17578
J. Phys. Chem. 1995,99, 17578-17585
Photoinduced Electron Transfer in Charge-Transfer Crystals by Diffuse-Reflectance (Picosecond) Time-Resolved Spectroscopy Stephan M. Hubig and Jay K. Kochi* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: August 7, 1995@
Solid-state photoexcitation of crystalline charge-transfer complexes of arene donors (including various benzenes, naphthalenes, biphenyls, anthracenes, and phenanthrene) and either tetracyanobenzene (TCNB) or methyl viologen (MV) with a 25-ps laser pulse generates short-lived charge-transfer excitons, the transient diffusereflectance spectra of which strongly resemble those of the corresponding arene cation radicals. The firstorder decays of the transients monitored on the picosecond and early nanosecond time scales are ascribed to back-electron transfer (BET) within the photogenerated ion pairs. Plots of the decay rate (In JCBET) versus the ) linear with slopes similar to those obtained from the same charge-transfer charge-transfer energies ( ~ Y C T are complexes that are either dissolved in solution or encapsulated in porous glasses @ = 2-3 eV-I). The data suggest that charge-transfer excitons generated by laser excitation of charge-transfer crystals can be considered as highly localized excited ion-pair states which behave similarly to contact ion pairs in solution.
SCHEME 1
Introduction Electron donor-acceptor interactions have been recognized as playing a key role in the development of unusual optical, electric, and magnetic properties of crystalline materials.’-5 For example, a “nonlinear” optical response has been found in molecular crystals of “push-pull” conjugated donor-acceptor systems such as (dimethylamino)(dicyanovinyl)stilbene and (dimethylamino)-N-methylstilbazoliumsaltsS6 Electron donor and acceptor moieties can also be assembled in crystalline form, if they form intermolecular charge-transfer (CT) complexes that can be precipitated from solution. Such charge-transfer crystals with well-defined structures represent an ideal medium to study electron-transfer processes under conditions of rigid orientation, high constraint, and close d i ~ t a n c e . ~ . ~ The spectroscopic and photophysical properties of CT complexes have been studied in solution t h o r o ~ g h l y . ~Ac.’~ cording to Mulliken theory,’ I the electronic wave function of such complexes includes a partial charge-transfer component in the ground-state. Excitation into the excited singlet state is predicted to result in electron transfer from the donor (D) to the acceptor (A), leading to the formation of an excited ionpair state, as depicted in Scheme 1. With the development of picosecond time-resolved spectroscopic techniques, experimental confirmation of Mulliken theory has been achieved as shortlived radical-ion pairs that are observed immediately attendant upon the photoexcitation of the CT absorption bands of electron donor-acceptor complexes in Photoinduced electron transfer leading to the ion-pair state as well as any subsequent thermal reaction of the ion pair (including the energy-wasting charge-recombination process) has been studied in detai1.I5-*O For solid-state studies, numerous CT complexes have been prepared either as molecular crystals or as ion-pair X-ray crystallographic studies confirm, in most cases, a close approach (d 3.5 A) of the electron donor and acceptor moieties. Whereas a wealth of time-resolved spectroscopic studies have led to a thorough understanding of the photophysics of CT complexes in solution,’2-20such studies with CT crystals are rather scarce and mostly restricted to emission experim e n t ~ . ~Only ~ . ~a few ~ reports address the direct detection of @
Abstract published in Advance ACS Abstracts, November 1, 1995.
1
[Do+,A”]’
excited ion-pair state
hcr
D+A
(D”, A%]
ground-state CT complex
exciton states in CT crystals by transient absorption spectrosThis lack is partly due to the experimental difficulties associated with transient absorption spectroscopy studies on crystalline samples. However, two techniques have been applied, namely (i) transient spectroscopy in the transmittance mode using optically clear single-crystals of appropriate size under a microscope30 and (ii) transient spectroscopy in the diffuse-reflectance mode using rather opaque microcrystalline p o ~ d e r s . ~ ’Recently, -~~ the diffuse-reflectance technique has been applied to the photophysics of phenanthrenelpyromellitic dianhydride (PMDA) charge-transfer crystals.** Upon photoexcitation, a transient absorption spectrum that strongly resembled that of the PMDA anion radical was obtained.28 Tetracyanobenzene (TCNB) and methyl viologen (MV) form charge-transfer complexes with a wide variety of organic electron donors.34 Several CT crystals with aromatic donors have been prepared and the X-ray structures have been As shown in Chart 1, the CT complexes of TCNB or MV with anthracene, naphthalene, durene, and biphenyl all crystallize in a similar fashion to form cofacial stacks of altemate donor and acceptor moieties with a closest donor-acceptor distance of about 3.5 A. In this study, we prepared a series of CT crystals of TCNB and various arenes of differing donor strengths, and we then recorded the transient absorption spectra upon the photoexcitation of the charge-transfer absorption bands. The lifetimes of the excited (ion-pair) states were determined on the picosecond time scale, and they were correlated with the thermodynamic data associated with one-electron transfer between the arene donor and tetracyanobenzene. To study the effects of charged species on electron-transfer processes in the solid state, a second series of CT crystals was also prepared from the same arene donors and methyl viologen (MV) hexafluorophosphate as the electron acceptor, since the latter has been shown to form CT crystals with various types of electron donors such as 2,6-dimetho~ynaphthalene,~~ bromide
0022-365419512099-17578$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 49, 1995 17579
Photoinduced Electron Transfer in CT Crystals
CHART 1: Crystal Structures of Charge-Transfer Complexes of Various Arene Donors with Tetracyanobenzene or Methyl Viologen (as the Bis(hexafluorophosphate)Salt), Showing the Alternate Stacking of Donors and Acceptors According to the Crystallographic Data in the Literature>*-"
1
0.00 360
460
660 WAVELENOTH
TCNB / Naphthalene
Anthracene / TCNB
-
z==z
TCNB /Biphenyl
=
Durene / TCNB
MV / Dimethaxynaphthalene
and iodide,35 metal d i t h i ~ l e n e s etc. , ~ ~ The solid-state results are compared to the analogous experiments in solution with a view of interpreting the results within the context of current electron-transfer theories.
