-
3198
J. Phys. Chem. 1981, 85, 3198-3205
Cross sections for u j u)j’ transitions are calculated from the asymptotic forms of IEvj) using
N
e-iK’g[ju)+ R-lCG ’u’)eiK’RfYUtju(fi)
8, KR - 7 2 1 ~ (C-14) where K’ = [2p(E - Edf)]1/2,etc. Using eq C-14, one may relate the scattering amplitudes to the transition matrix elements as fyutju(fi) = 2 ~ i ( K K 1 - ’ / ~ C [ l ] ’ / ~ Y , , ( a ) ~ ~ : , j(C-15)
The total cross section for the transition u j obtained as
-
u’j’is then
This expression should be compared with the cross section in the close-coupling representation, evaluated in the body-fixed frame namely j,
m
du?’,ui)
C [J1 C G]-’l7$~~,Ujfil2
=
J=O
(C-17)
Q=-j
*-% A + D + hvE (AD)* -% A + D k7 = kl + k2 X = k7 + k,[D] k8 = k6 + k6 Y = k6 + kh (AD)* 5 A*
In case of 6 excitation, the time dependence of the emission of the locally excited state I&) and the exciplex I E ( t )are given by
where ____
~~~~~~
~~
(4)(a) A. E. W. Knight and B. K. Selinger, Austr. J. Chem., 10,43 (1971); (b) M.H. Hui and W. R. Ware, J. Am. Chem. SOC.,98,4722 (1976);(c) D. O’Connor and W. R. Ware,ibid., 98,4706(1976).
0 1981 Amerlcan Chemlcal Society
The Journal of Physical Chemistry, Vol. 85, No. 27, 1981 3190
Exciplex Formation In Terminally Substituted Alkanes h2,l
= 3',{X
+ Y f [ ( X - Y)' + 4k3k4[D]]1/2)
TABLE I: Emissive Properties of the Exciplex in THF
-urn=
The quantum yield for emission from the locally excited state $Jmand from the exciplex $JE are given by kl Y = X Y - k3k4[D] k&3[D]
" = XY - k3k4[D]
(3)
(4)
In the case of a strong electron donor and a strong electron acceptor, the following processes6are also possible in polar solvents (6 > 10):
+ D -% A-IID+
A*
(AD)*
A-IID+
A-IID' is the solvated, solvent-separated ion pair. At sufficiently low temperatures where k4 may be neglected compared to k8 N
X
hi
-
N
k,
Y
+ k,[D]
N
k8
@LE = ki/X
(5)
x lo3cm
fwmh X
lo3cm
T,"C P2NM
P3NM
P4NM P2NM P3NM P4NM
-90 -50 -10 t23 t50
24.88 25.73 26.20 26.53 27.24
24.45 25.09 25.77 26.88 28.60
25.300 25.97 26.75 26.93 27.17
4.92 4.93 4.03 5.32 5.52
4.48 4.59 4.72 4.85 5.31
4.98 4.92 5.23 5.31 (a)
The fwmh of the emission of P4NM in THF at 50 " C amounts 7 6 0 0 cm'' due to the presence of both exciplex and amine emission.
solvents the competition between exciplex formation (k,) and the direct formation of the separated ion pair could depend on the starting conformations. Similar results were obtained in intramolecular excimer-forming systems.1° For 2- (1-naphthyl)- 1-(N,N-dimethylamino)ethane in acetonitrile" the naphthalene decay matched neither the growth of the exciplex nor its decay. No interpretation of this behavior was given. Similar observations were made in the study of the exciplex formation of o-phenyl-a(Nfl-dimethy1amino)alkanes. It was suggested that when the orientation of the carbon-nitrogen bond was such that the nitrogen lone pair was trans in relation to the rest of the chain, no overlap between the nitrogen lone pair and the phenyl nucleus was possible and, therefore, exciplex formation was one to two orders of magnitude slower for these molecules.lla When both chromophores were separated by eleven methylene units, the deviations from the normal intermolecular scheme, which were interpreted as conformational effects,l' were not observed. In this contribution we report on the influence of solvent polarity on the relative importance of both conformations and on the different kinetic parameters. We furthermore want to determine the relative importance of the formation of the solvated ion pair in these systems.
