The Journal of Physical Chemistry, Vol. 83, No. 15, 1979 2019
Fluorescence Quenching of Phenylalkylamines
Oxygen quenching of fluorescence was investigated by first degassing (three freeze-pump-thaw cycles) solutions contained in 15-nm o.d, Pyrex cells equipped with teflon vacuum stopcocks. Oxygen was then admitted to the vacuum manifold to a given pressure and one cell opened to the manifold at 17 "C. The oxygen pressure, as recorded by a mercury filled U-tube manometer, was corrected for the vapor pressure of benzene (50 torr, 17 "C) and the mole fraction of oxygen in benzene ( X o J calculated from the corrected pressure (Po,)according to eq 4.23 This process
Xo, = 8.15 x
(4)
was repeated for different pressures over the range from 0 to 1.0 atm. Stilbene Isomerization. Degassed samples were prepared by three freeze-pump-thaw cycles and sealed under vacuum. Oxygenated samples were either bubbled with oxygen and closed with a syrum cap a t 1.0 atm (Tables 11 and 111) or oxygenated as described for fluorescence quenching studies (Figure 2). Samples were irradiated on a merry-go-round apparatus immersed in a water bath thermostated a t 20 f 1 "C. Monochromatic 313- and 365-nm irradiation was provided by a potassium chromate solution and Corning 7-54 and 0-52 filters, respectively. Light intensities were measured by benzophenonebenzhydrol a ~ t i n o m e t r y .The ~ ~ extent of trans-stilbene isomerization (and fumaronitrile isomerization) was determined with a Hewlett-Packard 5750A dual-flame ionization gas chromatograph with a 6 ft X 'Isin. column containing 5% SF96 on Chromasorb G. Acknowledgment. The authors thank Professors D. R. Arnold and R. A. Caldwell for helpful suggestions and permission to cite unpublished results. Support of this work by the National Science Foundation (CHE78-01120) is gratefully acknowledged. Funds for the purchase of the fluorescence spectrometer were provided by the National Science Foundation and the Northwestern University Research Committee.
References a n d Notes (1) N. Orbach and M. Ottolenghi in "The Exciplex", M. Gordon and W. R. Ware, Ed., Academic Press, New York, 1975, pp 75-112. (2) K. Schulten, H. Staerk, A. Weller, H.-J. Werner, and B. Nickel, Z. Phys. Chem., 101, 371 (1976). (3) (a) T. Nishlmura, N. Nakashima, and N. Mataga, Chem. Phys. Lett., 46, 334 (1977); (b) N. Mataga, M. Migita, and T. Nishimura, J . Mol. Structure, 47, 199 (1978). (4) G. G. Aloisi, U. Mazzucato, J. B. Birks, and L. Minuti, J. Am. Chem. Soc., 99, 6340 (1977). (5) H. Hayashi and S. Nagakura, Chem. Phys. Lett., 53, 201 (1978). 16) (a) D. Creed. R. A. Caldwell. and M. McK. Ulrich. J. Am. Chem. Soc.. 100, 5831 (1978); (b) R. A. Caldwell and D. Creed, J . Phys. Chem.; 82. - -, -2644 - . . 11978). . - . -,. (7) D. R. Arnold and P. C. Wong, J. Am. Chem. SOC.,101, 1894 (1979). (8) (a) F. D. Lewis, Acc. Chem. Res., 12, 152 (1979). (b) F. D. Lewis and C. E. Hoyle, J . Am. Chem. Soc., 99, 3779 (1977); (c) F. D. Lewis and D. E. Johnson, ibid., 100, 983 (1978); (d) F. D. Lewis et ai., ibid., in press. (9) F. D. Lewis and Ho, J . Am. Chem. Soc., 99, 7991 (1977). (10) (a) J. L. Charlton and J. Saltiel, J . Phys. Chem., 81, 1940 (1977); (b) M. Sumitani, N. Nakashima, Y. Yoshihara, and S.Nagakura, Chem. Phys. Lett., 51, 183 (1977); (c) 0. Teschke, E. P. Ippen, and G. R. Holtom, ibid., 52, 233 (1977); (d) L. A. Brey, G. B. Schuster, and H. G. Drickamer, J . Am. Chern. SOC.,101, 129 (1979). (11) Similar results have been obtained by Arnold and Wong' for the TS/fumaronitrile exciplex. (12) J. P. Petrovich, M. M. Baker, and M. R. Ort, J . Nectrochem. Soc., 116. 