Experimental Section
1. Materials. Methyl viologen (MV) hexafluorophosphate was prepared by adding an aqueous solution of KPF6 (Aldrich, 2 equiv) to an aqueous solution of 1 equiv of methyl viologen dichloride (Aldrich). The white precipitate formed immediately upon mixing was filtered, washed several times with water, and dried overnight in vacuo at 100 "C. 1,2,4,5Benzenetetracarbonitrile (tetracyanobenzene, TCNB) was used as received (Aldrich). The aromatic hydrocarbons (Aldrich) were purified by recrystallization from alcohol or alcohollether mixture^.^' Acetonitrile and dichloromethane were purified according to literature procedure^.^' Charge-transfer crystals were prepared by the slow evaporation of either an acetone solution containing equimolar amounts of the aromatic hydrocarbon and TCNB, or an acetonitrile solution containing equimolar amounts of the aromatic hydrocarbon and methyl viologen hexafluorophosphate. In the case of the TCNB acceptor, large crystals of the CT complexes could be readily grown. However, methyl viologen hexafluorophosphate yielded only microcrystals of the corresponding CT complexes. In both cases, the colors of the CT crystals depended on the electron donor-acceptor combination and varied from pale yellow to dark purple. For spectroscopic
660
760
[nml
Figure 1. Diffuse-reflectance absorption spectra (normalized) of anthracene crystals (dashed line) and anthracene/TCNB charge-transfer crystals (solid line).
studies, the samples were used as fine powders prepared from either pure CT crystals or those diluted with either neutral alumina (Fisher Scientific, 80-200 mesh) or other colorless powders, as noted individually below. 2. Apparatus. Steady-state diffuse-reflectance spectra of diluted samples of the charge-transfer crystals were recorded with a Perkin-Elmer 330 spectrophotometer equipped with an integrating sphere. The spectra were digitized using a UNPLOT-IvM scanner (Silk Scientific) and transferred to a personal computer for data storage and analysis. The picosecond time-resolved diffuse-reflectancespec&oscopyapparatus has been described p r e v i ~ u s l y .Briefly, ~~ the second (532 nm) or third (355 nm) harmonic of a mode-locked Nd:YAG laser (Quantel, YG501-C, 25 ps) was used as the excitation source. The residual fundamental (1064 nm) laser beam was focused onto a 10-cm cuvette containing a 5050 mixture of H20 and D20 to generate a white continuum pulse of 25-ps duration. The white light was split into two beams which were used as reference light and probe light for the crystalline sample stored in a 1-mm quartz cuvette. The diffuse-reflected probe light and the reference light were picked up by fiber optics and fed into a flat-field spectrograph, to which a dual-diode-may detector (Princeton Instruments) was attached. Spectra are presented as percentage absorption (% ABS) as defined in eq 1, % ABS = lOO(1 - R/R,)
(1)
with R and R,-, representing the diffuse-reflectedand the reference light, respectively.
Results 1. Formation and Diffuse-Reflectance Spectral Data of Charge-Transfer Crystals. Slow evaporation of solutions containing a 1:1 mixture of various aromatic hydrocarbons and TCNB or MV led to colored crystals or crystalline powders, respectively. The absorption spectra of the powdered samples recorded in the diffuse-reflectance mode revealed that the colors were caused by an absorption band that was not observed in the powders of either pure component. As a typical example, the diffuse-reflectance spectrum of TCNB/B, lo-dimethylanthracene CT crystals and the spectra of the individual components are shown in Figure 1. The new absorption band at 570 nm was ascribed to the intermolecular charge-transfer transition. As listed in Tables 1 and 2, the maxima of the CT absorption bands varied over a wide wavelength range (385-570 nm) depending on the particular combination of the aromatic hydrocarbon and the electron acceptor. For comparison,
17580 J. Phys. Chem., Vol. 99, No. 49, 1995
Hubig and Kochi
TABLE 1: Spectral and Structural Data on TetracyanobenzendArene Charge-TransferComplexes in the Crystal and in Solution
arene donor 1. 2. 3. 4. 5. 6. 7. 8. 9.
IP"
db
(eV)
(A)
5.50
I
I
CT absorption maximum' (ACT, nm) CH3CN CH2C12 crystal
biphenyl naphthalene phenanthrene durene 4-methoxy-biphenyl 2,6-dimethylnaphthalene
8.27 3.55d -360h 380 8.15 3.43' 8.10 i 8.03 3.46r -370h 400 404 454 2,6-dimethoxynaphthalene 7.58 anthracene 7.55 3.489 464 492 9-methylanthracene 7.25 10.9,10-dimethylanthracene 7.11 504
310 400 389 385 420 434 476 496 522 554
385 410 420 400 420 430 475 510 530 570
Ionization potential of arene donor from ref 5 1. Closest distance between arene donor and tetracyanobenzene in the CT crystal as determined by X-ray crystallography. Spectral maximum of the charge-transfer absorption of the 1:1 complex of tetracyanobenzene (TCNB) and the arene donor identified in column 1. Reference 24. e Reference 22. f Reference 21. 8 Reference 23. Shoulder observed. 1 Spectral tail observed. (I
1.""
7.00
8.00
7.50
8.50
IP IN1
Figure 2. Mulliken correlation of the charge-transfer transition energies, ECT,and the ionization potentials, IP, of the arene donors in arene/TCNB charge-transfer crystals with a slope of 0.87. The error bars ( h 4 0 meV) reflect an uncertainty of k 1 0 nm in the determination of An of the charge-transfer crystals. (The numbering of the data points corresponds to the arenes identified in Table 1.)
