The Journal of Physical Chemistry, Val. 82, No. 3, 1978
Exciplex Formation of Fluorescence Quenching
367
Intramolecular Fluorescence Quenching and Exciplex Formation in the (Carbazole)-(CH,),,-(Terephthalic Acid Methyl Ester) System Yoshihiko Hatano, Masahide Yamamoto, * and Yasunori Nishijima Department of Polymer Chemistty, Kyoto University, Kyoto, Japan (Received October 4, 1976)
Effects of the geometric restriction and molecular motion on intramolecular fluorescence quenching and exciplex formation have been studied in the (carbazole)-(CH,),-(terephthalic acid methyl ester) systems ( n = 1-10). Static quenching is observed for n = 1 and 2, while dynamic fluorescence quenching is predominant for n = 10; the latter is caused by thermal motion of the methylene chain, but the former is hardly affected by thermal motion. Both the dynamic and static processes of fluorescence quenching occur for compounds with n = 3, 4, and 5. Intramolecular exciplexes are formed through both quenching processes, and the trimethylene chain is especially favorable for exciplex formation. Rather small activation energies of exciplex formation for these systems are attributed to the large charge transfer character of the exciplexes and to the large quenching distance across which the fluorescence of the carbazole chromophore is quenched by the acceptor.
Introduction There have been quite extensive studies on intermolecular excimers or ex~iplexes.'-~Studies on excimer emission of aromatic hydrocarbon crystal^,^ sandwich dimer^,^?^ and intramolecular excimers'is have shown that perfectly overlapping sandwich geometry is preferred for excimer formation. However, there have been only a few investigations9 about the geometric requirements for exciplex formation of fluorescence quenching. Recently several investigations on intramolecular exciplexes were reported, and the geometric requirements of the exciplex and the effects of structural restriction upon intramolecular exciplex formation were d i ~ c u s s e d . ~ &Inl ~these works, it was revealed that the exciplex may not have a strong geometric preference.lOJ1 Eisenthal et al. examined the effects of molecular motion on exciplex formation in the (anthracene)-(CH&-(dimethylaniline) system by means of picosecond spectroscopy. They found much slower (ca. 900 ps) exciplex formation for the intramolecular system than for the corresponding intermolecular system, and concluded that this is due to the effect of methylene bonds on the rotational motion of the donor and the acceptor residues. In the previous paper, we showed that the series of compounds (carbazole)-(CHz),-(terephthalic acid methyl ester) ( n = 1-5) form intramolecular exciplexes when the carbazole moieties were excited.14 In these compounds, the degree of normal fluorescence quenching, apparent quantum yield, and lifetime of the exciplex emission are all strongly affected by methylene chain length. Here, we have investigated the effects of structural restriction caused by methylene bonds and Brownian motion upon normal fluorescence quenching and exciplex formation, studying the temperature dependence of the emission behavior. Hereafter these compounds are denoted as I, 11, ..., X,
n=
1,2,3.4.5.10
indicating the number of methylene units.
Experimental Section The synthesis and purification of I-V were reported in the previous paper.14 X was prepared by the same pro0022-3654/78/2082-0367$01 .OO/O
cedure as V. For measurement of the fluorescence quantum spectra, a calibrated Shimadzu RF-502 spectrofluorophotometer was used. Fluorescence lifetime was determined by analyzing the decay curves measured by the single photon counting method (half-width of the light pulse -2 ns) a t room temperature using an Ortec, Inc. system with the Hitachi multichannel analyzer, and with a TRW pulse fluorometer (half-width of the light pulse 15 ns) a t low temperatures. A quartz dewar equipped with a thermocouple was used for low temperature measurements. Fluorescence quantum yields were determined relative to that of quinine sulfate in l N sulfuric acid whose reported quantum yield is 0.546.15 2Methyltetrahydrofuran (MTHF) was distilled over sodium metal after refluxing with sodium metal for several hours, and then vacuum distilled and stored in vacuo. All solutions were deaerated by several freeze-thaw cycles a t N Torr.
