J. Phys. Chem. 1994,98, 1532-1543
1532
Influence of the Substitution on Intramolecular Exciplex Formation between Pyrene and Indole Moieties Nicole Helsen, Lucien Viaene, Mark Van der Auweraer,. and Frans C. De Schryver' Departement Scheikunde, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee- Leuven, Belgium Received: June 25, 1993; In Final Form: October 25, 1993"
Using stationary fluorescence spectroscopy and single-photon timing, intramolecular exciplex formation between pyrene and indole, connected by a propyl chain, was investigated in isooctane, diethyl ether, and acetonitrile. Three different compounds, where the propyl chain was connected to the 1-, 2-, and 3-position of the indole chromophore, respectively were compared to evaluate the influence of the position of the linking chain on the energetic and kinetic aspects of the intramolecular exciplex formation. While 1-( l-pyrenyl)-3-( 1-indoly1)propane follows over a large range of solvents, a kinetic behavior analogous to that proposed for intermolecular exciplexes such a behavior is for 1-( l-pyrenyl)-3-(2-indolyl)propane only observed in isooctane. For 1-( 1pyrenyl)-3-(2-indolyl)propane in more polar solvents and 1-( l-pyrenyl)-3-(3-indolyl)propane in all solvents studied at least three different kinetic species can be distinguished in the excited state. Compartmental analysis in the presence of a quencher allowed one to determine without any a priori assumption the different decay parameters of 1Py2In in isooctane at room temperature. The more complicated excited state kinetics of 1Py3In did not allow a more detailed analysis.
Introduction
SCHEME 1
Since the first observation of a structureless emission band at the bathochromic side of the emission of the locally excited state due to a complex between a molecule in the ground state and a molecule in the excited state,' there has been a continuing interest in this field. Generally, the most favorable geometry for an excited complex formed between an aromatic donor and an aromatic acceptor is that of a sandwich in which the chromophores are at a distance of 3-3.5 8,in a plane parallel orientatioa2 But other geometries are also observed, and in an intermolecular complex the relative position and orientation of the chromophores are such as to maximize the Coulomb i n t e r a c t i ~ n . ~ . ~ To allow the observation of excited complexes at lower overall concentrations, the chromophores can be connected by a spacer. Originally, intramolecular excited-state complex formation was observed only when the chromophores were linked by a propyl chain, which led to the n = 3 rule.5 Later, intramolecular exciplex4-"'1 and excimeri2-I8formation was observed for chains of variable length, and the influence of the chain length was investigated systematically in systems consisting of an aromatic acceptor and an aromatic699J9-26 or a l i p h a t i ~ ~ - ~ donor. J',~~-~~ Linking the interacting moieties by an alkyl chain limits the rotational and translational freedom of both moieties and also reduces the conformations possible for the chromophores in the excited complex.18330 This influences the exciplex properties3JO~21-23J7 as well as the rate of exciplex formation.lOJ1 Sometimes a kinetic behavior different11-'3,15,'8,29-30,32-38from that observed for intermolecular complex formation in the excited state39-41was encountered and was attributed to a more complex kinetics. Also for systems where no deviations occur from the scheme proposed for intermolecular exciplex formation, the analysis of the fluorescence decay is often complicated by overlap of the emission of the locally excited state with the emission of the exciplex or by the excitation of molecules which are already in the exciplex geometry in the ground state.lL,42-45 A general kinetic scheme for intramolecular exciplex formation can be described by Scheme 1, where k2l is the rate constant of
exciplex formation, k12 the rate constant of exciplex dissociation, kol the rate constant of deactivation of the locally excited state and k02 the rate constant of exciplex deactivation. K is the equilibrium constant in the ground state between conformations of the molecule that yield the exciplex immediately upon excitation and molecules for which a conformational rearrangement is necessary to form the exciplex. In the framework of this scheme it is expected that the fluorescence decay of the locally excited state and the exciplex can be always described as a linear combination of two exponentials. The concentrations of the locally excited state (PI) and the exciplex can be considered as components of a time dependent vector X(t). The time derivative of X is given by
* To whom correspondence should be addressed. 0
Abstract published in Advance ACS Abstracts, January 15, 1994.
dX(t)/dt = AX(t) where A is the transfer matrix of the system
(1)
The solution of eq 2 is given by
bi, the ith component of the (2 X 1) vector b(LX),denotes the fraction of the concentration of the different excited species, x*1(0) a_nd X*2_0) a t time zero. The normalized values of bl and b2, bl and b2 are given by the following expressions:
0022-3654/94/2098-1532%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1533
Intramolecular Exciplex Formation
CHART 1: Molecular Structure of the Investigated Bichromophores 62
+ b2)
= b,/(b,
(4)
In a fluorescence experiment one does not observe P , ( t ) or
Pz(r) directly, but the composite spectral contours of the excited state species. Therefore the fluorescence &response function at an emission wavelength b,,, is expressed by f(hem,t) = cl[l*(t)l
+ c2[2*(t)l
(5)
1Pyl In
In eq 5 c1 and c2 are the components of a (1 X 2) vector c(X,,) of spectral weighting factors ci given by cj
= k:JAhp,(Aem)
dXem
(6)
kf is the fluorescent rate constant of species i and p,(b,,,) is the spectral emission density of species i, normalized to the complete emission band of species i and ALmis the emission wavelength interval over which the fluorescence is monitored. &, the normalized elements of the vector c are given by
E, = C , / C C j
'CH3
(7)
1PyPln
1
Equation 7 allows one to rewrite eq 5 as jTXem,t)
= KE(X,m) ~ X P ( A ~ ) $
(8)
or .tIXem,t)
KE(Xem)
P exp(rt)P-'6(Xem)
(9)
In eq 9 P and exp(Ft) are the matrix of the eigenvectors of A and a diagonal matrix with as elements exp(-yit), where yi the ith eigenvalue of A is given by 1Py31N 7 ' 1 , ~=
+ y2)
'/z((Yl
[ ( y l - y2)'
+ 4kzlk121'"j (10) (11)
YI = k01 + k21
(12) In eqs 8 and 9 K is a scaling factor. Usin Z1 and 81 allows one to link Zl at a given emission wavelength and 1 at a given excitation wavelength. 2.1 and bl are a function of respectively the emission and excitation wavelength. When the rate constants are time independent, the expression for X*(r) corresponds to a series of exponentials and eqs 8 and 9 can also be written as
yz = k02 + k l 2
I
f(W = q exp(7,t) + a2 exp(y2t)
(13) When 8 2 can be neglected, the expression for X*(t) can be simplified to
with
= bl(Y2 + X,)/(Y, - 71)
(15)
812 = --bl(Yl+ X,)/(Y, - 71)
(16)
PI1
021 = -b,k2,/(72
- 71)
(17)
(18) P22 = blk2l/(YZ - r1) Analyzing the fluorescence decay curves with global compartmental a n a l y s i ~allows ~ ~ , ~an ~ accurate determination of the kinetic parameters of intramolecular exciplex formation with a minimum of assumptions concerning spectral overlap or the conformational distribution in the ground state. This will allow one to distinguish between the possibilities given above.41-47 In this contribution, the photophysical and kinetic properties of bichromphores consisting of a pyrene moiety and indole moiety
connected by a propyl chain are described in isooctane, diethyl ether, and acetonitrile. The compounds differ in the position of the substitution of the propyl chain at the indole chromophore (Chart 1). The fluorescence decay curves of lPy2In in isooctane are analyzed by global compartmental analysis in order to investigate to which extent the kinetic scheme proposed for intermolecular exciplex formation remains valid. The analysis by single curve and global analysis of the fluorescence decays of 1Py3In in isooctane and of lPy2In and 1Py3In in other solvents suggests a more complex behavior for those systems.