TABLE 2: Spectral and Structural Data Pertaining to Methyl ViologedArene Charge-TransferComplexes in the Crystal and in Solution
arene donor
IP"
db
(eV)
(A)
2,6-dimethylnaphthalene
2,6-dimethoxynaphthalene 7.58 3.46d 9-methylanthracene 9,lO-dimethylanthracene
7.25 7.11
CT absorption maximum' (ACT. nm) CH3CN
crystal
-400' 450 496 508
415 455 540 570
a Ionization potential taken from ref 5 1. Closest distance between donor and acceptor molecule as determined by X-ray crystallography. Spectral maximum of the charge-transfer absorption of the 1:1 complex of methyl viologen (MV) and the arene donor identified in column 1. See ref 25. e Shoulder observed.
absorption spectra of the corresponding charge-transfer complexes dissolved in acetonitrile and dichloromethane solution were also recorded, and the CT absorption maxima are listed in Tables 1 and 2. The CT absorption maxima of the TCNB complexes uniformly experienced a successive red-shift in going from an acetonitrile solution to a dichloromethane solution and to the crystalline state (see Table 1). Indeed, the spectral change was more pronounced in proceeding from acetonitrile to dichloromethane than the corresponding change from dichloromethane to CT crystals. With some arene complexes (dimethylnaphthalene, dimethoxynaphthalene, and methoxybiphenyl), the absorption maximum of the CT complex in dichloromethane solution coincided with that of the crystalline powder. The spectra of the charge-transfer complexes of methyl viologen measured in acetonitrile showed a similar blue-shift relative to the spectra of crystalline samples (see Table 2). For the TCNB crystals, the charge-transfer energies ECT = hc/& were plotted versus the ionization potentials of the arene donors (see Figure 2). A slope of 0.87 was obtained for the linear correlation. A similar plot for the MV crystals yielded a slope of 1.19. Figure 3 shows the direct linear relationship between CT energies of TCNB complexes in crystals and those of the complexes in dichloromethane solution (with a slope of 1.07). Similarly, the linear plots of the CT energies of MV crystals versus the CT energies of the corresponding TCNB crystals yielded a slope of 1.2 (see Figure 4). Interestingly, the CT maxima of the pair of TCNB and MV charge-transfer crystals were quite similar (compare Tables 1 and 2). The same is true for the CT spectra of the TCNB and MV complexes
2.00 2.00
2.60
3.00
5.60
(CRYSTAL) [*VI
E,,
Figure 3. Direct correlation of the charge-transfer energies of arene/ TCNB charge-transfer complexes in dichloromethane solution, En(CH2Ch), and those of the corresponding CT crystals, &(crystal), with a slope of 1.07. (The numbering of the data points corresponds to the arenes identified in Table 1.) 3.20
/
+j+
/
2.80
u"
2.40
6
/+
. 2.00 2.00
y-y+
2.40
E,
2.80
3.20
(TCNB) I O V l
Figure 4. Direct correlation of the charge-transfer energies of arenel MV charge-transfer crystals, En(MV), and those of the corresponding arenefTCNB charge-transfer crystals, &(TCNB), with a slope of 1.2. (The numbering of the data points corresponds to the arenes identified in Table 1.)
measured in acetonitrile solution (see Tables 1 and 2). According to Mulliken theory, CT complexes of different electron acceptors with the same electron donor moiety are expected to exhibit similar CT absorption maxima when the electron
J. Phys. Chem., Vol. 99, No. 49, 1995 17581
Photoinduced Electron Transfer in CT Crystals -"
I
I
32
-
c
f
B m
24
16
8
TABLE 3: Transient Absorption Spectrum Attendant upon the Charge-Transfer Excitation of TetracyanobenzendArene Complexes in the Crystal and in Solution transient absorption maximum (nm) arene donor CH2C12 solution CT crystal biphenyl 68 1 72 1 naphthalene 695 72 1 2,6-dimethylnaphthalene 684 680 2,6-dimethoxynaphthalene 646 64 1 anthracene 728 74 1 9-methy lanthracene 708 710 9,lO-dimethylanthracene 685 716
n 1
480
680
680
780
10
r"\
WAVELEMQTH tnml rn
0
I
640
BOO
660
720
780
WAVELENQTH tnml 660
880
780
WAVELENQTH [nml 30
-z
I
C 20
3 E
B:
10
0 680
680
7 80
WAVELENGTH [nml