Intramolecularexciplex formation and quenching has been observed for an aromatic acceptor linked by a polymethylene chain to an aromatic amine? a methoxy derivative,' or an aliphatic amine.8 The kinetic scheme valid for the intermolecular process has been extended to intramolecular systems. However, Matagas" and EisenthaP observed that, for intramolecular Experimental Section exciplex formation in 3-(9-anthryl)-1-[4-(N,N-dimethyl2-Phenyl-l-(N,N-dimethylamino)ethane(P2NM), 3amino)phenyl]propane, the risetime of the absorption of phenyl-1-(N,N-dimethy1amino)propane(P3NM), and 4the anthracene anion was much shorter than the decay phenyl-1-(N,N-dimethy1amino)butane(P4NM) were obtime of the anthracene emission (5 ns) or of the exciplex tained from Professor A. Gilbert. Their purity was con(140ns). Eisenthal suggested that the anthracene emission trolled by TLC and GLC and exceeded 99.5%. Tetrawas due to a conformation which failed to give the exciplex hydrofuran (Merck, fiir die fluorescenzspektroskopie)was and not to the feedback from the exciplex. In a more refluxed over potassium and distilled under nitrogen prior recent paper, Mataga et al, observed no deviation from the to use. Acetonitrile (Merck fur Spektroskopie) was disintermolecular kinetics scheme for the exciplex formation tilled over sodium carbonate prior to use. Spectra were of 3-(9-anthryl)-l-[4-dimethylamino)phenyl]propanegbin recorded on a Fica FluorimGtre absolu et diff6rentiel. apolar solvents. Weller et al,% found no indication of this Excitation occurred at 265 f 5 nm and the emission slitbehavior in apolar solvents but suggested that in polar width was 7.5 nm. At this excitation wavelenght, 97% of the light absorbed by the phenylalkylamine is absorbed (5)(a) H.Knibbe, K. hllig, P. SchAfer, and A. Weller, J. Chern. Phys., by the toluene chromophore. Decay measurements were 47,1184(1961);(b) N.Mataga, I. Okada, and N.Yamamoto, Chern. Phys. executed with the single-photon technique with a modified Lett., 1, 119 (1967). Applied Photophysics instrument. Excitation occurred at (6) (a) T. Okada, K. Kuyjito, Mm. Kubota, and N. Mataga, Chem. Phys. Lett., 24,563(1972);(b) T. Okada, T. Saito, and N. Mataga, Bull. 260 f 7 nm. The emission slitwidth was 15 nm. At this Chern. SOC.Jpn., 50,331(1977); (c) Y. Hatano, M. Yamamoto,and Y. excitation wavelength, over 90% of the light absorbed by Nishijima, J . Phys. Chern., 80,367(1978). the phenylalkylamine was absorbed by the toluene chro(7)(a) J. H. Borkent, Ph.D. Thesis, Amsterdam, 1976; (b) J. H. mophore. Data processing was done on the IBM 380 Borkent, J. Verhoeven, and Th. J. De Boer, Tetrahedron Lett., 32,3363 (1972). computer of the University, using a nonlinear leasbsquares (6) (a) C. R. Beddard and R. J. Davidson, J . Photochern., (1972/73), method12 and a Laplace transform method.13 1,491, (1972/73);(b) E.A. Chandroas and H. T. Thomas, Chern. Phys. Lett., 4,393 (1971); (c) H.Shizuka, M. Nakamura, and T. Morita, J. Chern. Phvs.. 83. -.2019 - -- 11979). - -, (9)(a) K.'Gniidig and K. B. Eisenthal, Chern. Phys. Lett., 46, 339 (10)The authors thank K. Zachariasse and G. Striker for information (1977); (b) T. Okada, S. Saito,and N. Mataga, Bull. Chern. SOC.Jpn., prior to publication on similar deviations from the kinetic scheme ob50,331 (1977); (c) M.Mgita, T. Okada, N. Mataga, T. Nakaahima, K. served in diarylalkanes. ~~~
~
Yoshihara, Y. Sakata, and S. Misami, Chern. Phys. Lett., 72,229(1980). (d) The authors thank Professor A. Weller for communication of this results during hie stay aa a visiting professor in Leuven (1979). Part of this work ie represented in the Ph.D. Thesis of M. Schulz, Gattingen, 1974.
