J. Phys. Chem. 1993,97, 13099-13106
13099
Photoinduced Giant Charge-Separated States in a New Series of Completely Rigid Covalently Linked Triadst James M. Lawson and Michael N. Paddon-Row' School of Chemistry, University of New South Wales, P.O. Box I , Kensington, NSW 2033, Australia Wouter Schuddeboom and John M. Warmad Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft. The Netherlands Andrew H. A. Clayton and Kenneth P. Ghiggino' Photophysics Laboratory, School of Chemistry, University of Melbourne, Parkville 3052, Australia Received: July 1, 1993; In Final Form: September 18, 1993'
Preliminary fluorescence and time-resolved microwave conductivity (TRMC) measurements are reported for the novel triad, DMA[4] DMN[8] DCV, in which the three chromophores, DMA (N,N-dimethylaniline), DMN (dimethoxynaphthalene), and DCV (dicyanovinyl), are connected via bridges comprising linearly fused norbomyl and bicyclo[2.2.0]hexyl units, four and eight bonds in length. The triad, DMA[4]DMN[8]DCV, exists in two noninterconvertible syn and anti diastereomeric forms, the former having a U-shaped geometry and the latter having an S-shaped geometry. Samples of the two stereoisomers, labeled Hand L, were obtained, but the actual stereochemistry, syn or anti, of each isomer is presently unknown. Time-resolved fluorescence studies in benzene reveal that photoinduced electron transfer (ET) in the model dyad, [2]DMN[8]DCV, to generate the chargeseparated (CS) species, [2]DMN+[8]DCV-, occurs rapidly (3.3 X 1Olo s-l) and with near unit efficiency. Similar rates of quenching were found for triads H- and LDMA[4]DMN[8]DCV. TRMC studies on the flash photolysis of H-DMA[4]DMN[8]DCV in benzene suggest formation of the giant C S state, DMA+[4]DMN[8]DCV-, with near unit efficiency. Intriguingly, the L-DMA[4]DMN[8]DCV diastereomer gave little or no TRMC signal following flash photolysis. This observation is tentatively explained in terms of a rapid benzene solvent-mediated charge-recombination process in the L-isomer which is assumed to possess the syn, or U-shaped, geometry. This geometry should encourage congregation of benzene molecules in the molecular cavity spanning the DMA and DCV chromophores.
Introduction CHART I Electron transfer (ET) continuesto remain the focus of intense research activity.'-'3 In this respect, studies on intramolecular ET in rigid, covalently linked donor-bridgeacceptor dyads are playing a major role because the attachment of donor and acceptor groups to a fairly rigid bridge enables the dependence of ET dynamics on donor-acceptor distance and orientation to be determined with minimum ambiguity! Many types of bridge have been used, ranging from modified proteins and peptide~~C~e~f~~ DMN[G]DCV \CN to hydrocarbon bridges of varying degrees of structural complexity.613 Studies on dyads possessing a saturated hydrocarbon bridge have amply demonstrated that rapid intramolecular ET can take place over interchromophore separations as great as 13 A, presumably by way of a through-bond coupling mechanism involving the bridge u and u* orbital^.'^ To date, our work has mainly focused on studying long-range intramolecular ET in dyads, such as the series DMN[n]DCV ,CN (Chart I), in which the dimethoxynaphthalene (DMN) donor and thedicyanovinyl (DCV) acceptor are attached to a completely DMN[lO]DCV KCN rigid norbornylogous bridge, n C-C bonds in length, comprising linearly fused norbornyl and bicyclo[2.2.0]hexyl units. The members of the DMN[n]DCV series were found to display extraordinarily rapid rates of thermal ET (in the corresponding anion radicals)'* and photoinduced ET." For example, the rate DMN(l2lDCV of photoinduced intramolecular ET, from locally excited DMN DMN-DCV interchromophore edge-to-edge separation in this donor to DCV acceptor, was found to be ca. lo9 s-1 for the 12molecule is ca. 13.5 A.130.15 The lifetimes, T ~for, intramolecular bond sytem, DMN[12]DCV, notwithstanding the fact that the charge recombination of the resulting charge-separated (CS) states, DMN+[n]DCV-, were found to increase quite markedly t In memory of Gerhard L. Closs, 1928-1992; master chemist. with increasing bridge length, for example, from a moderate 32 *Abstract published in Advunce ACS Abstracts, November 15, 1993. 0022-3654/93/2097-13099$04.00/0
0 1993 American Chemical Society
Lawson et al.