Figure 5. Diffuse-reflectance (transient) absorption spectra obtained 25 ps after the photoexcitation of (A) anthracenemCNE3, (B) naphthalene/ TCNB, and (C) biphenyUCNB charge-transfer crystals. The transient absorption bands are comparable to those of the corresponding arene cation radicals.40 acceptors have comparable electron affinities. However in acetonitrile, the reduction potential of tetracyanobenzene is about 250 mV more negative than that of methyl v i ~ l o g e n .It~ ~is worth mentioning that in the case of 9,lO-dimethylanthracene, the CT absorption bands of the crystals from TCNB and MV show the same maximum (570 nm), although the MV ciystals exhibit a much broader CT absorption band that tails beyond 700 nm. As a result, the MV crystals appear gray, whereas the TCNB crystals are dark purple. 2. Diffuse-Reflectance Transient Absorption Spectra Following the Photoexcitationof Charge-TransferCrystals. Samples of the CT crystals (ground to a fine powder and then diluted with neutral alumina) were excited with a 25-ps laser
Figure 6. Diffuse-reflectance(transient) absorption spectrum obtained 25 ps after the photoexcitation of CT crystals from 9,lO-dimethylanthracene and methyl viologen. The transient spectrum is the composite of the absorption bands of reduced methyl viologen4' and the dimethylanthracene cation radical.4o pulse, and the transient absorption spectra were detected in the diffuse-reflectance mode. Immediately upon the photoexcitation of the anthracene/TCNB CT crystals at 532 nm, a transient absorption spectrum with an absorption maximum at 741 nm was obtained (see Figure 5A). This transient spectrum bore an unmistakable resemblance to the absorption spectrum of the anthracene cation radical that was previously generated in solution,39low-temperature matrix,'"' and zeolite.33 Similarly, the photoexcitation of naphthalene and biphenyl CT crystals at 355 nm yielded transient spectra that corresponded to those of the respective arene cation radicals (see Figure 5B and C). As shown in Table 3, the maxima of the transient absorption bands were, in some cases, somewhat red-shifted in the solid-state when compared to the corresponding spectra in dichloromethane solution. The absorption band centered at 470 nm (corresponding to the spectrum of the anion radical of TCNB40)could not be detected in the crystalline samples owing to experimental limitations. Thus, in order to avoid scattered light in the case of 532-nm photoexcitation, a cutoff filter was placed in front of the probing fiber optics, which precluded detection of any transient absorption below 550 nm. In the case of 355-nm photoexcitation, strong charge-transfer emissions severely interfered with the detection of transient absorption signals in the 400-500-nm region. Laser-flash photolysis of the methyl viologen CT crystals yielded transient spectra consisting of composite absorptions that corresponded to the superposition of the spectra of the cation radical of the donor and the reduced methyl viologen cation radical (MV'+). For example, the transient spectrum shown in Figure 6 consists of a pair of absorption maxima centered at 600 and 720 nm for the reduced methyl viologen4' and the oxidized dimethylanthracene cation radical,"'' respectively.
17582 J. Phys. Chem., Vol. 99, No. 49, 1995
Hubig and Kochi
30
c
c
< e
e-
-e '
10 0
a< o bI . -100
"
100
'
'
'
300
.
SO0
700
'
I
a00
1100
TINE Ipal
Figure 7. First-order decay of the arene cation radical observed at 7 10 nm upon the photoexcitation of 9-methylanthracenelTCNBchargetransfer crystals.
A
TABLE 4: Kinetic Data for Arene Charge-Transfer Crystals with Tetracyanobenzene and Methyl Violoeen arene donor ECT'(eV) ~ D E C(s-') Rb BET' (s-') (A) 1:1 Arene Complex with TCNB 3.22 8.8 x lo8 0.30 6.2 x lo8 biphenyl naphthalene 3.03 3.0 x lo9 0.26 2.2 x lo9 phenanthrene 2.95 2.1 x 109 0.50 1.1 x 109 durene 3.10 1.6 x lo9 0.31 1.1 x lo9 4-methoxybiphenyl 2.95 4.7 x 109 0.56 2.1 x 109 2,6-dimethylnaphthalene 2.89 5.7 x lo9 0.25 4.3 x lo9 2.6-dimethoxynaphthalene 2.61 5.0 x lo9 0.18 4.1 x lo9 anthracene 2.43 5.9 x 109 0.21 4.7 x 109 9-methylanthracene 2.34 5.8 x lo9 0.25 4.4 x lo9 9,lO-dimethylanthracene 2.18 1.4 x 1O1O 0.20 1.1 x 1Olo (B) 1:l Arene Complex with MV 2,6-dimethylnaphthalene 2.99 6.9 x lo9 0.10 6.2 x lo9 2,6-dimethoxynaphthalene 2.72 8.8 x lo9 0.10 7.9 x lo9 9-methylanthracene 2.30 7.5 x 109 0.23 5.7 x 109 9,lO-dimethylanthracene 2.18 2.9 x 1O'O 0.22 2.3 x 1OIo Charge-transfer energy: ECT(eV) = hc/lm with the CT maximum taken from Tables 1 and 2. Relative residual absorbance: R = Abs(4 ns)/Abs(25 ps). Back-electron-transferrate constants calculated using eq 3.
'
\lo
23
'
21 2.00
e
I 2.40
2.80
E,
3.20
[.VI
Figure 8. Linear correlation of the back-electron-transfer rate constants, In kBET, and the charge-transfer transition energies, Em, in TCNB (0) and MV (A) charge-transfer crystals with slopes of 1.1 and 1.7, respectively. (The numbering of the data points corresponds to the arenes identified in Table 1.)