-
Results and Discussion Absorption and fluorescence spectra for these compounds were previously reported and no appreciable ground state interaction was observed between carbazole (CZ) and terephthalic acid methyl ester (TPM) moieties for II-X.14 The exciplex emission bands for all these compounds show considerably large solvent shifts compared with that of normal fluorescence. The fluorescence spectra of V in several solvents a t room temperature are given in Figure 1. The exciplex emission of other compounds shows behavior similar to that of V. From the solvent shift of the exciplex emission maxima, the dipole moments in the fluorescent exciplex state of I-X are estimated to be 16-17 D, by the same procedure as described by Okada e t a1.lob It is interesting to compare this result with theirs for the dipole moments in the fluorescent state for the (anthracene)-(CHz),-(N,N-dimethylaniline)systems which are 27, 25, and 15 D for n = 1, 2, and 3, respectively. The quantum yield of normal fluorescence for CZ (am), the apparent quantum yield of exciplex emission (a,), the lifetime of exciplex emission (T,), and the maximum wavenumber of exciplex emission (E,""") measured in MTHF solvent a t 25 "C are shown in Figure 2. The largest 9,and the longer T , for I11 indicate that the trimethylene chain is especially favorable for intramolecular exciplex formation. This may be due to the favorable steric situation of the trimethylene chain upon the exciplex configuration. As for the sample I, exciplex emission is 0 1978 American Chemical Society
368
The Journal of Physical Chemktty, Vol. 82,No. 3, 1978
Y. Hatano, M. Yamamoto, and Y. Nishijima
.C VI
4
c
-c
"c " W
0
v)
z -3
LL
400
300
I
Wavelength
500
nm)
Wavelength
Figure 1. Fluorescence spectra of V in several solvents at room temperature: concentration, M; excitation wavelength, 344 nm. The solvents were (1) cyclohexane, (2)ethyl ether, (3) tetrahydropyran, (4) tetrahydrofuran.
700
600 (nm)
Figure 3. The fluorescence spectrum of I1 (lo4 M in deaerated MTHF, excited at 340 nm) at various temperatures: (1) -125,(2)-75, (3) -27,
(4)14 O C .
c a
._ v)
c W
c
-c " c " W W
4-
v)
L W
3 0
01
2
e
3-
LL
2300
0-
400
-10
I 1
2
3
4
5
"
' 0 10
Figure 2. Effect of methylene chain length on normal fluorescence quantum yield, apparent quantum yield of exciplex emission, lifetime of exciplex emission, and maximum wavenumber of exciplex emission. The number of methylene units is denoted by n .
observed, even though the parallel overlapping configuration between the two rings is not attainable. Therefore, it seems that the geometric requirements of the exciplex are not very critical. In I, I1 and IV, the strain in the methylene chain in the exciplex configuration may be larger than that for 111, and this makes the exciplex unstable, so 7s: are shorter and accordingly @s: are smaller than those for 111. In the longer methylene chain compounds such as IV, V, and X, the conformational strain becomes smaller with an increase in methylene chain length. Hence, the exciplex becomes more stable and 7, becomes longer in that order. The probability that the CZ moiety exists in the vicinity of the TPM moiety may be smaller in the longer methylene chain compounds, so am's are larger in the longer chained compounds, as can be seen in Figure 2.16 It is noteworthy that for I, 11, and IV, ijemax lies in the smaller wavenumber region compared with the others; the energy gap between the fluorescent exciplex state and the ground state is smaller under the conformational restriction of the methylene chain in these compounds. Emission spectra for all of these compounds are quite temperature dependent, while the absorption spectra show no significant change over the temperature range from 30 to -120 " C except for the narrowing of the vibrational structures a t low temperatures. Figures 3 and 4 show the temperature dependence of emission spectra for I1 and V, respectively, which are typical emission behavior. In V, exciplex emission decreases accompanied by an increase in normal fluorescence with lowering temperature. In 11,
500
600
Wavelength
( nm
700
1
Figure 4. The fluorescence spectrum of V (lo4 M in deaerated MTHF, excited at 344 nm) at various temperatures: (1) -125, (2) -78, (3) -48, (4) -7, (5) 25 O C .
"
- I20 -100
-50 Temp.