743 11969). (13) T. Forster'in r e i 1, p 1-22. (14) E. J. J. Groenen and P. N. T. van Velzen, Mol. Phys., 35, 19 (1978). (15) (a) H. Beens, H. Knibbe, and A. Welier, J . Chem. Phys., 47, 1183 (1967); (b) N. Mataga, T. Okada, and N. Yamamoto, BUN. Chem. SOC.Jpn , 39, 2562 (1966). (16) J. Saltiei, D. E. Townsend, B. D. Watson, P. Shannon, and S. L. Finson, J . Am. Chem. Soc., 99, 884 (1977). (17) (a) J. Saltiel et ai., Org. Photochem., 3, 1 (1973); (b) J. Saltiei et al., Pure Appi. Chem., 41, 559 (1975). (18) Estimated from 23.06 TS) - E,,,d") (D. Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970)) by using E,,;* from Table Iand E,/? = 1.50 V vs. SCE in acetonitrile (R. Dietz and M. E. Peover, Discuss. Faraday Soc., 45, 154 (1968)). (19) For a recent discussion of electron transfer in radical ion pairs see: R. P. Van Duyne and S.F. Fischer, Chem. Phys., 5, 183 (1974). (20) 0. L. Chapman, R. D. Lura, R. M. Owens, E. D. Plank, S.C. Shim, D. R. Arnold, and L. B. Gillis, Can. J . Chem., 50, 1984 (1972). (21) R. Foster, "Organic Charge-Transfer Complexes", Academic Press, 1969, New York, Chapter 6, and references therein. (22) J. N. Demas and G. A. Crosby, J . Phys. Chem., 75, 991 (1971). (23) J. E. Jolley and J. H. Hiidebrand, J. Am. Chem. SOC.,80, 1050 (1958). (24) W. M. Moore and M. Ketchum, J . Chem. Soc., 84, 1368 (1962). I
.
1
Intramolecular Fluorescence Quenching of Phenylalkylamines Haruo Shiruka, * Makoto Nakamura, and Toshifumi Morita Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received February 13, 1979) Publication costs assisted by Gunma University
Intramolecular normal fluorescence quenching of phenylalkylamines has been studied by means of fluorometry and nanosecond time-resolved spectroscopy. No intramolecular exciplex emission was observed. The quenching involving intramolecular electron transfer and subsequent proton transfer is significantly dependent upon the number of methylene units, solvent polarity, and temperature. In nonpolar and weakly polar solvents, static quenching resulting from a sandwich-like or holding structure occurs markedly for C3, but not for C1 and C2. In strongly polar solvents, dynamic quenching takes place in the order of C1, C3, and Cz. The quenching rate constants in EtOH can be elucidated in terms of intramolecular electron transfer as a function of rotational diffusion (Iz, = CY'TV-' + p). Large activation energies for electron transfer in the excited state observed in EtOH may be attributed to slight electron overlapping between the benzene ring and amino group in the excited state and to negatively large activation energy for solvent viscosity of EtOH. Through-bond CT interaction was scarcely observed in the sample molecules. Introduction It is well known that sandwich structures are favorable for excimer formation as have been shown by studies on the Since the original works of Forster and Kasperl on pyrene crystal e ~ c i m e r , sandwich ~,~ dimer~,~JO and inexcimers and of Leonhardt and Weller' on exciplexes, tramolecular excimers.l1JZ There have been several inintra- and intermolecular excimers or exciplexes have been extensively studied and their structures well e ~ t a b l i s h e d . ~ ~ vestigations about intramolecular exciplex formation which 0022-3654/79/2083-2019$01 .OO/O