3. Picosecond Decay Kinetics in Charge-Transfer Crystals and in Solution. The diffuse-reflectance transient absorption spectra decayed rapidly on the picosecond and early nanosecond time scales to a residual spectrum that featured essentially the same absorption maxima as the earlier spectra and remained unchanged over several nanoseconds. Accordingly, the kinetic traces (monitored at the absorption maxima of the transients) were fitted as a monoexponential decay to a raised baseline (see Figure 7). The first-order decay rate constants (~DEC)for the TCNB and MV charge-transfer crystals are listed in Table 4. Since all CT crystal samples were examined in diluted form with alumina, control experiments were also carried out to identify any spurious effects of the diluent on either the transient spectra or the transient lifetimes. Different types of alumina (various mesh), silica, barium sulfate, and magnesium oxide were used and compared with pure powdered CT crystals. No effect of the diluent on either the transient spectra or the transient lifetime was observed. Also listed in Table 4 are the relative yields (R) of the residual absorbance as compared to the initial (transient) absorbance. According to Mulliken theory,' charge-transfer energies may be used to gauge the relative donor strength of arenes in a series of related CT complexes with the same acceptor moiety. Accordingly, the charge-transfer energies, ECT = hc/&, as calculated from the CT absorption maxima reported in Tables 1 and 2, are listed in column 2 of Table 4. Inspection of the data shows that the decay rate constants ( ~ D E C ) within a particular series of charge-transfer crystals generally decreased
with increasing CT energies. However, for a particular arene donor, the decay rate constant for the MV crystal was significantly larger than that obtained from the corresponding TCNB crystal, despite CT energies which remained about the same (vide supra). Transient spectra which did not decay completely to the baseline indicated that longer-lived species were also formed. Accordingly, we employed the following kinetics model to extract the back-electron-transfer rate constants (see Discussion section): The first-order decay to a raised baseline was, as a first approximation, attributed to a pair of competing processes, (a) back-electron transfer BET) within a local ion pair to the ground-state CT crystal and (b) the formation of a longer-lived charge-separated state ( ~ s E P ) , as depicted in eq 2:
[D, A]
BET
[D'+,A'-] local ion pair
kSEP
+
[Do+ A'-] (2) longer-lived charge-separated state
On the basis of eq 2, the back-electron-transfer rate constant BET in Table 4) was computed from the overall decay rate constant (~DEC)and the relative yield (R) of the longer-lived charge-separated state according to eq 3:17
(3) As presented in Table 5 for TCNE? charge-transfer complexes, the back-electron-transfer rate constants measured in dichloromethane solution were slightly larger than those measured in the corresponding CT crystals. Furthermore, the values of BET were about 1 order of magnitude larger in acetonitrile as compared to the rate constants determined in the corresponding CT crystals. The correlation of the back-electron-transfer rate constant BET with the charge-transfer energy (EcT) in TCNB crystals is illustrated in Figure 9. For comparison, the same correlations of the back-electron-transfer rate constants in acetonitrile and dichloromethane solutions are also included. Figure 8 shows that there was also a significant increase in backelectron-transfer rate constants in MV crystals relative to those measured in the corresponding TCNB crystals. Thus, in crystals with similar charge-transfer energies ECT, those derived from MV crystals consistently exhibited back-electron-transfer rate
J. Phys. Chem., Vol. 99, No. 49, 1995 17583
Photoinduced Electron Transfer in CT Crystals
TABLE 5: Back-Electron-TransferRate Constants for TetracyanobenzendArene CT Complexes in the Crystal and in Solution BET rate constanto BET, s-I) arene donor crystal CHZC12 CHsCN biphenyl 6.2 x lo8 2.0 x 108 5.6 x 109 2.2 x 109 7.4 x 1 0 9 6 naphthalene 4.5 x 1 0 9 6 1.1 x 109 phenanthrene 1.1 x 109 2.0 109 2.0 x i o 1 0 b durene 4.3 x 109 2.1 x 109 1.4 x 1010 2,6-dimethylnaphthalene 2,6-dimethoxynaphthalene 4.1 x 109 > 4 x 10'0 >4 x 10'0 anthracene 4.7 x 109 7.9 109 >4 l o l o b 4.4 109 1.2 x 10'0 24 x 10'0 9methylanthracene 1.1 x 1010 1.5 x 1O'O >4 x loio 9,lO-dimethylanthracene Back-electron-transfer rate constants following the CT irradiation of the TCNB/arene complex (calculated using eq 3). Reference 16. (I
26 I
I
c
c
X
9
1
to the stacking axis confirms the charge-transfer character of the electronic transitions. Although Mulliken' s electron-transfer model was developed for isolated CT complexes in solution," it may also be applied to CT crystals if the CT interaction in the crystal is considered to be rather localized. Thus in the first approximation, only the strong interaction between two nearest neighbors is considered, and additional weak interactions over longer distances are negle~ted.4~ The validity of such an "oriented-gas'' model' to predict CT band positions is shown in our study by the linear Mulliken plot in Figure 2 with a slope close to unity for TCNB/ arene crystals. Linear Mulliken plots are also obtained for MV/ arene crystals. Furthermore, the plots of the CT energies of crystals versus CT energies of the corresponding complexes in solution (dichloromethane) and the plots of the CT energies of TCNB crystals versus the CT energies of MV crystals are linear with unit slopes (see Figures 3 and 4). In solution, the position of the CT absorption maximum (hvm) is controlled not only by the ionization potentials and electron affinities of electron donors and acceptors, respectively, but also by the solvation energy that is included in the constant term in eq 4.11-44Solvation is manifested in Table 1 by the change from dichloromethane to acetonitrile, which causes a significant blue-shift of the CT absorption maximum. In energy units, the average blue-shift is calculated to be 200 f 40 meV. Such a solvent effect is included in the Marcus equation for the CT absorption energy asa
1 l9
I
2.00
0
3.00
2.60
E,,
5.60
IOVl
Figure 9. Linear correlations of In BET and ECT for areneRCNB charge-transfer complexes in acetonitrile (A),dichloromethane (0),and CT crystals (0). The linear fits (dashed lines) for the acetonitrile, dichloromethane, and CT crystal data yield slopes of 2.1, 2.3, and 2.1, respectively. (The numbering of the data points corresponds to the arenes identified in Table 1.)
constants that were larger by a factor of 2 than the BET values observed in TCNB crystals.
where I , is the energy term for first-shell solvation of the ionpair state, Ii is the vibrational reorganization energy of the ionpair state, A0 is the solvent reorganization energy excluding firstshell solvent molecules, and AGOis the free energy term for the transitions from the ground-state to the excited ion-pair state. The solvent reorganization energy terms 2.0 and I , in eq 5 are most likely to be affected by solvent changes. The value of I O can be calculated from the solvent-continuum model?