Experimental Section Synthesis. IPylIn. The synthesis of 1-(l-pyrenyl)-3-(3methyl- 1-indolyl)propane is performed as described elsewhere.'g IPy2In. 1Py2In is prepared starting from 1-acetylpyrene and 1-methylindole-2-carboxylicacid. 1-Methylindole-2-carboxylic acid is first converted into 1-methyl-2-methylindolealdehydewhich is coupled to 1-acetylpyrene by an aldol condensation. The resulting chalcon is subsequently reduced to 1Py2In. The details related to the synthesis, purification, and identification are reported in additional separate material. IPy3In. 1Py3In is prepared by aldol condensation of l-methylindolecarboxaldehyde with 1-acetylpyrene followed by reduction of the resulting chalcon to 1Py3In. The details related to the synthesis, purification, and identification are reported in additional separate material. The Solvents. All solvents (Janssen, Merck or Rathburn) were of spectroscopic or fluorescent grade and were used as supplied. Their purity is checked before use by measuring the background fluorescence. Spectra. Absorption spectra were determined on a PerkinElmer Lambda 5 UV-vis spectrophotometer and a Perkin-Elmer Lambda 6 spectrophotometer.
Helsen et al.
1534 The Journal of Physical Chemistry, Vol. 98, No.6, 1994 1
0.51
E20
4
0.4
I
0.8
E9 w
f
5 2
0.6 0.4 \
0 360
WAVELENGTH (NM)
Figure 1. Absorption spectra of lPyZIn, 1-MePy, 1,2-dimethylindole, and the sum spectrum in isooctane at room temperature: (. .) lPyZIn, (- * -) 1-MePy, (- - -) 1,2-dimethylindole,(-) sum spectrum.
.
Stationary excitation and emission spectra were recorded on a Spex Fluorolog Model 1691 and on a SLM 8000 C spectrofluorimeter. The spectra were corrected for the wavelength dependence of the sensitivity of the detection system. The fluorescence quantum yield was determined using quinine sulfate in 0.1 N HzSO4 as a standard (a = 0.55).49 All solutions were degassed by four freeze-pumpthaw cycles. Between 20 and 90 O C the temperature was controlled by a Lauda K4RD thermostat using water as heat carrier. Between 20 and -90 O C the temperature was controlled using a Lauda Ultra Kryomat thermostat filled with methanol. FluorescenceDecays. The fluorescence decays were determined by time-correlated single-photon timing using the setup described e l ~ e w h e r e .The ~ ~ fluorescence decay curves were analyzed by single-curve analysis, global analysis, and global compartmental analysis.46*47 Optoacoustic Experiments. The experimental setup for the laser-induced optoacoustic spectroscopy (LIOAS)50is described in detail el~ewhere.51.5~The absorbance of the sample solutions used in the LIOAS experiments was always below 0.1, and the samples were deoxygenated by bubbling argon through the solution for 15 min. 2-Hydroxybenzophenone was used as a reference compound.
Results Influenceof the Solvent Polarity on the Stationary Fluorescence Spectra. As reported for lPy2In in Figure 1 the absorption and excitation spectra of lPyIn, lPyZIn, and 1Py3In resemble the sum spectra of the two chromophores. This suggests that negligible charge-transfer or exciton interactions occur between the chromophores in the ground state. The emission spectra of l P ~ l I n 1Py2In , ~ ~ (Figure 2), and 1Py3In (Figure 3) consist of a structured emission band, whose features resemble those of the emission of 1-methylpyrene and of a structureless emission band a t longer wavelengths. Upon increase of the polarity of the solvent, the emission maximum of the structureless band shifts to longer wavelengths, and the intensity of the structured emission is decreased. The excitation spectra do not depend upon the emission wavelengtband resemble the absorption spectra when the absorbance of the solution does not exceed 0.3 over the wavelength range studied. Those data suggest that the structureless bathochromic emission is due to an intramolecular exciplex (Table 1). According to the Lippert Mataga equation (eq 19) the energy of the emission maximum of the exciplex depends in a linear way
*
I
I
0.2
.. 400
440
480
520
560
600
640
680
WAVELENGTH (NM)
Figure 2. Influence of the solvent polarity on the emission spectra of lPy2In at room temperature;the spectra are normalized at the maximum
of the locally excited state in isooctane (IO). . ~. diethvl ether (EZO), acetonitrile (MCN).
6
5
5 z w $
4
w $ 3
w
a 2 1
0 360
400
440
480
520
560
600
640
WAVELENGTH (NM)
1Py3In at room temperature;the spectra are normalized at the maximum
of the locally excited state in isooctane (IO), diethyl ether (.E20),
acetonitrile (MCN).
on the solvent parameterf’(e,,n)
(Figure 4):
with
and in, correspond to the emission frequency in a solvent with dielectric constant el and refractive index n and the emission frequency in vacuum respectively. eo, h, c, and p correspond to the permittivity of vacuum (8.85 X V C-1 m-I), Planck’s constant (6.6 10-34J s), the velocity of light in vacuum (3.0 X lo8 m s-l), and the radius of the solvent cavity (in meters) respectively. ke (C m-1) is the permanent dipole moment of the excited state. This relationship is based on the assumptions that solvation occurs only by dipolar interactions and that the dipole moment of the ground state is negligible. From the slope of this linear relationship values of 10 0 0 0 , l l 700, and 9900 cm-1 could be obtained for the ratio 2re2/4irtohcp3 for lPylIn, lPyZIn, and 1Py3In, respectively. Assuming a value of 5 A for p, exciplex dipole moments of 11.1, 12.0, and 1 1.O D can be obtained from this ratio for 1Pyl In, 14.2111, and 1Py3In, respectively. According to eq 19 a linear relationship is observed between the emission maximum of the exciplex of lPylIn, 1Py2In, and 1Py3In and that of the reference exciplex anthracene/N,N-diethylaniline2
-
Intramolecular Exciplex Formation
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1535
TABLE 1: Summary of the Photophysical Properties of lPylIn, lPyZIn, and 1Py3Ina lPylIn solvent hexane isopentane isooctane dibutyl ether diethyl ether butyl acetate ethyl acetate tetrahydrofuran acetone acetonitrile 2pc24utohcp3(cm-I)
f
lPy2In
ap
%
Xmx/nm
0.67 0.67 0.75 0.88 0.885 0.87 0.93 0.96 0.994
0.38 0.38 0.57 0.57 0.46 0.44 0.46 0.18 0.038
424
:*
f
7s
Xmax/nm
424 438 440 454 454 46 1
0.66 0.79 0.87
0.55
426 440 457
0.90
0.56
47 1
474
0.96 0.93 11700
0.44 0.30
487 503
488
10000
0.65 0.56
1Py3In
*.fp'
f TC
0.92
0.5 1
X,/nm 435
0.98
0.58
438
0.96
0.59
464
0.93 0.92 0.96 0.98 9900
0.59 0.52 0.44 0.50
475 478 493 510
Ofm and ?:correspond to the quantum yield of exciplex formation and the quantum efficiency of exciplex fluorescenceat room temperature. X,,/nm is the emission maximum of the exciplex, 2pC2/4utohcp3 is related to fie, the exciplex dipole moment. 24000
