295.0 to ca. 300 nm is due to the reabsorption of fluorescence. The fluorescence lifetime is shorter than 0.5 ns. These observations indicate that in the 2-EP-TFA system, the 325-nm fluorescence and the 370-nm one, which appear for the P2VP-TFA system,I are not observed. No fluorescence was observed for pyridine, 4-n-alkyl- and 2-methylpyridines in DCE in the presence of TFA. In order to clarify the species emitting the 295-nm fluorescence, we measured the fluorescence spectrum of 2-EP-HC1 system in DCE, which was prepared by a direct bubbling of dry HCl gas into a 1.0 X lo4 M 2-EP solution in DCE. The fluorescence spectrum from this sample was identical with that of 2-EP-TFA system in Figure 2. This result indicates that the 295-nm fluorescence originates from a ( m r * ) character of the protonated 2-EP. The peak wavelength (Amx), quantum yield (4), and lifetime ( 7 ) of other molecules are given in Table I. The 4 values of
-
- -
(26) Coppens, G.; Gillet, C.; Nasielski, J.; Vander Doncket, E. Spectre chim. Acrai962, 18, 1441. (27) Mataga, S.;Mataga, N. 2.Phys. Chem. (Wiesbaden) 1959,19,231. (28) Brown, R. D.;Hefferman, M.L. Ausr. J. Chem. 1957, 10. 211. (29) Yoshida, Z.; Kobayashi, T. Theor. Chim. Aero 1971, 20, 216.
280 I
I
I
I
320 1
I
I
I
360 I
I
I
I
1
400 nm I
I
I
l
i
l
tc L
2.6
Figure 3. Fluorescence (Fl) and fluorescence excitation (Ex) spectra of 2-EP in MP in the presence of TFA at 77 K. Fl and Ex: TFA 4.3 X 10" M. Ex: emission 310 nm.
r
1
1
I
3.8
240
400 nm 1
u x 1 0 - ~ (cm-') Figure 4. Absorption (Ab), fluorescence (Fl),and fluorescence excitation (Ex) spectra of 4-MP in THF-MeOH-MTHF in the presence of TFA at 77 K. Ab: TFA (1) 0 M; (2) 4.3 X 10" M; F1 (b): TFA (1') 0 M; (3) 1.2 X lo4 M; (4) 2.2 X 10" M; (5) 4.3 X 10" M; Fl(a): Difference spectra between the spectra of 4-MP-TFA systems and background signal (4-MP solution). Ex (- -): TFA ( 6 ) 4.3 X loT4M emission 330
-
nm. r
280
240
4.2
3.8
320
3.4
3.0
360
400r
n
2.6
vx Figure 5. (a) Absorption (Ab), fluorescence (Fl), and fluorescence excitation (Ex) spectra of 2-EP in THF-MeOH-MTHF in the presence of TFA at 77 K. Ab: TFA (1) 0 M; (2) 4.3 X 10" M. Fl: TFA (3) 1.7 X M; (4) 4.3 X M; (5) 8.6 X M; (6) 2.2 X lo" M; (7) 4.3 X lo4 M. Ex (---): TFA (8) and (9) 4.3 X 10" M; (8) emission 300 nm; (9) emission 330 nm. (b) Fluorescence polarization spectrum: TFA 4.3 x 10-4 M.
monoalkylpyridines are on the order of while those of dialkylpyridines are on the order of except for 2,4-dimethylpyridine. These values are found to be about 1 order of magnitude smaller than those of ethylbenzene (4 = 6.6 X and 2The 7 values of mono- and diethyltoluene (4 = 8.9 X alkylpyridines are shorter than 0.5 ns, and found to be significantly shorter than those of ethylbenzene (T = 11.4 ns) and 2-ethyltoluene (T = 10.5 ns). Absorption and Fluorescence Spectra at 77 K. The fluroescence and fluorescence excitation spectra of 2-EP in M P in the presence of TFA are shown in Figure 3. 2-EP exhibits no observable fluorescence in M P in the absence of TFA. As shown in figure, the fluorescence spectrum of 2-EP solution containing a saturated TFA concentration of 4.3 X lo4 M resembles that of the 2-EP-TFA system in DCE at room temperature and no other fluorescence is observed. Likewise, the fluorescence excitation spectrum obtained by monitoring the fluorescence wavelength of 310 nm is similar to that of the 2EP-TFA system in DCE at room temperature except for an appearance of weak vibronic bands. No fluorescence was observed
2592
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Handa et al.
TABLE I: Absorption and Fluorescence Data for Pyridine and Mono- and Dialkylpyridines in DCE at Room Temperature absorption, A,, nm (c,,,)~ fluorescence (protonated molecule)“ compound free molecule protonated molecule“ h” nm 102@ T , ns pyridine 257.0 (1.7 X 10’) 256.5 (4.7 X lo3) 264.0 (6.8 X lo3) 2-methylpyridine 263.0 (2.7 X lo3) 3-methylpyridine 264.5 (2.5 X lo’) 264.0 (5.5 x 103) 287.0 0.15