Discussion The formation of charge-transfer complexes in solution is usually accompanied by the spontaneous development of a new color. In other words, the absorption spectrum of a CT complex in solution does not represent merely the sum of the absorption spectra of the components but exhibits one or several additional absorption bands. Mulliken proposed that the new absorption bands are due to charge-transfer transitions; Le., light absorption causes an electron transfer from the donor to the acceptor moiety of the complex, and the excited electronic state may be described as a charge-separated or ion-pair state." Thus, the spectral position of the CT absorption band depends on the ionization potential (IP) of the donor and the electron affinity (EA) of the acceptor as given by
ECT = hc/Ac, = IP - EA -t- const
(4)
Mixed crystals that contain n-donor and n-acceptor molecules also exhibit intense colors that are not common to the individual components. The additional absorption bands have been ascribed to charge-transfer transitions.' For best overlap of the n-orbitals, the electron donors (D) and acceptors (A) in CT crystals are usually arranged in infinite molecular stacks (e.g. in alternate heterosoric stacks ... D A D A ...), as illustrated in Chart l.21-25The fact that the new absorption bands are polarized perpendicular to the plane of the molecules and parallel
where r+ and r- are the radii of the donor cation and the acceptor anion, respectively, d is the center-to-center distance between the ion-pair partners, n is the refractive index, and E is the static dielectric constant of the solvent. By taking n = 1.344145and E = 37.546 for acetonitrile, n = 1.424145and E = 8.946 for dichloromethane, and d = 3.5 A, we calculate the difference A& in the solvent reorganization energy between acetonitrile and dichloromethane'to be 600 meV. This value is obviously critically affected by the assumed distance between donor and acceptor moiety (e.g. with d = 7 A the value of Mo is reduced to 300 meV). The discrepancy between the experimental blueshift of the CT absorption maxima of.200 meV (vide supra) and the calculated MOvalue of 600 meV (for d = 3.5 A) is most likely attributed to the neglected solvent dependence of 11, and d . Solvation according to eqs 5 and 6 predicts the charge-transfer absorption maxima to be strongly affected by the polarity of the surrounding medium. In this view, the red-shift in the CT absorption maxima in proceeding from acetonitrile to dichloromethane and to CT crystals (see Tables 1 and 2) indicates a decreasing polarity in the immediate charge-transfer environment. In the case of TCNB/arene crystals, the aromatic neighbors of a local CT exciton can be considered rather nonpolar, which explains the red-shift of the CT absorption band.
17584 J. Phys. Chem., Vol. 99, No. 49, 1995
Hubig and Kochi
In the case of CT crystals with methyl viologen hexafluorophosphate, the magnitude of the red-shift of the CT absorption band depends on the arene donor involved (see Table 2); e.g., with naphthalene donors a smaller red-shift is observed as compared to that for CT crystals with anthracene donors. Apparently in this case, the red-shift cannot satisfactorily be explained merely in terms of the polarity of the surrounding medium. The model of highly localized charge-transfer exciton states being populated in CT crystals upon photoexcitation of the CT absorption bands may be scrutinized further by analyzing the picosecond transient absorption spectra obtained upon laser excitation. In fact, for all CT crystals examined in this study, the transient spectra of the CT exciton states are strongly reminiscent of the absorption spectra of the corresponding arene cation radicals (see Figure 5 and Table 3). In the case of the MV crystals, the direct observation of the reduced acceptor MV'+ (see Figure 6) confirms the ion-pair character of the exciton state. Furthermore, the lifetimes of the CT excitons generated in the crystals are similar to those of the corresponding contact ion pairs in dichloromethane solution. According to Asahi and Mataga4' the contact ion-pair lifetime in solution is related to the back-electron-transfer rate constant, which depends on the electron-transfer driving force AGp according to the following linear relationship:
(7) In a recent study,I7 it has been demonstrated that the p values obtained from the linear plots of In RBET versus AGp are independent of the solvent (e.g. acetonitrile, ethyl acetate, or acetone) for CT complexes with various arene donors and pyromellitic dianhydride and phthalic anhydride as acceptors. An average value of p = 2.8 eV-I was reported. The driving force AGIPwas calculated according to the following equation:
-AGIp = Eo(D/Df) - Eo(A/A-)
+ W + AGs
(8)
where Eo(D/D+)is the oxidation potential of the donor, E0(N A-) is the reduction potential of the acceptor, W = e2kR is the Coulombic work term, and AGs is the sum of all correction terms of the solvation energies for the ions calculated from the Born equation:48
A G ~= e2/2(1/r+ - 1/r-)(l/n2 - I/€)
(9)
As general as the Mataga equation (eq 7) for back-electrontransfer rate constants may be in solution, it is inapplicable to the solid state, since the definition of the electron-transfer driving forces in eq 8 cannot be calculated, owing to oxidation and reduction potentials as well as Coulombic work terms which are unknown or not well-defined. Therefore, let us consider the charge-transfer energy ECT = hd1c.r as an alternative measure for the electron-transfer driving force, since it can be determined experimentally in the crystal (as well as in solution). In accord with eq 7, the correlation of the back-electron-transfer rate constants with the CT transition energy should then follow a modified linear relationship, i.e. In kBET= a' - ,&ECT Indeed, Figure 8 shows that the back-electron-transfer rate constants (RBET) for TCNB and MV crystals do have a linear dependence on the charge-transfer transition energies. Let us now turn to the direct comparison in Figure 9 of the back-electron-transfer rate constants in TCNB charge-transfer
I'."\
\
23 -
I
X
1 21
-
;'\ 1s 1.70
2.20
2.70
E,
3.20
I 5.70
IN1
Figure 10. Global Mataga correlation of In k e and ~ ~ECT for arenel TCNB charge-transfer complexes in acetonitrile (A),dichloromethane (O), and CT crystals (0).The acetonitrile data points in Figure 9 were arbitrarily shifted along the abscissa by -0.9 eV (see text).