,
/ I
1 17500
18wo 18500
19ooo
19500 Vre!
Zoo00 20500 21000 21500
Figure4. Lippert-Mataga plots of the emission maximum of (m) 1Py 1In, (0) 1Py2In,and (+) 1Py3In. The emission maximum is plotted versus that of the reference exciplex anthracene/N,N-diethylaniline.2
360
400
440 480 520 Wavelength (nm)
0
u
350
(cm-1)
560
600
640
Figure5. Influence of the temperature on the emission spectra of lPy2In in isooctane; the spectra are normalized at the emission maximum of the locally excited state. which is characterized by a dipole moment pc of 12.5 D. In this way dipole moments of 11-5,12.2, and 11.4 D can be calculated for the exciplex of lPylIn, lPyZIn, and 1Py3In, assuming that the cavity radius is the same for all exciplexes considered. An increase of the solvent polarity leads for lPy2In to an increase53 of the fwhm of the exciplex emission from 4000 cm-I in isooctane to 4500 cm-I in acetonitrile. A similar solvent change barely affects the fwhm of 1Py3In which equals 4300 cm-1 in isooctane and 4200 cm-I in acetonitrile. Influence of the Temperature on the Stationary Fluorescence Spectra in Isooctane. In isooctane the relative emission of the locally excited state of lPy2In and 1Py3In decreases in favor of the emission of the exicplex when the temperature is increased from -92 and -97 "C to -36 and 10 "C,respectively (Figures 5 and 6). For 1Py2In a further increaseof the temperature decreases
450
550
650
WAVELENGTH (NM)
Figure 6. Influence of the temperature on the emission spectra of 1Py3In in isooctane; the spectra are normalized at the emission maximum of the locally excited state. the ratio of the quantum yield of the exciplex emission versus that of the emission of the locally excited state (Figure 5). Due to the small relative quantum yield of the emission of the locally excited state and the overlap of the emission of the locally excited state and t h e exciplex, it is no longer possible to determine ln(@:/@e,) accurately at temperatures above 10 oc for 1~y31n. Influence of the Temperature on the Stationary Fluorescence Spectra in Polar Solvents. In di-n-butyl ether, the emission spectrum of 1Py2In consists at -90 OC mainly of the emission spectrum of the locally excited state. Upon increasing the temperature, the relative intensity of the exciplex is increased. Between 0 and 5 1 O C a further increase of the temperature induces an increase of the relative intensity of the emission of the locally excited state. At temperatures between -25 and -90 "C ln(@f/@ec)depends in a linear way upon 1/T. lPy2In in diethyl ether shows a n increase of the ratio (a:/&) between -91 and -9.0 "C, followed by a decrease a t higher temperatures (Figure 7). On the other hand for 1Py3In was observed in diethyl ether an increase of the ratio (Figure 8) over the complete temperature range that is experimentally accessible. In acetonitrile the ratio increases from -50 to 22 "C for 1Py2In. At higher temperatures the spectral overlap and the very small relative intensity of the locally excited state no longer allow an accurate determination of (@:/@Le). While for 1Py3In in acetonitrile the ratio also increases between 4 2 and -5 "C no further change of the relative intensity of the exciplex emission is observed at temperatures above -5 "C. Fluorescence Decay of lPy lIn in Isooctane. The fluorescence decays of lPylIn a t 25 "C could at different wavelengths be analyzed as a single exponential with a decay time of 125.3 f 0.1 ns using global analysis. The fluorescence decays obtained a t
(@:/@k)
(@L/@k)
Helsen et al.
1536 The Journal of Physical Chemistry, Vol. 98, No. 6, I994 ._
I
1
SCHEME 2
1
ii5
0.8
z W
59
5
W
a
0.6
0.4
U
0.2 0
360
400
440
480
520
560
600
640
WAVELENGTH (NM)
Fipre7. Influence of the temperature on the emission spectra of 1Py2In in diethyl ether; the spectra are normalized at 377 nm. 2.8
2
:.
-
...-..
\, ‘3.
2.4
25°C
z w
5- 1.6
w
2 0.8 0.4 0 360
400
440
480
520
560
600
640
WAVELENGTH (NM)
Figure& Influence of the temperature on the emission spectra of 1Py3In in diethyl ether; the spectra are normalized at the emission maximum of the locally excited state.
-50 OC at different wavelengths could be analyzed as a linear combination of two exponentials with a decay time of 92.0 and 42.4 ns (Zxl = 0.842) using global analysis. This means that global analysis confirms the previous single curve analysis48of fluorescence decays in isopentane. Fluorescence Decay 1Py21ninIsooctaneat Room Temperature. According to eqs 8 and 13 the fluorescence decay of the emission of 1Py2In in isooctane at room temperature can be analyzed as a linear combination of two exponentials with decay times of 1.7 and 108.6 ns, respectively, at all wavelengths. The ratio of the preexponential terms amounts to -0.80 at 500 nm where the emission is mainly due to the exciplex. The deviation from -112J3.@,scs* can be attributed to the overlap of the emission of the locally excited state and the exciplex or to the excitation of molecules which are already in the exciplex conformation in the ground state. By global compartmental analysis the kinetic scheme of a twocompartmental system, where none of the rate constants depends upon the concentration of one of the components, can be investigated with a minimum of a~sumptions.4~JIn global compartmental analysis the fluorescence decay curves are fitted directly to the rate constants of the system, given in Scheme 1, and to bl and El. 61 and 2.1 correspond to the relative absorbance and the normalized spectral emission weighting factor of species 1, respectively. Previously,45 it was demonstrated that in a twocompartmental system global compartmental analysis needed at least three independent parameters for the determination of all relevant kinetic and spectroscopic information. Addition of quencher, which quenches the species in compartment 1 and 2 with a rate constant kQ1and kQz (Scheme 2). respectively, to the
U
intramolecular system reduces the number of assumptions, necessary to make the system identifiable, to where the following conditions are satisfied: (1) At least three different quencher concentrations must be used, one of which may be equal to zero. (2) At least one parameter must be known. This parameter can be one of the rate constants, with the exception of kQl or kQ2,one or one zl. (3) The quenching rate constants must be different, kQ1 # kQ2. The decay curves collected for different quencher concentrations at two emission wavelengths and one excitation wavelength suffice to determine the system. Assuming that 1-methylpyrene is a model compound for lPy2In (kol and kQ1 are the same for the two systems), the simultaneous analysis of the two compounds in the presence of the same quencher concentrations can lead to the analysis of the lPy2In without any further assumptions about the rate constants. To analyze the photophysics of lPy2In with a minimum of a priori assumptions the fluorescence decay is determined at three different wavelengths in the presence of different concentrations of 1,4-dibromobenzene. The fluorescence decay of l-methylpyrene is determined for the same quencher concentrations at 377 nm. The quencher concentrations used amounted to 0,0.05, 0.1,0.2, and 0.3 M. The quencher concentrations were chosen to lead to values of Z/Zo between 1 and 3, where Z and ZOare the intensity of the fluorescence in the presence and absence of 1,4dibromobenzene. For one wavelength (377 nm) 2.1 is expected to be large, for another wavelength (500 nm) 2.1 is expected to be small, and for the third wavelength (420 nm) E l and & are expected to contribute equally. For each wavelength and each concentration the fluorescence decay was determined with two different time increments, one time increment leading to a decay over 2 decades and one time increment yielding a decay over 0.5 decades. In the absence of 1,Cdibromobenzene the fluorescence decays were determined every 10 nm throughout the emission spectrum and analyzed simultaneously to obtain the emission spectrum of the two species (species associated spectra). The global compartmental analysis without any assumption about the rateconstants yields anacceptablevalueofZ,z (4.456), but the relative errors on the parameters are very large. This is due to the small difference between the quenching rate constant of the two species (kQl = 4.28 X lo7 M-I s-l and kQ2 = 4.70 X lo7 M-1 s-1) which reduces the identifiability of the system. The value of 61 is equal to 1.055. This shows that in the ground state only a negligible fraction of the molecules is present in a g+gconformation.30-59 A second analysis with the assumption if1 = 1 at 367 and 377 nm yields also an acceptable value for Z,z (4.645). The obtained rate constants and the values of 61 and El at different wavelengths are summarized in Table 2. They are very similar to the values obtained by the analysis without assumptions. From the values of Z1, the separate fluorescence spectra of the two species are calculated (Figure 9). The fluorescence spectrum of the locally excited state has the same features as that of 1-methylpyrene. The fluorescence spectrum of the exciplex consists of a broad, structureless band with a maximum at 420 nm and a fwhm of 3900 cm-1.