13100 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
CHART I1
syMMA[4]DMN[8]DCV
b N
CN CN Me2N CN
Me2N
\cN
syn-DMA[ 61DMN[8]DCV
Me2N
CN
Me2N
entCDMA[8]DMN[8]DCV
ns for the eight-bond DMN+[8]DCV- to an impressive 740 ns for the 12-bond DMN+[121DCV- (benzene solvent).13c The remarkablephotoinduced ET rates observed for the DMN[n]DCV series convincingly demonstrate the efficacy of the norbornylogous bridge for mediating rapid intramolecular ET processes over large distances. This typeof bridge therefore offers promise in the construction of molecular electronic devices, such as photovoltaic systems,lg since it possesses the double advantage of fixing the spatial arrangement of the redox centers with respect to each other and controlling the dynamics and direction of ET processes between the redox centers. However, the successful constructionof such devices must simultaneouslysatisfy the dual criteria of efficientcharge separationand longevity of the resulting CS state toward charge recombination.18 Covalently linked dyads are unsuitable for this purpose since, whereas the charge-recombination lifetime, T ~ increases , with increasing bridge length, the quantum yield for the chargeseparation step diminishes with increasing bridge 1ength.lg The series,DMN[n]DCV, is noexception to thisdilemma for, although T~ for DMN+[12]DCV- is about 25 times greater than that for DMN+[8]DCV-, the quantum yield for formation of the former CS state is only 75%, compared to near-unity for the latter state.lk,c This problem can be circumvented by using polychromophoric systems, Le.,triads, tetrads, pentads, etc., that constitutea gradient of redox centers arranged within a spatially well-defined array.lg.2*16 The principle behind this strategy is illustrated in eq 1 for the case of the covalently linked triad, DrDI-A, in which D1 is initially locally excited. D2-*Dl-A
-
D,-D,+-A-
-
D:-D,-A-
(1)
In this system, the ET process takes place in a sequence of rapid "hops" between adjacent chromophores that are spanned by a bridge that is short enough to guarantee that the hop occurs with near unit efficiency. The final result is charge separation over a sufficiently large distance that the unwanted chargerecombination step becomes acceptably slow enough. Considerable progress has been made using this approach, notably by the Wasielewskiand the Gust and Moore groups, who have studied photoinduced ET processes in a variety of triads,
tetrads, and pentads.lg& Theseelegant studieshavedemonstrated the feasibility of the polychromophore approach, namely that stepwise photoinduced ET can take place over substantialdistances with a high degree of efficiency to produce CS species with lifetimes in the microsecond regime. Encouraged by these findings, we have been exploring ways of constructing triads, 1, based on the norbornylogous bridge. These systems offer several significant advantages over other polychromophoric systems studied to date.lg.h.16 In particular, the symmetry and complete rigidity of the norbornylogous framework, combined with our ability to append a wide range of chromophore^^^ to the bridge and to alter systematicallyboth the bridge length13cJ7fand config~ration,~3~J~~ confer upon 1 the
potential for providing unprecedented insight into the factors governing the dynamics of stepwise photoinduced ET processes leading to the formation of giant CS states. Our endeavors in this area are recently bearing fruit in the form of the successful synthesis of three triads, DMA[nl]DMN[8]DCV (Chart 11) (DMA = N,N-dimethylaniline).l* In this paper we present the preliminary results of our photophysical (fluorescence)and time-resolved microwave conductivity (TRMC)13*19investigationson theshortest membersof theseries, namely, thesyn and anti stereoisomersof DMA[4]DMN[8]DCV.