crystals with those measured in solution (see Table 5).49 Although the results in Figure 9 show a widely scattered m a y of data points, a close scrutiny reveals that the plot of In kBET versus ECT is indeed linear, but it is shown as separate correlations for the acetonitrile solutions, for the dichloromethane solutions, and for the crystalline state. Thus, in contrast to Mataga's results in solutionI7 (where all data points lie on one single line), our data are best fitted on three parallel lines with essentially the same slopes of B' = 2.1, 2.3, and 2.1, respectively (as indicated by the dashed lines). In order to accommodate all data on a single line, let us consider the selective displacement of some of the data along the abscissa (Le. by the variation of EcT). Whereas the shift in the fitted line of the dichloromethane correlation to that of the solid-state data is within the variance of the scattered points, a significant shift is required for the acetonitrile data. The latter is not surprising if one considers the various solvent-dependent parameters that affect ECT,as presented in eq 5.44 (Note that the solvation terms are more important in acetonitrile relative to less polar solvents such as dichloromethane.) The comparison of AG1p (as defined in eq 8)17 and ECT(as defined in eq 5)44 as two measures of the electron-transfer driving force indicates that the primary difference between AGPand ECTis given by the sum of LO and Ai. If we consider the solvent reorganization energy AIto be the predominant parameter, the ECT values in acetonitrile and dichloromethane can be adjusted by taking into account the difference Mo in the solvent reorganization energies of acetonitrile and dichloromethane (vide supra). Thus, when the acetonitrile data points in Figure 9 are merely shifted by the amount of Mo = -0.6 eV (vide supra) along the ECTaxis, a more narrow comdor for the combined data in acetonitrile, dichloromethane, and crystals is obtained. However, for an optimum fit of all data points onto a single line, the acetonitrile data must be shifted by -0.9 eV along the abscissa (see Figure 10) and a slope of 2.9 eV-' was computed. The discrepancy between this empirical shift of -0.9 eV and the calculated Mo value of -0.6 eV indicates that, besides the solvent dependence of 10,solvent-dependent corrections for other energy terms in eqs 5 and 8 (such as the first-shell solvation energy ,Ip) need also to be taken explicitly into account. When the back-electron-transfer rate constants in the TCNB and MV crystals are compared, a significant difference in rate is noted (see Figure 8 and Table 4). In solution, electron-transfer rates are affected by the ionic strength and the polarity of the environment. In a similar context, we expect ionic species, such as MV2+ and PF6- that are surrounding the local CT exciton,
Photoinduced Electron Transfer in CT Crystals to promote back-electron transfer within the CT exciton-in a manner akin to the ionic-strength effect defined for solutions. In conclusion, our results indicate that the linear correlations commonly observed between back-electron-transfer rate constants and electron-transfer driving forces in solution can also be generalized to charge-transfer crystals in which electrontransfer processes occur in the absence of solvent under conditions of high constraint and fixed distance between the electron-transfer partners. Interestingly, the /3’ values, i.e. the slopes of In BET versus driving force, obtained in this study are quite similar to those obtained in solution” and in porous glasses.50 Furthermore, the linear Mulliken plots, together with the cation radical-like transient absorption spectra, the fast firstorder decay rates, and the linear dependence of In k e on~ the ~ driving force, all indicate that the charge-transfer excitons may be considered, to first approximation, as highly localized excited ion-pair states.
J. Phys. Chem., Vol. 99, No. 49, 1995 17585 (19) Burget, D.; Jacques, P.; Vauthey, E.; Suppan, P.; Haselbach, E. J . Chem. Soc., Faraday Trans. 1994, 90, 2481. (20) Kochi, J. K. Acta Chem. Scand. 1990, 44, 409. (21) Lefebvre. J.; Miniewicz, A.; Kowal, R. Acta Crysfallogr. 1989, C45, 1372. (22) Kumakura, S.; Iwasaki, F.; Saito, Y. Bull. Chem. SOC.Jpn. 1967, 40, 1826. (23) Stezowski, J. J. J . Chem. Phys. 1980, 73, 538. (24) Pasimeni, L.; Guella, G.; Corvaja, C.; Clemente, D. A.; Vicentini, M. Mol. Cryst. Liq. Cryst. 1983, 91, 25. (25) Yoon, K. B.; Kochi, J. K. J . Phys. Chem. 1991, 95, 3780. (26) (a) Kozankiewicz, B.; Prochorow, J. Chem. Phys. 1989, 135, 307. (b) Kozankiewicz, B.; Prochorow, J.; Corvaja, C.; Maniero, A. L. J . Lumin. 