(11)(a) M. Van der Auweraer, A. Gilbert, and F. C. De Schryver, J .
Am. Chern. SOC.,102,4007(1980);(b) M.Van der Auweraer, A. Gilbert, and F. C. De Schryver, Nouo. J. Chirn., 48,153 (1980).
(12)A. Grinwald and I. 2.Steinberg, Anal. Biochern., 59,583 (1974). (13)A. Gafni, L.Modlin, and L. Brand, Biophys. J., 15,263 (1975).
3200
77m Journal of Physical Chemlsfty, Vol. 85, No. 21, 1981
Van der Auweraer et al.
2
n
z1
.-c VI C
0 c 8 I
QI
.-c>
-m
0)
K
20000
25 000
30 000
35 OOOcn
W aven urn ber
____)
Flgure 1. Emission spectra of P2NM in THF; A, = 265 f 5 nm: (1) -91 O C , (2) -45 O C , (3) -10 O C , (4) +23 O C , (5) +50 O C .
Results and Discussion Spectra. The emission spectrum of P2NM at room temperature in acetonitrile consists of a structureless band with a maximum at 25250 cm-'. The quantum yield of the emission equals 0.04. In THF between 50 and -72 O C , the emission spectrum of P2NM consists of a broad structureless band. The maximum undergoes a bathochromic shift, and the fwmh decreases upon cooling (Table I). Below -72 " C a second band appears at 35 700 cm-' with an emission quantum yield of 0.002. The intensity of this band increases upon further cooling. The quantum yield of the bathochromic emission changes from 0.21 at 50 "C to 0.13 at -102 "C (Figure 1). The emission spectrum of P3NM in acetonitrile at room temperature consists of broad structureless band with a maximum at 25 900 cm-'. In THF at room temperature, the emission maximum is situated a t 26 570 cm-l; upon cooling, the band maximum undergoes a bathochromic shift, and the fwmh decreases (Table I). The quantum yield of exciplex emission in THF decreases from 0.17 a t 50 " C to 0.13 at -95 "C. Below -40 "C,a second band appears at about 35 700 crn-l. This emission at 35 700 cm-' becomes more intense upon further lowering of the temperature (Figure 2). The emission of P4NM in acetonitrile at room temperature is nearly completely quenched. In THF, the emission consists at room temperature of a structureless band at about 26 880 cm-'. Its maximum shifts to 15 700 cm-' a t -50 "C and a second band is observed at 35700 cm-'. This emission at 35 700 cm-' becomes more intense when the temperature decreases (Figure 3). As shown in Table I, the fwmh of the 26880-cm-' band increases strongly, and its maximum shifts strongly hyposochromic at temperatures above 23 OC. Previous work14 indicates that the bathochromic emission is due to an exciplex and that the emission at 35700 cm-' (280 nm) is due to (14) (a) D. Bryce Smith, M. F. Clarke, and A. Gilbert, Chem. Commun., 330 (1976); (b) M.Van der Auweraer, F. C. De Schryver, A. Gilbert, and S. Wilson, Bull. Chem. SOC.Belg., 88, 227 (1979).
20.000
25.000
30.000
35.000c m
Figure 2. Emission spectra of P3NM in THF; A,, = 265 h 5 nm: (1) -92 O C , (2) -74 O C , (3) -40 O C , (4) +14 O C , (5) +56 O C .