0 1979 American
Chemical Society
2020
The Journal of Physical Chemistry, Vol. 83,No. 15, 1979
is not so restricted by geometric requirement~.l~-'~ Dynamic behaviors of intramolecular exciplex formation have been studied by means of picosecond spectroscopy by Eisenthal et al.16J7and also Mataga et a1.l8 However, few studies on the geometric requirements for exciplex formation of fluorescence quenching have until recently been rep~rted.~~~~~ I n the present paper, solvent, temperature, and methylene unita effects upon the intramolecular quenching of phenylalkylamines are studied by means of fluorometry and nanosecond time-resolved spectroscopy. The title compounds were chosen because of the following: (1)There was no intramolecular exciplex emission from the molecules, though the normal fluorescence quenching occurred effectively. ( 2 ) Simple kinetic treatments were, therefore, expected to analyze the intramolecular quenching process involving electron transfer. (3) Molecular structures of the samples are very simple in comparison with those of (carbazole)-(CH,),-(terephthalic acid methyl ester) studied by Hatano et a1.,20and the relaxation processes in the excited state might be compared with those of toluene. (4) Phenylalkylamines have a r-electron system (benzene ring) and a lone pair of electrons (amino group) separated from each other by methylene bond(s). Are the quenching behaviors different from those of the other intramolecular electron donor and acceptor systems having a couple of r-electron systems or not? (5) Is there through-bond CT interaction22in the present molecules? Hereafter phenylalkylamines are denoted as C1, C2,and C3 indicating the number of methylene units:
n = 1, 2, 3
Experimental Section The sample molecules C1-C3 (G.R. grade products from Tokyo Kasei) were purified by repeated vacuum distillations. Toluene (a spectrograde product from Wako) was used without further purification. Cyclohexane (CH), methylcyclohexane (M), and isopentane (P) (G.R. grade products from Tokyo Kasei) were purified by passing them through a silica gel column. Acetonitrile (AN) was purified by the usual methodsz3 Ethanol and ether (Kanto Kagaku), sulfuric acid (Junsei Kagaku), and sodium hydroxide (Wako) were G.R. grade products, and they were used without further purification. Distilled water was used. All samples ( 2 X M) were thoroughly degassed by freeze-pump-thaw cycles. The pK, values of the sample molecules were measured with a Hitachi-Horiba pH meter (M-7E). The experimental procedures were the same as reported p r e v i o u ~ l y .The ~ ~ fluorescence quantum yields were determined by comparison with that of toluene at 300 K ( a ~ = 0.14).3 The fluorescence decays were recorded with a Hitachi nanosecond time-resolved spectrometer (pulsewidth 11 ns). The convolution method was applied to determine the fluorescezice lifetimes when the lifetimes were shorter than 20 ns.25 Results and Discussion ( 1 ) Absorption and Fluorescence Spectra of Phenylalkylamines. Figure 1shows absorption and fluorescence spectra of sample molecules C1-C3 in CH (a) and EtOH (b) a t 300 K, which are very similar to those of toluene. The first absorption band at 259 nm corresponds to the
H. Shizuka, M. Nakamura, and T. Morita wavelength
400
/ nrn
300
350
250
3.0 1.0
25
(a)
in^^
40
35
30
45
wavenumber / k K
Figure 1. Absorption and fluorescence spectra of phenylalkylamines in CH (a) and EtOH (b).