The Journal of Physical Chemistry, Vol. 98, No. 6,1994 1537
Intramolecular Exciplex Formation
TABLE 2 Rate Constants of the Photophysical Processes Occurring in 14.2111 in Isooctane at Room Temperature kol = (4.17 f 0.01) X lo6 s - ~ k2l = (2.01 f 0.03) X 10' S-' kql = (4.28 f 0.01) X lo7 M-' s-l & 0.994 f 0.006
x
k02 = (2.02 f 0.03) X lo7 s-l kl2 = (5.19 f 0.04) X 10' S-' kq2 = (4.65 f 0.03) X lo7 M-Ls-I
x
TABLE 3 Influence of the Temperature on the Decay Times of lPy2In (y1-l and y2-l) and 1-MePy (TO-') in Isooctane and on the Ratio of the Preexponentials ( a l / a ~at) 377 and 490-500 nm*
T TI-' ("C) (ns)
x
(nm)
?I
(nm)
i.1
(nm)
i.1
367 377 380 390 400
1.ooo 1.ooo 0.768 f 0.019 0.538 f 0.009 0.327 f 0.006
410 420 430 440
0.144 f 0.005 0.106 f 0.004 0.067 & 0.005 0.051 f 0.005
450 460 470 480
0.081 f 0.005 0.055 f 0.005 f0.002 f0.003
-90 -80 -70 -60 -50 -40 -30 -20 -10 -1 10 22 32 41 51
E The rate constants were obtained by global compartmental analysis of the fluorescence decays using the assumption that E1 = 1 at 367 nm and 377 nm. Zxz8= 4.645.
47.57 43.50 36.48 30.29 22.36 15.91 10.79 7.86 5.14 3.79 2.49 1.68
~ 2 - l
4
~
(ns)
(377 nm)
173.07 132.97 108.18 93.58 88.92 85.95 87.53 89.53 95.35 98.55 102.90 108.60 115.18 119.82 125.18
0.10 0.11 0.21 0.37 0.59 0.68 0.71 0.63 0.62 0.51 0.38 0.42
2
4a2
(500nm)
(Zy&
-0.94 -0.91 -0.93 -0.94 -0.96 -0.92 -0.90 -0.89 -0.84 -0.81 -0.77 -0.80
2.786 2.684 2.256 2.742 2.648 2.293 2.725 2.860 1.220 0.948 1.365 2.634 2.985 1.231 2.653
YO-'
(ns) 294.4 290.5 286.9 283.6 280.6 277.9 275.4 273.1 271.0 269.2 267.2 265.2 263.6 262.4 260.9
(Z)xz)g corresponds to the global Zxzfor the decay at 377 and at 490
or 500 nm. 20 19
0.3
18
0.2
-
h
0.1
LY
Y
340
380
420
460 500 540 WAVELENGTH (NM)
580
Figure 9. SAEMS of the locally excited state (0)and the exciplex state (+) of lPy2In in isooctane at room temperature.
As shown in Table 2,&1is equal to one within the experimental error. Therefore the deviation from -1 the ratio of the preexponentials of fluorescence decays obtained at the red end of the spectrum must be due to the overlap of the emission of the locally excited state with the emission of the exciplex. TemperatureDependenceof the FluorescenceDecay of 1Py2In. The fluorescence decay curves of lPy2In in isooctane were collected in function of the emission wavelength and temperature. At temperatures between -90 and 22 OC,the fluorescence decay curves of the locally excited state and the exciplex state can be analyzed using either single-curve or global analysis as a twoexponential decay depending on the temperature. The ratio of the preexponential factors approaches -1 at long wavelengths (Table 3). At temperatures higher than 22 OC the fluorescence decay of lPy2In can be analyzed as a one-exponential decay. The fluorescence decay of the model compound can be analyzed as a one-exponential decay at all temperatures and the decay time increases upon decreasing the temperature. From the decay parameters of lPy2In and 1-methylpyrene and the ratio of the preexponential factors at 377 nm, kol, k21, ko2, and kl2 can be obtained at temperatures between 22 and -90 OC. The value of klz shows the largest temperature dependence and approaches the value of k02 near -40 OC. This agrees with the fact that the maximum of ln(@L/@L)is situated near -40 OC. From the Arrhenius plots (Figure lo), the activation energies and the preexponential factors of the different rate constants can be obtained (Table 4). At low temperatures, a deviation of In k12 in function of 1 / T is observed. An analog deviation was already observed in other systems.45360 FluorescenceDecay of 1Py3In in Isooctane. The fluorescence decays of 1Py3In in isooctane were obtained as a function of temperature and of the emission wavelength. At low temperatures the individual decay curves can be analyzed in the framework of eqs 8 and 13 with bl equal to 1. At temperatures lower than -60
-C
v
17 16 15
\\
14 13 3.7
4.1
4.5 1O O O i l ( K
-')
4.9
5.3
Figure 10. Arrhenius plots of the rate constants of lPy2In in isooctane. (m) In k21, ( 0 )In k12, ( X I In k02. TABLE 4 Activation Energies and Preexponentials of the Different Rate Constants and Thermodynamic Properties of the Exciplex of lPylIn in Isopentane and 1Py2In in Isooctane lPylIn lPy2In 0.4 f 0.3 kJ mol-' 0.4 f 0.03 kJ mol-' Ea01 k'oi Ea21 k021 &a
k"12
4.50 X lo6 s-' 15.1 kJ mol-' 7.6 X loLos-l 22.83 kJ mol-' 1.13 x 1013 s-1
&a
k002 AH0
As" E,, k:
-8.0 kJ mol-' -41.1 J mol-' K-' 26.0 kJ/mol 2.0 x 107 S-1
4.50 X lo6 s-l 15.4 & 0.25 kJ mol-' 8.5 X 1Olo s-l 26.5 & 0.45 kJ mol-' 1.9 x 1013S-1 1.5 f 0.22 kJ mol-' 4.2 x 107 s-1 -10.1 f 0.41 kJ mol-' 4 1 . 4 f 1.7 J mol-' K-1 37.9 kJ/mol (1.3 f 0.1) X lo7 s-'
OC the single-exponential decay of the emission a t 377 nm suggests that the dissociation of the exciplex to the locally excited state ( k I 2 )can be neglected versus the other decay processes of the exciplex (kO2).In the exciplex region, a two-exponential decay is observed and the ratio of the preexponential factors is within the experimental error equal to -1. The decay parameters 71 and 7 2 gradually decrease with increasing emission wavelength. At higher temperatures, however, the fluorescence decay of the locally excited state can no longer be analyzed as a linear combination of two exponentials as prescribed by the kinetic scheme used for intermolecular exciplex formation. When it was attempted to analyze the fluorescence decays obtained at different wavelengths as a linear combination of two
1538 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 TABLE 5 Global Analysis of the Fluorescence Decay of 14.3111in Isooctane at -64 OC as a Linear Combination of Two Exponentials with Decay Times y1-l and yj-lr X (nm) 378 410 420 430 440 460 470 480 490 500 510
71-I(ns) 55.70 55.70 55.70 55.70 55.70 55.70 55.70 55.70 55.70 55.70 55.70
YZ-' (ns)
ff2/ffI
ZXZ
18.89 19.45 2 1.66 23.54 24.99 25.81 25.94 29.20 27.25 28.00
0.135 -0.62 -0.72 -0.87 -0.87 -0.94 -0.95 -0.95 -0.96 -0.96 -0.96
1.818 0.797 0.597 0.959 0.119 0.057 0.535 3.052 0.729 0.114
OOne of the decay times (71-l) is linked through the range of wavelengths. a2/a1corresponds to the ratioof the preexponentials. (Zxz)g = 2.539, channel width = 1.316 ns.