1994, 62, 179. (27) Betz, E.; Port, H.; Schrof, W.; Wolf, H. C. Chem. Phys. 1988,128, 73. (28) Fukazawa, N.; Fukumura, H.; Masuhara, H.; Prochorow, J. Chem. Phys. Lett. 1994, 220, 461. (29) Kuwata-Gonokami, M.; Peyghambarian, N.; Meissner, K.; Fluegel, B.; Sato, Y.; Ema, K.; Shimano, R.; Mazumdar, S.; Guo, F.; Tokihiro, T.; Ezaki, H.; Hanamura, E. Nature 1994, 367, 47. (30) Tamai, N.; Porter, C. F.; Masuhara, H. Chem. Phys. Lett. 1993, 211 364. (31) For the microsecond time-resolved diffuse-reflectance technique, see: Wilkinson, F.; Willsher, C. J. Tetrahedron 1987, 43, 1197 and references therein. (32) For the picosecond time-resolved diffuse-reflectance technique, see: (a) Kelly, G. P.; Leicester, P. A.; Wilkinson, F.; Worrall, D. R.; Ferreira, L. F. V.; Chittock, R.; Toner, W. Spectrochim. Acta 1990, 46A, 975. (b) Ikeda, N.; Imagi, K.; Masuhara, H.; Nakashima, N.; Yoshikara, K. Chem. Phys. Lett. 1987, 140, 281. (33) Yoon, K. B.; Hubig, S. M.; Kochi, J. K. J . Phys. Chem. 1994, 98, 3865. (34) (a) Iwata, S.; Tanaka, J.; Nagakura, S. J . Am. Chem. SOC. 1966, 88, 894. (b) Jones, G.; Malba, V. Chem. Phys. Lett. 1985, 119, 105 and references therein. (35) Russell, J. H.; Wallwork, S. C. Acta Crystallogr. 1972,828, 1527. (36) Kisch, H.; Diimler, W.; Niisslein, F.; Zenn, I.; Chiorboli, C.; Scandola, F.; Albrecht, W.; Meier, H. Z. Phys. Chem. 1991, 170, 117. (37) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon: Oxford, 1988. (38) For the reduction potential of TCNB, see ref 12. For the reduction potential of MV2+,see: Bockman, T. M.; Kochi, J. K. J . Org. Chem. 1990, 55, 4127. (39) Masnovi, J. M.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. J . Phys. Chem. 1985, 89, 5387. (40) Shida, T. Electronic Spectra of Radical Ions; Elsevier: Amsterdam, 1988. (41) Bockman, T. M.; Kochi, J. K. J . Org. Chem. 1990, 55, 4127. (42) Soos, Z. G.; Klein, D. J. In Molecular Association; Foster, R., Ed.; Academic Press: London, 1976; Vol. 1, p 1. (43) Mikhelashvili, M. S.; Michaeli, A. M. J . Phys. Chem. 1994, 98, 81 14. (44) Marcus, R. A. J . Phys. Chem. 1989, 93, 3078. (45) Murov, S. L., Carmichael, I., Hug, G. L., Eds. Handbook of Photochemistry; Marcel Dekker: New York, 1993. (46) Budavari, S., Ed. The Merck Index; Merck: Rahway, 1989. (47) Asahi, T.; Mataga, N. J . Phys. Chem. 1989:93, 6575. (48) Interestingly, Mataga uses values of d = 6 A and r+ = r- = 3 A for his calculation based on spectral and theoretical evidence for a structural change going from the plane-parallel sandwich-like geometry of the chargetransfer complex with d * 3.5 A to a more loose geometry of the contact ion pair with d * 6 (49) For MV charge-transfer complexes a comparison between kBET values in solution and in the solid state was not possible because backelectron transfer in acetonitrile solution occurred within the time resolution (25 ps) of our laser flash photolysis apparatus. (50) (a) Miyasaka, H.; Kotani, S.; Itaya, A. J . Phys. Chem. 1995, 99, 5757. (b) Braun, M.; Gafney, H. D. J . Phys. Chem. 1994, 98, 8108. (51) (a) Takahashi, Y.; Sankararaman, S.; Kochi, J. K. J . Am. Chem. Soc. 1989, I 1 1 2954. (b) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989. ~
Acknowledgment. We thank J. Korp for the calculation of the X-ray structures and the National Science Foundation, Robert A. Welch Foundation, and the Texas Advanced Research Program for financial support. References and Notes (1) Wright, J. D. Molecular Crystals; Cambridge University Press: New York, 1987; pp 131 ff. (2) Miller, J. S.; Epstein, A. J. Angew. Chem., In?. Ed. Engl. 1994, 33, 385. (3) Kanis, D. R.; Ratner, M. A.; Marks, J. J. Chem. Rev. 1994, 94, 195. (4) Law, K.-Y. Chem. Rev. 1993, 93, 449. ( 5 ) Carter, P. W.; DiMagno, S. G.; Porter, J. D.; Streitwieser, A. J . Phys. Chem. 1993, 97, 1085. (6) For further examples, see: (a) Chemla, D. S.,Zyss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: Orlando, FL, 1987; Vols. 1 and 2. (b) Williams, D. J., Ed. Nonlinear Optical Properties of Organic and Polymeric Materials; ACS Symposium Series 233; American Chemical Society: Washington, D.C., 1983. (7) Ramamurthy, V. Photochemistry in Organized and Constrained Media; VCH Publishers: New York, 1991. (8) Kalayanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (9) (a) Foster, R., Ed. Molecular Association; Academic Press: London, 1975. (b) Foster, R. Organic Charge Transfer Complexes; Academic Press: New York, 1969. (10) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970. (1 1) (a) Mulliken, R. S. J . Am. Chem. SOC. 1950, 72,600. (b) Mulliken, R. S. J . Am. Chem. SOC. 1952, 74,811. (c) Mulliken, R. S. J . Phys. Chem. 1952, 56, 801. (d) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New York, 1969. (12) (a) Miyasaka, H.; Ojima, S.; Mataga, N. J . Phys. Chem. 1989, 93, 3380. (b) Ojima, S.; Miyasaka, H.; Mataga, N. J . Phys. Chem. 1990, 94, 4147. (c) Ojima, S.; Miyasaka, H.; Mataga, N. J . Phys. Chem. 1990, 94, 5834. (13) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Phys. Chem. 1991, 95, 2068. (14) Ebbesen. T. W.: Manrine. L. E.: Peters. K. S. J . Am. Chem. SOC. 1984,106, 7400. (15) Asahi. T.; Matapa. N.; Takahashi, Y.; Mivashi, T. Chem. Phys. Lett. 1990, 171, 309. (16) Ojima, S.; Miyaska, H.; Mataga, N. J . Phys. Chem. 1990,94,7534. (17) Asahi, T.; Ohkohchi, M.; Mataga, N. J . Phys. Chem. 1993, 97, 13132. (18) (a) Gould, I. R.; Young, R. H.; Mueller, L. J.; Farid, S. J . Am. Chem. Soc. 1994, 116, 8176. (b) Chung, W.-S.; Turro, N. J.; Gould, I. R.; Farid, S . J . Phys. Chem. 1991,95, 7752. 1
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