TABLE I: Molar Extinction Coefficients ( e ) at Absorption Maxima (Amm) of the 'B, Bands of Phenylalkylamines in CH Am=/
samples
nm
toluene Cl
262 259 259 259
c* c,
in EtOH
€ / l o 2M-' cm-'
a
2.80~ 1.86 1.66 1.89
Amax/
nm 262 259 259 259
e/lO*
M-'
cm-'
a
2.04 2.32 1.82
1.73
a Errors within i 3%. Data taken from I. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1965, p 47.
-
lBz lAl transition in toluene, and fluorescence emission originates from the 'Bz state. The molar extinction coefficients (4at the lBz band are not much different from those of toluene both in CH and EtOH (Table I). Pasman et a1.22have reported that broadening and apparent inlAl transition in tensification of the aromatic lB2 D-(CHJ2-A, where D and A denote dimethoxyphenyl and 1,l-dicyanomethylene respectively, are observed and these phenomena are attributed to intramolecular CT interaction leading to absorption in a region overlapping the lBz band. However, no broadening and intensification of the lB2 lAl transition in phenylalkylamines were observed (Figure 1). It can be said from study of the absorption spectra data that no appreciable CT interaction occurs between the benzene ring and the amino group in the ground state of the sample molecules both in nonpolar and polar solvents. No intramolecular exciplex emission from phenylalkylamines was observed in various solvents, although normal fluorescence quenching took place effectively. (2) Solvent and Temperature E f f e c t s on the Normal Fluorescence Quantum Yields (ar)and Lifetimes (7).The values of afand T were markedly dependent upon solvent polarity and also temperature. For example, their values in various solvents at 300 K are shown in Table 11. In nonpolar and weakly polar solvents (CH, MP, and ether), the values of af and T at 300 K are in the order toluene, Cz, C1, and C3. In strongly polar solvents [EtOH, AN, and a mixture of HzO-EtOH (4:l in volume) at pH 133, they are in the order toluene, Cz, C3, and C1. Figure 2 shows the temperature dependences of the normal fluorescence lifetimes T . The values of T increase with decreasing temperature both in nonpolar and polar solvents.
-
-
The Journal of Physical Chemistry, Vol. 83, No. 15, 1979
Fluorescence Quenching of Phenylalkylamines
2021
TABLE 11: Normal Fluorescence Lifetimes (r) and Quantum Yields ( O f ) of Phenylalkylamines in Various Solvents at 300 K r/ns r$fc/lo-z solvents
ea
nb/10-2 P
CHd ether EtOH AN H,O-EtOHe
2.02 4.33 24.5 37.5 67
0.93 0.230 1.043 0.335 1.742
toluene 36.1 1 33.1 ? 1 28.0 i 1 25.2 f 1 5 . 2 ? 0.8
*
CI 27.5 i 1 23.0 * 1 2.5 i 0.5 7.2 i: 0.8 2.0 t 0.5
c, 28.8 f 1 27.2 * 1 22.7 t 1 13.1 t 0.8 4.6 i. 0.5
-
C, 2 . 3 * 0.3 1.2 t 0.4 6.2 ?: 0.5 2.8
f
0.4
toluene 14 12.5 11.4 10.2 2.6
C, 9.7 8.1 0.87 2.5 0.88
Cl 11 10
8.2 5.0 2.1
c3 0.91 0.45 2.7 1.3
Data taken from “Handbook of Chemistry”, Chemical Society of Japan, Maruzen, Tokyo, 1966, a Dielectric constants. The values of and @ f in CH were equal t o those in MP ( 3 : l ) within experimental errors. p 505. Errors within i10%. e In a H,O-EtOH ( 4 : l ) mixture at pH 13. 60
40 VI \
P 20
0 150
200
2 50
300
T / K
150
200
2 50
300
T / K
Flgure 2. Temperature dependences of the normal fluorescence lifetimes (7)of phenylalkylamines in MP (a) and EtOH (b).