TABLE 6: Global Analysis of the Fluorescence Decay of 1Py3In in Isooctane at -64 OC as a Linear Combination of Three Exponentials with Decay Times ~ l - l yz-l, , and y3-l.
377 410 420 430 440 460 470 480 500 510
71-1
72-1
73-1
(ns)
(ns)
(ns)
55.92 55.92 55.92 55.92 55.92 55.92 55.92 55.92 55.92 55.92
20.80 20.80 20.80 20.80 20.80 20.80 20.80 20.80 20.80
26.74 26.74 26.74 26.74 26.74 26.74 26.74 26.74 26.74
a1
a2
1.ooo 0.758 -0.717 0.804 -0.797 1.000 -0.744 1,000 -0.454 1.000 -0.253 1.000 -0.136 1.000 -0.107 0.941 0.059 0.863 0.137
a3
0.242 0.196 -0.142 -0.454 -0.694 -0.821 -0.859 -0.970 -0.975
Z-/Z+
zxz
-0.479 1.092 -0.717 1.268 -0.797 0.710 -0.886 1.450 -0.908 -0.947 -0.196 0.525 -0.957 0.720 -0.966 -0.970 0.955 0.405 -0.975
a The three decay times are linked through the range of wavelengths. ThepreexponentiaIsal,a2anda3arenormalizedtothesumofthepositive preexponentials (2+). X / 2 + is the ratio of the sum of the negative preexponentials to the sum of the positive preexponentials. (ZX& =
1.657.
exponentials using global analysis the values of Zxzexceed 5.0 at all temperatures. At low temperatures, acceptable global analyses were however obtained by linking the monomer decay time throughout the whole emission region and by linking the exciplex decay time for decays obtained at different time increments at a single wavelength. In this case the decay time of the exciplex increased in a systematic way at longer emission wavelengths. This was observed independently of the number of channels or of the channel width in the experiment. To control this observation, the fluorescence decay curve of 1Py3In was determined every 10 nm at - 6 4 and 22 OC. Linking only the decay time of the locally excited state, the global analysis of the decay curves at - 6 4 OC as a two-exponential decay is acceptable (Table 5). The increase of the exciplex decay time with increasing wavelength suggests that a triple-exponential decay is analyzed as a double-exponentialdecay. This is confEmed by the fact that it is possible to analyze the decay curves as a triple-exponential decay using global analysis and linking the three decay times throughout the whole emission spectrum (Table 6). At 377 nm the contribution of two of the components with a decay time of 20.8 and of 26.7 ns is negligible, suggesting that under those conditions the decay can be described by a singleexponential decay. A global analysis, linking all decay times, which combines a one-exponential decay at 377 nm with a threeexponential decay at the other emission wavelengths yields acceptable statistical parameters. The ratio of the preexponential factors of the two components that do not contribute to the emission at 377 nm vary in a systematic way from 1.O at 440 nm t o 4 1 4 at 510 nm. At temperatures below -64 O C fluorescence decays obtained for different wavelengths at the same temperature can be analyzed in a similar way.
Helsen et al.
TABLJE 7: Global Analysis of the Fluorescence Decay of lPy3In io Isooctane at 22 OC as a Linear Combination of Three Exwnentiab with Decay Times 71-l.y2-l. and ~8~
377 390 420 430 450 460 470 490 500 510 520
30.97 30.97 30.97 30.97 30.97 30.97 30.97 30.97 30.97 30.97 30.97
2.99 2.99 2.99 2.99 2.99 2.99 2.99 2.99 2.99 2.99 2.99
14.56 14.56 14.56 14.56 14.56 14.56 14.56 14.56 14.56 14.56 14.56
0.10
0.69 0.28 -0.77 -0.82 -0.91 -1.02 -0.90 -0.72 -0.74 -0.81 1.00 -0.65
0.26 0.49 0.61 0.81 0.89 1.00 1.00 1.00 1.00
0.21 0.46 0.51 0.39
0.19 0.11 -0.01 -0.12 -0.22 -0.19 -0.31
-0.77 -0.82 -0.91 -1.02 -0.84 -0.84 -0.96 -1.00 -0.96
-0.507 2.038 1.068 0.734 0.540 1.567 0.343 2.254 1.322 -0.512 1.894
a The three decay times are linked through the range of wavelengths. Thepreexponentialsal, azanda3 arenormalised to thesumofthepositive preexponentials (E+). Z-/Z+ is the ratio of the sum of the negative preexponentials to the sum of the positive preexponentials. The channel width is equal to 0.363 ns and ZXz6= 2.922.