20.000
25.000
30.000
35.000c m
Flgure 3. Emission spectra of P4NM in THF; X, = 265 f 5 nm: (1) -86 O C , (2) -55 O C , (3) -20 O C , (4) +23 O C , (5) +50 O C .
fluorescence of the toluene chromophore. For P4NM above room temperature a back reaction also occurs in which the exciplex returns to the singlet excited state of the amine which emits at about 30 300 cm-' in THF. This was also observed for 5-phenyl-l-(N,N-dimethylamino)pentane and 11-phenyl-1-(N,N-dimethy1amino)undecane in THF.'lb Decay Measurements. P2NM. The emission of the exciplex of PSNM in acetonitrile decays exponentially with a 4.0-11s lifetime. The lifetime of the exciplex of PSNM
Exclplex Formation In Terminally Substituted Alkanes
The Journal of Physical Chemistry, Vd. 85, No. 21, 1981 5201
TABLE 11: Kinetic Parameters in THF P2NM 1 0 9 ~ s~ ,
1 0 9 s~ ~ ~ 107~,,5 107~,,s E F ,mol k J - ' E,, mol kJ-' E,, mol kJ-' E,, mol kJ-' a
2.8 32 6 3.5
P3NM
P4NM
P5NM
PllNMa
1 . 3 x 104 1 . 3 X 10' 6.0 18 15.9 14.5 3.8 3.4
1 . 6 X lo*
1.1 x l o a
7.0
17.0 1.6 15 12.3
15.5 16.0
12.2
3.2 2.5
For P l l N M there is no difference between fast and slow conformations; the obtained h , must be compared with kF.
16
15 LO
Figure 4. k,.
Arrhenlus plot
50
60
70
3 1 / T x 10 of k6 and k8 of PPNM in THF (V) k,; (V)
seems to be nearly independent of the solvent in isopentane, dibutyl ether, diethyl ether, THF,14 and butyl acetate,15 and equals 8 ns. Therefore, it is reasonable to assume that the shortening of the lifetime observed on going to acetonitrile is due to dissociation of the exciplex into a solvated ion pair. Thus, for P2NM in acetonitrile at room temperature, klo should equal (1.2 f 0.2) X lo8 s-l. The quantum yield of exciplex emission decreases more than twice as rapidly as the exciplex lifetime on going to acetonitrile. The quenching of the locally excited state leads, with only 40% yield, to the exciplex (k3). Therefore the quenching leads, with about 60% yield, to another species, probably the solvated ion pair (Ice). The only other explanation for these phenomena would be a dramatic decrease of k5 upon increasing the solvent polarity, which is estimated very unlikely by Weller and Beens."sc The direct formation of solvated radical ions in competition with exciplex formation has been observed by Schultz.lGe Furthermore, when k5 is solvent dependent due to influence of the solvent on the mixing of the charge transfer and a locally excited state or on the exciplex geometry, this solvent effect should be found over the complete range of solvent polarities. However, we found for P3NM that k5 does not change on going from isopentane to THF. The emission of P2NM in THF decays exponentially at all temperatures between 60 and -104 O C ; the emission of the locally excited state is, even at the lowest temperature, too weak to be recorded. Since the toluene emission is nearly completely quenched at all temperatures, the ~~
(16) M.Van der Auweraer and F. C. De Schryver, unpublishedresults. (16) (a).R. A. Marcus, J. Chem. Phys., 29,966-89 (1956); (b) R. A. Marcus, ibid., 38,1858 (1963);(c) R.A. Marcus, 39,1734(1963); (c) ibid., 26,867 (1.958); (e) M.Schultz, Ph.D. Thesis, Gcittingen, 1974.
16 L d
30
40
3
50
1/T x10
F re 5. Decay parameters of PSNM in THF I,, = 260 (u) AeaB0,(0) A";, (0)X,3O0.