In a nonpolar solvent (MP), the temperature dependence of C3 is quite different from those of C1, C2, and toluene, and the T values of C3 are very small compared with those of the other molecules (Figure 2a). This result may be caused by static quenching resulting from a sandwich-like or holding structure in the ground state of C3 in MP. In spite of no appreciable change in the absorption spectrum of C3 in MP, for C3 a holding structure seems to be much more stable than an extended structure in the ground state in a poor solvent such as MP. The intramolecular fast quenching of anthryl-(CH,),-N,Ndimethylaniline in nonpolar solvents due to a sandwich-like configuration has been demonstrated with picosecond pulses by Gnadig and Eisentha1.l’ It is noteworthy that the T value of C3in M P gradually increases with decreasing temperature. Therefore, a slight dynamic change in the holding structure of C, must be needed for quenching. As for C1 and C2, it is restricted to form a holding structure in the ground state according to geometrical requirements. The intramolecular quenching due to static and dynamic CT interactions was scarcely observed for C1 and C2 in nonpolar media as will be shown later. Hatano et a1.20 have reported intramolecular fluorescence quenching of (carbazole)-(CH2),-(terephthalic acid methyl ester) in methyltetrahydrofuran, MTHF; static quenching
is observed for n = 1 and 2, but both static and dynamic processes of quenching occur for n = 3. The difference in the number of methylene units n between phenylalkylamines and (carI~azole)-(CH~),-(terephthalicacid methyl ester) may be attributed to that of electron overlapping between donor and acceptor in the excited state. A large charge transfer character of the exciplexes for the latter compoundsm is known in contrast to the former molecules. The presence of the oxygen atom between methylene and carbonyl groups of the latter compounds may also relate to formation of a holding structure. On the other hand, the 7 values of the sample molecules in EtOH decreased sharply in the order of C1, C3, Cat and toluene with elevating temperature as shown in Figure 2b. Geometric requirements for the intramolecular CT interaction in the excited phenylalkylamines in polar media were not restricted as have been originally shown by Chandross and Thomas13 and also Okada et al.14 The structures of phenylalkylamines in the ground state may be extended configurations in a good solvent such as EtOH. Dynamic quenching due to a CT interaction was, therefore, expected. The experimental results in polar solvents can be accounted for by eq 1,where k,, k f , and kn are the rate constants for electron transfer, radiative, and radiationless transitions (internal conversion hi, plus intersystem crossing kiSJ of the locally excited state of phenylalkylamines Ph*-(CH2),-NH2, respectively, k,, k,,, lz If, and k ’,, the rate constants for back-electron transfer, proton transfer, radiative, and radiationless transitions of the charge transfer state Ph--(CH2),+NH2, respectively, and k’h the rate constant for annealation processes of the biradical PhH-(CH2),-NH. A similar scheme for intermolecular fluorescence quenching of pyrene by amines, Ph*-(CH,),-NH,=
k.5
+
kpt
Ph--( CH,),-NH,yWLc
Ph-( CH,),-NH, P~H-(cH,),-NH
(1)
involving electron transfer and subsequent proton transfer, has been reported by Okada et the value of k,, is estimated to be 1 2 X 10’O s-’. It is known that intramolecular proton transfer in the excited state is very fast ( -1010 s-1).24No exciplex emission of the sample molecules is due to the large value of k,, compared with those of the other processes in the charge transfer state. That is, eq k,, >> k’, k:, k-, (2)
+
+
2 holds. Single exponential decay of the normal fluorescence of phenylalkylamines observed with pulse excitation supports eq 2. Of course, no intermolecular
2022
The Journal of Physical Chemistry, Vol. 83, No. 15, 7979
a
H. Shizuka, M. Nakamura, and T. Morita
9
9 (c)
7
8
8
Q
73
U
6'ul
-
Y
x
0 7
-0 7
0
0
0
w
5
4
6
3
5
4
in EtOH
~-l/ 10-3 K
6
5
I
3
4 T-'/
5 IO-3~
6
I
I
6
4
3
5 ~ - 1l / 0-3~
Figure 3. Arrhenius plots of log kdvs. T-' in MP (a), log kdvs. T-' in EtOH (b), and log k , vs. T-' in EtOH (c).