At temperatures ranging from - 6 4 to 45 OC the fluorescence decays obtained at wavelengths between 377 and 520 nm can be analyzed as a sum of three exponentials using global analysis (Table 7); in this case however the decay of the emission at 377 nm must also be described by a sum of three exponentials. NonradiativeDecay of theExciplex. Optoacoustic spectroscopy allowed one to determine a, the fraction of the energy of an absorbed photon that is converted into heat within the integration time of the detecting system. While those experiments were performed earliers3.61for 1Py2In and 1Py3In, comparing the photophysical properties of the exciplexes of lPyl In, 1Py2In, and lPy3Inrequired thedeterminationof afor lPylInin solvents of different polarity. As reported in Table 8, a increases upon increasing the solvent polarity. This is at least partially due to an increase of the quantum yield of exciplex formation or electron transfer and to a decrease of the exciplex energy upon increasing the solvent polarity. To know whether the increase of ci is also due to more efficient internal conversion of the exciplex, one has to calculate also 8, the fraction of the nonradiative decay of the exciplex or an eventual solvent-separated ion pair that occurs by internal conversion. As derived earliers1 8 can be calculated starting from the emission energy and the fluorescence quantum yieldof theexciplex and the locally excited state. The calculation of requires furthermore the knowledge of the triplet energy of the locally excited state and the fluorescence quantum yield of a model compound. Extrapolation of the data obtained at lower temperature yields a value of 4.8 X lo7 s-l for k02 for lPylIn in isooctane at room temperature. Hence a value of (1.8 f 0.3) X lo7 s-l and (2.9.0 f 0.5) X lo7 can be obtained for k: and kiw, respectively. k;, the rate constant for the internal conversion of the exciplex, is within the experimental error equal to zero. Combining the optoacoustic data with the stationary and nonstationary fluorescence of lPylIn in diethyl ether allowed one to obtain k:, k?, and k?. The variation of ki is within the experimental error, and a decrease of k? is accompanied by an increase of k?. In the more polar solvents, the complex did not allow one to obtain the individual rate constants; however, combining theoptoacoustic data with the stationary fluorescence still allowed the determination of 8. As observed for other exciplex-forming systems 8 increased in a systematic way upon increasing the solvent polarity.5*,61~6~
Discussion Spectra. The structureless features and the shift of the bathochromic emission as a function of the solvent polarity, suggest that for 1PylIn,48 lPyZIn, and 1Py3In the structureless batho-
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1539
Intramolecular Exciplex Formation
TABLE 8: Determination of the Exciplex Decay Parameters of 14.1111 Using Optoacoustic Spectroscopy isooctane diethyl ether tetrahydrofuran CY
B @ic a +!vc b
k! C k: k2
0.32 f 0.016 0.014 f 0.08 0.002 f 0.08 0.41 f 0.12 (0.1 f 5.8) X lo6 s-l (2.9 f 1.0) x 107 s-1 (1.8 f 0.3) X lo7 s-l
0.38 i 0.02 0.20 f 0.20 0.08 f 0.13 0.31 f 0.08 (2.1 f 3.3) x 106 s-l (8.0 f 2.3) X lo6s-l (1.3 f 0.2) x 107 8-1
acetonitrile
0.43 f 0.02 0.19 i 0.20 0.10 f 0.17 0.40f0.11
0.82 f 0.04 0.71 f 0.12 0.68 f 0.10 0.28 f 0.08
e e e
e e
e
Quantum yield of internal conversion of the exciplex. Quantum yield of intersystem crossing of the exciplex. Rate constant of internal conversion of the exciplex. Rate constant of intersystem crossing of the exciplex. Due to the complex fluorescence decay, it was not possible to obtain the individual rate constants. (I
chromic emission can be attributed to an exciplex. The absorption spectra and the excitation spectra do not suggest an interaction between the two chromophores in the ground state. For the lesspolar solvents the quantum efficiency of the exciplex emission 7;depends only slightly upon the solvent polarity for lPylIn, lPyZIn, and 1Py3In. In acetonitrile, however, a decrease of the quantum efficiency of the exciplex emission, 75 is observed for l P y l I n and to a smaller extent for lPy2In (Table 1). This could be explained by the formation of radical ion pairs either directly from the locally excited state or by the dissociation of the exciple^.^' This decrease of 7;is however less important than the decrease observed for exciplexes where the donor moiety is an aliphatic amine.64 The solvent dependence of the emission maximum suggests that the exciplex dipole moment of lPy2In is larger than that of 1Py3In or lPylIn which is considerably larger than that of the intermolecular exciplex of 1-methylpyrene and 1,Zdimethylindole characterized by a value of 2pu,2/4?rhctop3equal to 7550 cm-I. This is due to fact that in the intramolecular systems the alkyl chain restricts the approach of the bary centers (average position) of the charges on the two chromophores. For lPy2In increasing the solvent polarity does not only shift the emission maximum of the exciplex to longer wavelengths, it increases also the fwmh of exciplex emission (Av) as indicated in53
where but corresponds to the outer sphere reorganization energy, Ainl is the intramolecular reorganization energy associated with vibrations for which hvinl < k T and Ainh is the intramolecular reorganization energy associated with vibrations for which hvinh > kT. For the transition from an excited state with a permanent dipole moment p e to a ground state without a permanent dipole moment, A,,, is given by 2
(22) Using eq 22 and the values of p e 2 / 4 ~ t o pdetermined 3 using the Lippert-Mataga plot lout can be calculated for isooctane and acetonitrile. As Ainl, Ainh, and vinh can be expected to be identical in isooctane and acetonitrile, the fwhm (Av) in isooctane can be calculated from the value of Av in acetonitrile. In this way, based on fwhm of 4500 cm-1 in acetonitrile a value of 4050 cm-' was obtained for the fwmh of the exciplex emission of 1Py2In in isooctane. This corresponds within the experimental error to the observed fwmh in isooctane (4000 cm-I). This correspondence as well as the linear Lippert-Mataga plots suggest that the electronic structure of the species emitting in acetonitrile is similar to that emitting in isooctane. For 1Py3In starting from an fwhm of 4300 cm-l in acetonitrile a value of 3800 cm-' was obtained for the fwmh of the exciplex emission in isooctane. This is 500 cm-l smaller than the experimentally observed fwmh, suggesting a structural change of the exciplex or the presence of more than one polar-emitting species (see below).
Kinetic Aspects of Exciplex Formation of 1Py2In and 1PylIn in Isooctane. For single curve as for global analysis the fluorescence decay of 1 P ~ l I and n ~ ~lPy2In isooctane can be described as a linear combination of two-exponential decays, indicating there are twodifferent kinetic species, the locally excited stateand the exciplex state. In agreement with the interpretation of the stationary fluorescence and fluorescence excitation spectra, the biexponential decay suggests the presence of two kinetically different excited species: a locally excited state and an exciplex. Those results suggest furthermore than at room temperature in isooctane the equilibrium between the different conformations in the ground state is faster than the formation of the exciplex.1IJsJO Thevalue of 61 obtained for 1Py2In by performing compartmental analysis of fluorescence decays obtained in the presence of a quencher is almost equal to 1, indicating that in the ground state there are no molecules in the exciplex conformation. For lPy2In in isooctane the temperature dependence of ln(@L/@Le)suggests that below -36 O C the dissociation of the exciplex to the locally excited state (klz) is slower than the rate of the other decay processes of the exciplex (koz). Under those conditions and assuming that k02 and kf do not depend upon the temperature the slope of a plot of ln(@,/@Le)versus 1 / T allows one to determine a value of 19.1 kJ mol-' for the activation energy E12. This value corresponds within the experimental error to that obtained from the analysis of the fluorescence decays. For lPy2In in more polar solvents the deviations from Scheme 1 suggested by the fluorescence decays no longer allow one to determine the activation energy on k21 from the temperature dependence of the stationary fluorescence spectra. In isooctane a t room temperature the rate constant of the decay of the exciplex (k02) of both lPylIn and lPy2In is negligible compared to the rate constant of the dissociation of the exciplex to the locally excited state (k12). The values of the activation energies on k21, Ezl, amount to 15.1 and 15.4 kJ mol-' for lPylIn and 1Py2In, respectively. Those values resemble thevalues found for other intramolecular exciplexes9J1~20~21~37 and excimersI7,6a8 and are compatible with the rotation around a single bond. The difference of the activation energies on k12,E12, observed between lPylIn and lPyZIn, reflects the difference of AHo. Upon decreasing the temperature, positive deviations of In k12 from an Arrhenius plot are observed for kl2. When fluorescence decays at low temperature are constructed by extrapolating the values of k12,obtained at higher temperatures to low temperatures values of al/az (377 nm) are obtained that are 2 orders of magnitude smaller than the values of a~/az(377 nm) that are determined experimentally. It is unlikely that this deviation is due to errors in the analysis. In an intuitive way one could expect that spectral overlap between the emission of the exciplex and the emission of the locally excited state could give too large values of a I / a 2 (377 nm). Although this hypothesis cannot be excluded completely the low intensity of the exciplex emission at low temperatures makes it improbable. Furthermore between -90 and -70 O C the values of k12 determined as a parametric function of El ranging between 0.7 and 1 exceed those obtained for E1 equal to 1. This unexpected behavior can be attributed to the
a
Helsen et al.