0
30
1°C)
Figure 5. Temperature dependence of normal fluorescence quantum yield in deaerated MTHF. Concentrations are M for all samples: (2)11; (3) 111; (4) IV; (5) V; (10)X. a,,, of I lies between those of I1 and 111.
however, exciplex emission decreases, but normal fluorescence intensity hardly increases with lowering temperature. Samples I and I11 show behavior similar to 11; samples IV and X behave like V. No isoemissive point is, however, observed in all samples. The temperature dependences of a, and @ e are shown in Figures 5 and 6, respectively. am'sof I and I1 are practically negligible regardless of temperature (from 30 -196 "C). This fact indicates that the normal fluorescencesof these compounds
-
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
Exciplex Formation of Fluorescence Quenching
^.
369
PIO,
1 3
1
4 2 0 -100
-50
0
30
Temp ( " C )
Figure 7. Plots of @,,,(-120 "C)/@,,,(T) (0)and T,(-120 "C)/T,,,(~ (0) against temperature for sample X. The solvent was 2-MTHF. -120 -100
-50
0
30
T e m p ('C)
Figure 6. Temperature dependence of apparent quantum yield of exciplex emission in deaerated MTHF. Concentrations are all samples: (1) I; (2) 11; (3) 111; (4) IV; (5) V; (10) X.
M for
are almost entirely quenched in the static process which is independent of the thermal motion of the methylene chain. The normal fluorescence of I11 increases slightly with lowering temperature. Nevertheless, the static process is predominant in the normal fluorescence quenching for 111. This static quenching may be caused by an electron transfer from the CZ moiety to the T P M moiety instantaneously after excitation of the CZ moiety, if the CZ moiety is located within a certain distance of the T P M moiety. This mechanism is consistent with the following fluorescence decay characteristics. In I, 11, and 111, normal fluorescence is so strongly quenched that its rise and the decay profile cannot be measured under these experimental conditions. The risetime for exciplex emission is instantaneous, and its decay profile is represented by a single exponential over the whole range of temperature indicating very rapid exciplex formation. Extended planar zigzag conformation of the methylene chain gives the distances between two moieties, ca. 8.3, 9.3, and 10.8 A for 11,111,and IV, respectively. Therefore, the upper limit for the quenching distance across which the fluorescence quenchin takes place by electron transfer may not exceed about 9 . In contrast to the static quenching for 1-111, it is notable that the dynamic process caused by thermal motion of the methylene chain plays an important role in the normal fluorescence quenching of IV, V, and X, since the am'sfor these compounds markedly increase with lowering temperature. This dynamic quenching can be explained as follows: if on excitation the two moieties are situated farther apart than the quenching distance, fluorescence quenching does not occur until the two moieties approach each other within the quenching distance by the thermal motion of the methylene chain. As for X, the decay profile for normal fluorescence is represented by a single exponential at any temperature, and its lifetime ( T ~ increases ) from ca. 1 (25 "C) to 16 ns (-120 "C) in a manner similar to the increase of amwith lowering temperature from 25 to -120 "C. In Figure 7 , the values of @,(-120 "C)/@,(T) and 7,(-120 "C)/T,(~') are plotted against temperature, where @.,(T) and T,(T) are quantum yield and lifetime of the normal fluorescence of X a t T "C. Both plots give identical curves, indicating that the normal fluorescence is quenched in the dynamic process. Further a,,, and T , of X a t 77 K are nearly equal to those of N-ethylcarbazole at this
R
temperature (arn = 0.5 and r, = 18 ns). This means that the normal fluorescence of X is scarcely quench thermal motion is suppressed. These results evidence that the normal fluorescence of X is quenched almost entirely by a dynamic process. am'sof 111, IV, and V are 0.09, 0.31, and 0.40, respectively, a t 77 K. These values are smaller than those of N-ethylcarbazole a t 77 K by 82% for 111,38% for IV, and 20% for V, while the rm'$ for all of these compounds are ca. 18 ns a t 77 K. Hence, the normal fluorescencesof these compounds are quenched in the static process by 82, 38, and 20% for 111, IV, and V, respectively. These values imply that the percentage of the separation between the CZ moiety and the TPM moiety is within the quenching distance. The normal fluorescence of CZ moieties, which are separated farther from T P M than the quenching distance on excitation, is