TABLE 111: Frequency Factors (A) and Activation Energies ( A E ) for Deactivation Process ( k d ) and Electron Transfer (k,) in the Excited State of Phenylalkylamines Ad/lO" samples
MP
toluene Cl
44i 3 58i 5 532 4
c2
c,
a
S-'
EtOH 0.5 k 1.0 i 3.4 t 2.5 i
0.1 0.2 0.3 0.3
AEd/kCal mol-' MP EtOH 9.1 i 0.6 9.2 i 0.8 9.2 i 0.7
6.0 i 4.7 i 7.0 i 5.9 i
0.4 0.4 0.6 0.5
A,/lO'* s - ' EtOH
AE,/kcal mol-' EtOH
1.4 r 0.3 (63Y 8.1 i 0.5
4.8 t 0.4 (9.5)U 6.6 i 0.5
Errors within 30%.
fluorescence quenching occurred under the experimental condition, since the values of af and T at sample concentrations S10-3M were the same as those at 2 X M. For intermolecular electron donor and acceptor systems, electron transfer both in singlet and triplet states have been studied and discussed ~ e l l . ~ ' - ~ l From the usual steady-state approximation, the fluorescence quantum yields (af) for the radiative transitions S1(lB2) So(lA ) is simply given by
-
(3) where h d and T are the rate constant for the total radiahe) and the lifetimes of tionless processes (hd = h, Ph*-(CHz),-NH2 a t temperature T , respectively. At first, the values of h d in M P and EtOH, except for that of C3 in MP, were estimated from the following equation:
+
hd 5
7-1
-
7,-1
(4)
where 7, is the constant value for the fluorescence lifetime at low temperatures. The 7, values for C1, C2,and toluene in M P were 39, 43, and 57 ns within 3% error at 203 K, respectively. In EtOH, the 7 , values for C1, C2, Cs, and toluene were about 60 ns at 140 K. From the assumption of the Arrhenius equation, eq 5 is derived, where A d and (5)
are the frequency factor and activation energy for h d , respectively. Plots of log h d vs. T1in MP and EtOH, which agree with eq 5, except for that of C3 in MP, are shown in Figure 3. The Ad and a d values obtained are listed in Table 111. The values of Ad and LEd for C1 and
C2 in M P are close to those of toluene, suggesting that features of relaxation processes in MP for C1 and C2 are similar to that for toluene. Therefore, the rate constants of k, for C1 and Cz in MP can be negligible compared with those of k,(= hi, kiBc).For C3 having a sandwich-like or holding structure in nonpolar and weakly polar solvents, the intramolecular static quenching due to electron transfer occurs effectively as described above. The value of k, for C3 in MP is greater than those of kf plus k,, and it can be roughly estimated from the following relation: he x T - ~for C3 in MP. From the plot of log k, vs. T1,the activation energy (LE,) for electron transfer was evaluated to be 1.9 kcal/mol for C3 in MP. The activation energy for solvent viscosity of M P was about -1.8 kcal/mol. The ,1E, value for C3 in MP is very close to the absolute value of solvent viscosity of MP. Similarly, the values of he for phenylalkylamines in EtOH were estimated from eq 6, where T and Ttol are the
+
he x
7-l
-
Tto?