1540 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 SCHEME 3
T
-50"C 1Pyl In
1Pysln
T > -50"C
E \
Figure 11. Two-dimensional representation of the exciplex geometries in the different systems. The exciplex configuration a is assumed to be more stable than the exciplex configuration b.
convergence at decreasing temperature between the short and thelong decay timeand (ko2 + klz)-I and (kol+ k21)-11respectively. Only for values of El between 0.5 and 0.55 at -90 O C to values of El between 0.7 and 1 at room temperature do the recovered values of k12 follow an Arrhenius behavior. In principle the deviations of an Arrhenius behavior observed for k12 could be due tovalues of 6, smaller than one$ low temperatures. Calculations suggest however that making bl slightly smaller than 1 leads to an increase rather than a decrease of th_eapparent values of klz at low temperatures. Only for values of bl close to 0.5, the values of k12 at low temperature correspond to those obtained by extrapolating the Arrhenius relation to low temperatures. As the analysis of the fluorescence decays obtained in the presyxe of 1,4-dibromobenzene at room temperature suggests that bl is close to 1 at room temperature, this suggests that bl decreases drastically upon decreasing the temperature, indicating that the "ground-state complex" would be at lower energy in spite of its g-g+ conformation. It is also unlikely that the deviation is due to the formation of microcrystals, characterized by an emission of the locally excited state. This would result in larger values of a2 and an analysis leading to too small values of ( Y I / ~ Z (377 nm). Furthermore, at the concentration of 5 X 10" M that is used in the experiments described here, the formation of microcrystals is not probable. A possibleexplanation is the formation of a third excited species, 3*. This would lead to the situation represented in Scheme 3. Scheme 3 would yield a triple-exponential decay for the emission of the locally excited state, which is in contradiction to the statistically acceptable global analysis of fluorescence decays as a linear combination of two exponentials. To reduce a threeexponential decay to a two-exponential decay, one of the following conditions must be met: (1) The preexponential factor of the third component is very small or equal to zero. (2) One decay time is smaller than the time resolution of the single-photon timing setup. (3) The decay time and the emission spectrum of two species resemble each other, making it impossible to resolve them by global compartmental analysis.
This third species could be an exciplex with a smaller stabilization due to a less favorable overlap of the chromophores or to a larger distance between the chromophores. At temperatures higher than -50 O C the equilibrium between the locally excited state and this exciplex will be shifted completely to the locally excited state, making it possible to analyze the date in the framework of Scheme 1. Extrapolation of the values of kl2 and kzlobtained a t higher temperatures suggests that a t temperatures below -50 OC the value of k12will be smaller than the value of kZ1and the equilibrium will be in the direction of the exciplex. However, at temperatures below -50 O C k13 will also be reduced relative to k3I and the equilibrium between the locally excited state and the second exciplex shifts toward this second exciplex. This will result in a more important quenching of the locally excited state, a larger value for the ratio of q/az(377 nm), and an apparent increase of kl2 at low temperatures. FluorescenceDecays of 1Py3In in Isooctane. The decay curves of 1Py3In can be analyzed at all temperatures only when a threeexponential decay is used. The single-exponential decay observed for the locally excited state at -64 OC and the wavelength dependence of the preexponential factors suggest the presence of one locally excited state and two exciplexes emitting at different wavelengths, one which has an emission maximum at 450-460 nm and another with an emission maximum a t longer wavelengths. Below -64 OC the emission at 377 nm can be analyzed as a singleexponential decay; therefore, the deviations from the kinetic scheme proposed for intermolecular exciplex formation cannot be due to the presence of different starting conformations in the ground state.llJ0 It also suggests that a t -64 O C the exciplex formation is not reversible. Using space-filling molecular models, the two exciplex conformations can be attributed to different rotational isomers of the bond between the indolyl group and the alkyl chain, leading to a different overlap and relative positions of the indolyl and the pyrenyl group. A two-dimensional representation of those structures is given in Figure 11. For 1Py3In, one exciplex (a) gains the necessary stabilization by optimizing the Coulomb attraction between the negative pyrene radical anion and the positive indole radical cation, while in the second exciplex
Intramolecular Exciplex Formation
TABLE 9 Fluorescence Decay Parameters of 1Py3In in Isooctane at Low Temperatures T (oc) kO2(107 S-1) kos (107 S-1) kzl + k31(107 S-I) -64 3.77 4.69 1.45 -85 3.67 1.64 0.83 ~~~
~
SCHEME 4
conformation (b) the larger distance between the bary centers of the two radical ions is compensated for by an increased overlap between the chromophores, leading to a more extensive stabilization by configuration interaction with locally excited states. Both exciplex conformations are stabilized sufficiently to be formed during the lifetime of the locally excited state and cannot interconvert once formed. The kinetic scheme proposed for 1Py3In is presented in Scheme 4. Since the emission of the locally excited state and the exciplexes always overlap, the kinetic parameters cannot be obtained individually. However, at -64 and -80 "C the absence of exciplex dissociation makes it possible to obtain k21 + k31, k02, and k o ~(Table 9). The validity of the hypothesis suggesting the formation of two exciplexesupon excitationof lPy3Inissubstantiated by thechange of the fwhm of the exciplex band when the solvent polarity is increased. In acetonitrile the exciplex conformation with the greatest dipole moment which is hence characterized by the smallest Coulomb stabilization will have the largest stabilization by solvation. This will result in a decrease of thedistance between the energy levels of the two exciplexes and lead therefore to an apparent decrease of the fwhm of the exciplex band as is observed experimentally. The deviations from Scheme 1, observed in all solvents for 1Py3In, on the basis of the fluorescence decays no longer allow one to determine the activation energy on k2l from the temperature dependence of the stationary fluorescence spectra. Structural and Thermodynamic Aspects. The quantum yield of exciplex formation, amounting in isooctane to 0.41,0.66, and 0.98 for lPylIn, 1Py2In, and 1Py3In, respectively, suggests an increasing efficiency of exciplex formation from l P y l In over 1Py2In to 1Py3In. From the temperature dependence of In(@:/&) in isooctane, it can be concluded that the temperature where the dissociation of the exciplex becomes more important than the decay of the exciplex increases from lPylIn (-45 "C) over 1Py2In (-36 "C) to 1Py3In (>90 "C). Time-resolved experiments suggest that this occurs a t -50 and -40 "C for 1PylIn48 and lPy2In (Figure lo), respectively. Actually, although for 1Py3In a leveling of the plot of ln(@:/@L) versus l/Tcould beobservedat 15 "C,evenat90°C thereisnodecrease of this ratio, suggesting that klz is larger than k02. To the extent that k02 has similar values for those three systems, this suggests that klz increases from 1Py3In over lPy2In to 1PylIn. Comparing the three bichromophores, it can be concluded that the extent of exciplex formation as well as the exciplex emission maximum is sensitive to the substitution position of the alkyl chain on the indole chromophore. Neither molecular modeling nor the experimentally obtained values of Erep in apolar solvents, equal to 26 and 37.6 kJ/mol in lPylIn and 1Py2In, respectively, suggest that the nonbonded repulsion between both chromophores