quenched in a dynamic process. As €or IV and V, the normal fluorescencesare strongly quenched and they decay rapidly following the decay of the exciting light pulse above temperatures around -100 "C for IV and -60 " C for V. However, in the lower temperature region, the normal fluorescence behaviors of these samples are the same as that of X. The rise curve for exciplex emission in X is slower than that of the exciting light pulse and its decay profile is represented by the difference of two exponential functions at any temperature. As for IV and V, in low temperature region the decay profile for exciplex emission is similar to that of X, but in higher temperature region its risetime is instantaneous and the decay profile is a single exponential. From the above results, we can consider the following mechanism for intramolecular fluorescence quenching and exciplex formation: D*-A+ D-A + hu D*-A .+ D-A + heat D*-A + (D-A)* D*-A+ (D-A)* (D-A)" D-A t hue (D-A)* + D-A t heat +
kf
k, k,, kid kfl
k,)
where D, A, and (D-A)* are the CZ moiety, T P M moiety, and fluorescent exciplex state, respectively. The parameter k l s is the rate constant for the static quenching process due to a short-range electron transfer, and hld is that of dynamic quenching involving the thermal motion of methylene chain preceding electron transfer. Using the steady-state approximation, the following relations are derived:
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
370
hf/(hf + h n + h1s + hid) hfv(hlst h l d ) / [ ( h f + k n +
@m = @e=
(1)
his+ kld)(hf' f (2) (3)
kn1)l 7,
= l/(hft t hnl)
@e/(@rnTe)=
Y. Hatano, M. Yamamoto, and Y. Nishijima
(41
hf'(hls+ hld)/kf
Then, the activation energy, Eld, of hld is evaluated under the following assumptions: hf, h,, and kl,are independent of temperature, that is, hf/(kf + h, k1J = @.,(77 K). Hence
+
a In (1/Qm- 1/@,(77 K))/a(l/T)= a In (hld)/a(I/T) = -Eld/R (5) Since k l d 0 and kl, >> (hf + h,) for I and 11, and kld >> hl, for 111-X in the temperature range above -70 "C, eq 4 is simplified as follows:
kfl/k, @e/(@m~e= ) hldhf'/kf @e/re =
for I a n d I1 for 111-X
Then the activation energy, Eft,of hfl is given by
a In (ae/Te)/a(1/T)= -Efl/R a In ( @ e / ( @ m T e ) ) / a ( l / T ) = -(Eld
Ef')/R
for I and I1
(6)
for 111-X
(7)
The activation energies obtained are summarized in Table I. Ef is almost constant (-1 kcal/mol) for all compounds. A considerable part of the temperature dependence of kp can be explained in terms of the temperature dependence of solvent polarity; the dielectric constant of MTHF increases by a factor of 1.6 on lowering the temperature from 25 to -75 OC, and this change in solvent polarity induces the corresponding decrease of hp.17 As for the activation energy of exciplex formation, Eld, it is very small for 111, while the IV-X, E l d increases with methylene chain length, and for V and X, it exceeds the activation energy of the solvent viscosity indicating a restriction of the methylene chain energy on the thermal motion of the CZ and T P M moieties. The smaller activation energies of I11 and IV show that a slight movement of the two rings leads to the fluorescence quenching. Chandross and Dempster* calculated the activation energy El for intramolecular excimer formation as 3.3 kcal/mol for 1,3-(a,a'-dinaphthy1)propanein methylcyclohexane-isopentane (9:l) solvent and 4.0 kcal/mol for 1,3-(P,P'-dinaphthyl)propane.They assigned these energies to the rotational barrier in trimethylene. Itoh et reported smaller activation energies for intramolecular exciplex formation for the two 1-(9,10-dicyano-2anthryl)-3-(naphthyl)propanes (DCAN) than that for the excimer of dinaphthylpropane. They attributed the smaller El for exciplex formation to the charge transfer character in the fluorescent state of the exciplex. It is noteworthy that Eld for 111 is considerably smaller in contrast to the activation energies for the above two systems. Charge transfer character may be greater in the CZ-TPM pair than in the dicyanoanthracene-naphthalene pair. In addition, the fluorescence quenching for I11 occurs over a larger distance than for DCAN. These circumstances result in the smaller activation energy for the exciplex formation of 111. These arguments are also consistent with the following results. The normal fluorescence of I11 is almost temperature independent while the fluorescence of the dicyanoanthracene moiety
TABLE I: A c t i v a t i o n Energies, E, a n d Efi, fork, and kf! (kcal/mol)a Samples
Eld
I I1
I11
0.2 1.3 2.4 3.2
IV V X a E(viscosity
Ef'
0.9 0.8 1.1 1.0 1.0 0.9
of MTHF) = -1.8 k c a l / m o l .