AFo,
60
* 7 nm: (0)
quantum yield of exciplex emission can be approximated by eq 7 assuming k3 >> k,. Knowing ke and d i allows ~ one (7) +E = k57E = k5/k8 to calculate kS and k,. An Arrhenius plot (of k5 and k6 (Figure 4) gives the preexponential factors anid activation energies of k5 and k6 (Table 11). P3NM. Above -70 "C, the exciplex emission of P3NM monitored at 25640 cm-l decays exponentially; the risetime of the emission, however, is less than the time resolution of our single-photon counting equipment. Ai; lower temperatures the decay curve of the exciplex emilasion can be analyzed as the difference of two exponentials, with a time constant (A2m)-1 for the negative and a decay time constant (X1390)-1 for the positive component of the decay curve (Figure 5). Below -75 O C it was possible to record the decay of the emission a t 35 700 cm-l (280 nm). It can be rmalyzed as a s u m of two exponentials. The ratio of the preexponential factors of the fast and the slow decaying components of the emission decreases upon decreasing the temperature. The decay time of the slow decaying component (AIm)-' is appreciably longer than (h13w)-1 and shows a different temperature dependence. These results do not agree with Scheme I. As demonstrated previously," such behavior can be explained by
3202
The Journal of Physical Chemistry, Vol. 85, No. 21, 1987 Ink
21
20
19
Van der Auweraer et ai.
i
Ink
20
-
19
-
18 -
17
-
16 I F-
16
15
4 .5
4.0
1/TxlO
14
6 .O
5.5
5.0 3
Flgure 8. Arrhenius plot for the rate constants of P3NM in THF: ( 0 ) kFdetermined from (M)kFdetermined from h:”, (0)ks, (V)
3
5
4 3 1 I T x 10
6
Figure 8. Arrhenius plot of the rate constant of P4NM in THF: (0) kF determined from A t w , (M)kF determined from (0)k6, (0)
Atrn,
k6.
.
bond in R-CH,-N-R’(-R”), R, R’, R” # H is 6-7 kcal/ mol. This makes this process in the temperature range studied slower than the lifetime of the excited toluene. Analogous results have been ~ b t a i n e d ”by ~ means of accoustic measurements for triethylamine. When the nitrogen lone pair is trans to R, it cannot reach the phenyl without rotation around the CH2-N bond, and thus such molecules can be identified with the slow conformations. The kinetics shown in Scheme I1 has been proposed when only the toluene chromophore is excited. k7, k8, kl,and ks have the same meaning as in Scheme I. F and F*are ground state and first excited singlet state of a set of conformations that gives the exciplex by a fast process; S and S* are ground state and first excited singlet state of a set of conformations that gives exciplex by a slow process; kF and ks are rate constants for the formation of the exciplex from the “fast” and the “slow” conformations; and kTFand ka are rate constants for back reaction from the exciplex to F* and S*.
-y
-
0
I
I
I
Scheme I1
4 5 6 3 1 / T x10 F re 7. Decay parameters of P4NM in THE A, = 260 f 7 nm: (0) 3
Aye, g)
(0)
AV
F C (267 nm)
F*
k7
%E A-C
k8
E
I T
(0) hv
assuming the presence of two different conformations for which exciplex formation occurs with different rates and between which interconversion is very slow on the time scale of the experiment. NMR experiments have demonstrated that, for dibenzylmethylaminel‘* or dimethyltert-butylamine, the barrier for rotation around the CH,-N (17)(a) Y.Aeaki, M. Wamato, and E. Muraka, Chem. Phurrn. Bull., 21,112 (1973); (b) C. A. Bushwell and W. I. Anderson, Tetrahedron Lett., 2,129 (1972); (c) E. Heasell and J. Lamb,R o c . R.SOC.London, A, 237, 233 (1956).
\
S -c (267 n m ) S*
*9 E*
E
/
For
k- S
k7
It is assumed that interconversion between S and F or
F* and S* is much slower than all other processes. When
it is more assumed that k -