The Journal of Physical Chemistry, Vol. 98, No. 6,1994 1541 in the exciplex conformation is larger for 1PylIn. Hence the more negative values of A P obtained for lPy2In are probably due to a larger Coulomb attraction between both radical ions in the contact ion pair. The combination of this larger Coulomb attraction and the larger exciplex dipole moment obtained for 1Py2In suggests that in lPylIn an important configuration interaction between locally excited states of the chromophores and a charge-transfer state leading to more extensive stabilization of the exciplex and a decrease in the exciplex dipole moment must occur. On the other hand, the similarity of the values of AS",which amount to-41.4and-41.0 Jmol-1 K-I, respectively, suggests that both exciplexes have a similar structure corresponding to a g + g conformation of the propyl chain. The total number of g + g conformations is equal to two, while the total number of chain conformations not leading to an exciplex equals 7. The ratio between the number of chain conformations yielding an exciplex to the number of chain conformations that does not lead to an exciplex amounts to 2/7, leading to a value of AS" equal to -10 J mol-' K-1. To explain the experimentally obtained value of AS",the exciplex formation must be accompanied by a restriction of the freedom of rotation of the end groups or a steeper potential of nonbonded interaction upon rotation around the carbon4arbon single bonds leading to a smaller partition function for the hindered rotation in the exciplex. The emission maxima of the exciplexes in isooctane shift to longer wavelength in the order lPylIn, lPy2Ir-1, and 1Py3In. Although it is possible that the larger shift observed for 1Py3In could be due to a larger repulsion energy in the ground state as suggested by molecular models, the shift of the fluorescence maxima in isooctane parallels the relative values of kl2 suggested by the temperature dependence of the stationary emission spectra in isoooctane. In lPy2In and to a larger extent in lPylIn the most stable conformations (a) are characterized by a smaller Coulomb stabilization than in 1Py3In. In the second exciplexconformation (b) of 1Py2In, the nitrogen is still situated in a more eccentric position compared to the center of the pyrene radical anion, and there is less overlap between the chromophores. This situation could perhaps correspond to the third excited species which could be formed a t low temperatures. For lPylIn the second exciplex (b) conformation becomes destabilized to such an extent that only the first exciplex is formed. The intermolecular exciplex between 1,2-dimethylindole and 1-methylpyrene is stabilized48 a t -21.3 kJ mol-' versus the locally excited state of 1-methylpyrene. This larger stabilizationenthalpy will be due to the absence of the chain which prevents the chromophores to reach the mutual configuration leading to an optimum of the Coulomb attraction and van der Waals repulsion between the r-electron clouds. The smaller distance between the bary centers of the positive and the negative charge, leading to a larger Coulomb attraction, is also reflected in the smaller dipole moment of the intramolecular exciplex. Matrix Elements between the Charge-Transfer State and the Ground State. The rate constant for exciplex fluorescence amounts torespectively 1.8 X 107and 1.3 X lO7s-Ifor lPylInand lPyZIn, while the sum of the rate constants of the radiationless decay channels of the exciplex amounts to 2.90 X lo7and 0.7 X lo7 s-1, respectively. As the combination of the fluorescence decay data with the optoacoustic data leads to similar values of k: in isooctane((l.Ok5.8) X lO'and(8.0k 10.0) X l O 5 ~ - ~ f olPylIn r and lPyZIn, respectively), the more important radiationless decay of the exciplex of l P y l I n must mainly be due to a larger value of kf. Actually the experimentally obtained values of k p increase from 6.2 X 106 in lPy2In to 2.9 X lo7s-l in 1PylIn. The fluorescent rate constant of the intramolecular exciplex is significantly larger than that of the intermolecular exciplex between 1-methylpyrene and 1,Zdimethylindolewhich amounts to 3.6 X 106 s-1 in MeTHF. The sum of the rate constants of the
1542 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994
radiationless decay channels of the intermolecular exciplex in MeTHF, which amounts to 0.9 X 107s-1 can be compared to that of the intramolecular exciplex of lPy2In in isooctane. The sum of the nonradiative decay rate constants of the exciplex of lPylIn is characterized by a small activation energy of 0.4 kJ mol-’. As the sum of the decay rate constants of the exciplex of lPy2In has a small activation energy of 1.5 kJ mol-’, it is likely that kiW2, which amounts to 0.7 X lo7s-l, is also characterized by a small activation energy. To the extent that the exciplex fluorescence is due to the mixing of a charge-transfer configuration in the ground ~tate,2.4.6~-~‘ the electronic matrix element, HAD, between the charge-transfer configuration and the neutral zero-order ground state can be calculated using eq 23 or eq 24.
HDA=
HDA=
[
3.58 X 1O-”ki]li2 (23)
n3p,2u
[
1.4 x 105k:
n3R:u
1
In eqs 23 and 24 n is the refractive index of the solvent, pe is the exciplex dipole moment, R,is the distance between the donor and the acceptor in the exciplex, v is the frequency of the emission maximum of the exciplex and ki is the fluorescence rate constant of the exciplex. If a distance between the donor and the acceptor of 3.3 %r. is assumed, a value for HADof 2240 and 1610 cm-I is calculated for lPylIn and lPyZIn, respectively. In thecalculation of this value for l P y l In the partial charge-transfer character of the exciplex has been taken into by the use of the experimentallyobtainedvalueofp?/4rtop3. Thevalueof l P y l In has been calculated with the assumption that the C I with locally excited states does not contribute to ki. An important contribution of C I with locally excited states to ki could lead to an overestimation HAD. The matrix element HADis important as it willalso be involved in theinternalconversionof the exciple^.^^-^^ Using the same procedure, a value of 890 cm-l is obtained for the intermolecular exciplex of 1-methylpyrene and 1,2-dimethylindole in MeTHF. Using optoacoustic c a l ~ r i m e t r y , ~it&has ~~ been observed that at room temperature the nonradiative decay of the exciplex occurs in isooctane mainly by intersystem crossing. As the latter process will be governed by the matrix element for spin-orbit coupling, it is not possible to relate the values ko2 obtained in isooctane to the matrix element HAD. In this respect it should be mentioned that the ratio k$/ki, which reflects the Franck-Condon weighted density of for the internal conversion process, is, despite of the smaller “energy gap”, considerably smaller than observed in isooctane for the intramolecular exciplexes involving aliphatic a m i n e ~ . ~This z - ~ should ~ be related to the larger reorganization energy for electron-transfer processes involving aliphatic amines.76 In all solvents used, the values of /3 obtained by combination of optoacoustic spectroscopy and stationary fluorescence spectroscopy increase in a systematic way from lPylIn over 1Py2In to lPy31n.52-61 The complex decay observed 1Py3In does however not allow one to determine the individual rate constants for this compound. Conclusions Upon excitation of lPylIn,a 1Py2In, and 1Py3In an intramolecular exciplex is formed. As the exciplexes of the three compounds are characterized by similar values of k02 (or k03), the quantum yield of exciplex formation (@E) in apolar solvents reflects the values of k21 (at temperatures where k02 exceeds kl2) or of the ratio k2l/k12 (at temperatures where klz exceeds k02). The decrease of the dipole moments from 1Py2In over lPylIn or 1Py3In to the intermolecular of 1-methylpyrene and 1,2-
Helsen et al. dimethylindole correlates with the decrease of the enthalpy of exciplex formation or the increasing trend to form an exciplex. The alkyl chain prevents the chromophores to reach a configuration where the barycentre of the positive charge on the indole radical cation is situated above the center of the pyrene radical ion. Molecular models suggest that this effect becomes more important from 1Py3In over lPy2In to 1PylIn. Also thevalues of AHo for the exciplex formation or (a?) in apolar solvents follow the same trend. For lPylIn the separation between the energy levels of the charge transfer state and the locally excited state becomes so small that important configuration interaction between those states occurs leading to a decrease of the exciplex dipole moment and to a decrease of AHo for exciplex formation. For 1Py3In the analysis of the fluorescence decays and the influence of the solvent polarity on the fwhm of the exciplex emission suggest the formation of two different exciplexes. For lPy2In the stabilization of this second exciplex is much smaller, and its formation is suggested only by indirect evidence from the analysis of the temperature dependence of the photophysical parameters. As an alternative interpretation, the apparent deviations from an Arrhenius plot can be due to overlap of the emission of the locally excited state and the exciplex at 377 nm. For lPylIn the analysis of the fluorescence decays allows only for the formation of a single exciplex.
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