of DCAN increases with lowering temperature. Moreover, the exciplex fluorescence of I11 shows a considerably larger solvent shift compared with that of the exciplex fluorescence of DCAN; the dipole moments of the exciplexes were reported to be 9.5 D for P,a'-DCAN and 8.7 D for P,P'-DCAN, while that for I11 is estimated to be 16 D. The results obtained in this study show that the exciplex does not have a strong geometric preference in a similar manner as was reported previously.lOJ1 The trimethylene chain is especially favorable for intramolecular exciplex formation. The normal fluorescence of 1-111 is quenched in the static process caused by an electron transfer from the CZ to T P M moiety immediately after the excitation of CZ. Furthermore, the upper limit of the quenching distance across which the fluorescence quenching due to electron transfer takes place is considered not to exceed about 9 A. Activation energy for intramolecular exciplex formation in this donor-acceptor pair is considerably small, e.g., the activation energy for the exciplex of I11 is smaller than that for the exciplex of DCAN or for the excimer of dinaphthylpropane. This can be attributed partly to the greater charge transfer character of this system and partly to the fact that fluorescence quenching occurs over greater distance in the CZ-TPM system than in the dicyanoanthracene-naphthalene system.
References and Notes (1) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, N.Y., 1970. (2) N. Mataga and T. Kubota, "Molecular Interactions and Electronic Spectra", Marcel Dekker, New York, N.Y., 1970. (3) H. Leonhart and A. Weller, Ber. Bunsenges. Phys. Chem., 67, 791 (1963); H. Knibbe, D. Rehm, and A. Weller, ibid., 72, 257 (1968). (4) J. Ferguson, J . Chem. Phys., 28, 765 (1958). (5) E. A. Chandross, E. G. MacRae, and J. Ferguson, J . Chem. Phys., 45, 3546 (1966). (6) E. A. Chandross and J. Ferguson, J. Chem. Phys., 45, 3554 (1966). (7) F. Hirayama, J . Chem. Phys., 43,, 3163 (1965). (8) E. A. Chandross and C. J. Dempster, J. Am. Chem. Soc., 92, 3586 (1970). (9) G. N. Taylor, E. A. Chandross, and A. H. Schiebei, J. Am. Chem. SOC.,96, 2963 (1974). (10) (a) R. Ide, Y. Sakata, S. Misumi, T. Okada, and N. Mataga, Chem. Commun., 1009 (1972); (b) T. Okada, T. Fuji&, M. Kubota, S. Sasaki, N. Mataga, R. Ide, Y. Sakata, and S. Misumi, Chem. Phys. Lett., 14, 563 (1972). (1 1) E. A. Chandross and H. T. Thomas, Chem. Phys. Lett., 9, 393 (1971). (12) (a) M. Itoh, T. Mimura, H. Usui, and T. Okamoto, J. Am. Chem. Soc., 95. 4388 11973): M. Itoh. T. Mlmura. and T. Okamoto. Bull. Chem. Sdc. Jpn:, 47,"1078 (1974). (13) (a) T. J. Chuang and K.B. Elsenthal, J. Chem. Phys., 59, 2140 (1973); (b) T. J. Chuang, R. J. Cox, and K. B. Eisenthal, J . Am. Chem. Soc.,
as - -, 8828 - ---
(1974) \.-.
.I.
(14) M. Yamamoto, Y. Hatano, and Y. NishiJima,Chem. Lett., 351 (1976). (15) J. N. Demas and G. A. Crosby, J . Phys. Chem., 75, 991 (1971). (16) am's of I and I 1 are negligibly small (C1 X and these weak normal fluorescences may be due to trace amounts of Impurity formed by decomposition of each sample. Even if thls weak normal fluorescence comes from the CZ moiety of each sample, the conclusion that the normal fluorescence is predominantly quenched in the static process Is not altered. (17) Y. Hatano, M. Yamamoto, and Y. Nishljima, to be submitted for publication.