(6)
normal fluorescence lifetimes for phenylalkylamines and toluene in EtOH at temperature T , respectively. Equation log k, = log A,
-
AEe 2.303RT
-
(7)
7 may also hold, where A, and AE,are the frequency factor and potential barrier for he, respectively. Plots of log k, as a function of T1are shown in Figure 3c, which agree with eq 7. The AE,values in EtOH are in the order of Cz, C3, and C1 (see Table 111). The small AEe value for C1 in EtOH may be due to the shortest distance between the benzene ring and amino group among C1-C3. The fact that the value of a,for C3 is smaller than that for C2 indicates that the conformational change favorable for electron
The Journal of Physical Chemistty, Vol. 83, No, 15, 1979
Fluorescence Quenching of Phenylalkylamines
5
I
2023
1
I
in EtOH
;;y;;;;;, c3
0
> ,;
10.2i0.1 10.lt0.2
2
4
8
6
10
12
14
PH Figure 4. Plots of k , as a function of
Tq-' in EtOH.
TABLE IV: Specific Values in the Dynamic Quenching Via Rotational Diffusion for Phenylalkylamines in EtOH'
a'1104 (TIV 10 I deg-' V/cmI3 l O ~ - ~ e gp / 1 0 8 samples P s - ' a / 1 0 - 3 mol' S-' -0.39 3.67 22 0.28 C, 1.39 44 2.20 -0.41 0.98 C, 0.19 -0.52 5.36 66 0.77 C, 0.68 a
See the text.
transfer in the excited state of Cz is not so easy in comparison with that of C,. The value of AE, (1.9 kcal/mol) for C3having a holding structure in M P is relatively close to those for intramolecular exciplex formation of 1-(9,1O-dicyano-2-anthryl)3-(naphthy1)propane (2.7-2.8 kcal/mol in 3-methylpentane; 2.0-2.2 kcal/mol in MTHF).15 However, the values of me (14.8 kcal/mol) for phenylalkylamines in EtOH are very large in comparison with those of activation energies for intramolecular excimer formation [3.3 kcal/mol for 1,3-(a,a'-dinaphthy1)propane and 4.0 kcal/mol for 1,3(P,p-dinaphthy1)propane in MP (9:1)]12 and for intramolecular exciplex formation [0.2-3.2 kcal/mol for (carbazole)-(CH,),-(terephthalic acid methyl ester) in MTHF].20 The difference in the activation energy for electron transfer may be caused by the difference in solvent viscosity: e.g., the activation energy (viscosity) of MTHF is -1.8 kcal/molZ0and that (viscosity) of EtOH is -3.2 kcal/mol. It seems that intramolecular electron overlapping between donor and acceptor in the excited state is also an important factor for electron transfer as well as solvent viscosity. The electron overlapping in the excited state of phenylalkylamines may be smaller than that of the other molecules, since the former compounds are not comprised of a couple of r-electron systems but a xelectron system (benzene ring) and a lone pair of electrons (amino group). ( 3 ) Intramolecular Quenching Involving Electron Transfer Via Rotational Diffusion. Let us further consider dynamic processes in the excited state of phenylalkylamines. We studied the relation of he to Tq-' in EtOH, where q is viscosity in poise. Figure 4 shows plots of k, as a function of T7-l. There are the thresholds at 0.28 X lo4 (Cl), 2.20 X lo4 (C2),and 0.77 X lo4 deg P-I (CJ. The values of k, increase linearly with increasing T f l after passing through the threshold (Tq-l),,. The value of he in the region Tq-l > ( T T - ~is) therefore ~ given by k, = a'(T/r)+ P (8) where a' and P are constants (see Table IV). The value
Figure 5. Fluorescence titration curves of phenylalkylamines in a H,O-EtOH mixture (4: 1).
of the rate constant for rotational diffusion in EtOH, k r d , can be expressed by the Debye rotational correlation time, T
~
:
~
~
=k,d-') = 47rao3?7/(3Kr) (9) where a. is the radius of the rotating particle and K the Boltzmann constant. We thus obtain k r d = RT/(Vq) (10) where R is the gas constant and V the specific volume of the rotating sphere. Substituting eq 10 into eq 8, we obtain (11) he = akrd + fi where a = a'VR-l. From the slopes and intercepts in Figure 4, the values of cy' and can be obtained, which are listed in Table IV. The values of a' relate to overall efficiencies for intramolecular electron transfer in the excited state of phenylalkylamines via rotational diffusion. The experimental values of a' are in the order C1, C,, and Cz, indicating that the order of electron transfer efficiencies due to dynamic motion (Brownian motion) of excited phenylalkylamines is the same as that for the cy' values. It is of interest that the larger slopes (a') become the smaller threshold values ( T V - ~as) shown ~ in Figure 4 and Table IV. These findings suggest that intramolecular electron transfer from the amino group to the benzene ring in the excited state of phenylalkylamines considerably depends upon the number of methylene units in polar solvents. If the specific volumes (V)of the rotating parts (-NH2, -CHzNHz, and -(CHJ2NH2) for C1, C2, and C3 are approximately assumed to be 22, 44, and 66 mL mol-', respectively, the a values which correspond to the efficiencies for electron transfer per a rotational collision in the excited state can be evaluated. The a value order (C, > C1 > C,) is in accord with that of the geometrical requirements (see Table IV). ( 4 ) pH Dependence of Fluorescence Quantum Yields (af).The pH effect on afwas examined in order to obtain information on the interaction between the benzene ring and the protonated amino group in the excited state. Figure 5 shows the fluorescence titration curves of phenylalkylamines in a H,O-EtOH mixture (4:l in volume) at 300 K. At lower pH values 1 5 , the values of CPf and 7 and 4.9 f 0.5 ns, respectively, for C1-C6 are about 2.3 X which are almost equal to those of toluene. This is evidence that a lone pair of electrons from the amino group takes part in the intramolecular fluorescence quenching involving electron transfer. The afvalues decrease with increasing pH values to give constant values of 9fand T (see Figure 5 and Table 11) at pH values 213. The af TO(
2024
The Journal of Pbysical Chemistry, Vol. 83, No. 15, 1979
values at p H 13 are in the order C2, C3, and C1, where the amino group of the sample molecules is free from protons. The excited state pK,* values were estimated approximately from the midpoints of the fluorescence titration curves.33 Both pK, (measured by a pH meter) and pK,* values are given in Figure 5. The pK,* values in the lowest excited singlet state of phenylalkylamines are equal to those of pK, in the ground state, indicating no CT interaction between the protonated amino group and benzene ring separated by methylene group(s) in the excited state. The intramolecular quenching of phenylalkylamines free from protons in the HzO-EtOH mixture occurs in the order C1, C3, and Cz.
Summary No intramolecular exciplex emission of phenylalkylamines was observed, and the normal fluorescence quenching took place effectively resulting from intramolecular electron transfer and subsequent proton transfer. The fluorescence quenching is considerably dependent upon the number of methylene units ( n ) , solvent polarity, and temperature. C1 shows little CT interaction in the S1state in nonpolar and weakly poIar solvents. Dynamic quenching due to electron transfer in the S1 state occurs markedly in polar media. Cz shows little CT interaction in the S1state in nonpolar and weakly polar solvents. Only a little dynamic quenching occurs in strongly polar solvents. C3 shows static quenching occurs effectively resulting from a sandwich-like or holding structure in the So state in nonpolar and weakly polar media (poor solvents). Dynamic quenching takes place in the S1state since C3has an extended structure in good solvents (strongly polar solvents). The rate constants for intramolecular quenching in EtOH can be elucidated in terms of electron transfer as a function of rotational diffusion ( k , = ~ ' T v+- 6). ~ Large activation energies for intramolecular electron transfer in the excited state of phenylalkylamines in EtOH may be attributed to small electron overlapping between the benzene ring and amino group in the excited state and also negatively large activation energy for solvent viscosity of EtOH. Through-bond interaction was scarcely observed in the sample molecules. Acknowledgment. The authors are grateful to Miss N. Tomiyasu for her assistance in part of the present work.
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