The Molecules Our triad synthesis results in the formation of two rigid, noninterconverting diastereomers, syn-DMA[4]DMN[8]DCV and anti-DMA[4]DMN[8]DCV, which differ in the spatial relationship between bridges of the bicyclic units directly fused to the DMN chromophore (Chart II).m It might seem from casual inspection of the structural drawings displayed in Chart I1 that both stereoisomershave similarshapesand should therefore display similar characteristics as far as the dynamics of ET and
Completely Rigid Covalently Linked Triads
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13101 n
P syn-DMA[4]DMN[E]DCV
antCDMA[4]DMN[E]DCV
Figure 1. Force-field optimized geometries of syn- and anti-DMA[rl]DMN[8]DCV. The hydrogen atoms in all perspectives and the dimethylamino and methoxy groups in the profiles have been omitted for clarity.
charge-recombination processes are concerned. However, the force-field optimized geometries21of these stereoisomers (Figure 1) reveal important structural differences in that the syn stereoisomerhas a U-shaped profile, whereas the anti stereoisomer has an extended, or S-shaped, profile. This difference is reflected in a smaller DMA-DCV centerto-center distance in the syn isomer (14.9 A), compared to the anti isomer (19.4 A). The closer proximity of the DMA and DCV chromophores in the syn diastereomer means that the syn-DMA+[4]DMN[8]DCV- CS state will enjoy greater Coulombic stabilization than the anti-DMA+[4]DMN[8]DCV- CS state, particularly in weakly polar solvents. Consequently, the dynamics of photoinduced ET and subsequent charge-recombination processes could be quite different in the two isomers. Separation of DMA[4]DMN[8]DCV into pure syn and anti forms proved difficult, but preparative TLC enabled us to obtain a pure sample of one diastereomer, labeled H, and a sample of the other diastereomer, labeled L, containing a small amount (ca. 596, by lH NMR) of the H diastereomer. Unfortunately, we are unable to assign the stereochemical configuration, syn or anti, to the Hand L isomers of DMA[4]DMN[8]DCV because their 1H NMRspectra arevery similar (see Experimental Section) and because we have so far been unable to obtain a suitable crystal of either stereoisomer for X-ray structure determination. Appropriate model monochromophoric and dyad systems were also synthesized (Chart 111). As for the case of DMA[4]DMN[8]DCV, the synthesis of DMA[4]DMN[2] dyad gave a 5050 mixture of the syn and anti diastereomers, which was used for the photophysical studies without separation into pure stereoisomeric forms. This is reasonable since ET and electronic energy transfer (EET) processes between the DMA and DMN chromophores in DMA[4]DMN[2] should not depend on the orientational relationship between the norbornyl and bicyclo[2.2.2]octyl units. Henceforth, we shall refer to this mixture of diastereomers simply as DMA[4]DMN[2].
Experimental Section Synthesis. Full synthetic details for syn- and anti-DMA[4]DMN[8]DCV molecules and for the model systems, shown in Chart 111, will be provided elsewhere;'* herein we merely present the *HNMR spectral data. The isomers are labeled Hand L, since unambiguous stereochemical assignment, i.e., syn or anti, cannot yet be made. H-DMA[4]DMN[8]DCV: 'H NMR (500 MHz, CDCl3) 6 0.94 (s, 6H), 1.42 (d, br, J = 8.5 Hz, 2H), 1.60-1.68 (m, 5H), 1.73 (s, 2H), 1.77 (d, J = 10.0 Hz, lH), 1.85 (d, J = 10.0 Hz, lH), 1.87 (s, 2H), 1.92 (d, J = 11.7 Hz, 2H), 2.02-2.09 (m, 4H), 2.08 (s, 2H), 2.31 (s, 2H), 2.48 (d, J = 10.0 Hz, lH), 2.84 (s, 6H),2.98 (s,2H), 3.21 (s, 1H),3.23 (s,3H),3.60 (s,2H),3.94 (s, 6H), 6.37 (dd, J = 8.0,l.g Hz, lH), 6.70 (d, J = 1.9 Hz, lH), 6.97 (d, J = 8.0 Hz, lH), 7.75 (s, 2H).
LDMA[4]DMN[8]DC'V: 'H NMR (500 MHz, CDCl3) 60.94 (s, 6H), 1.45 (d br, J = 7.8 Hz, 2H), 1.60 (s, 2H), 1.64 (s, 2H), 1.66 (d, J = 6.9 Hz, lH), 1.72 (s, 2H), 1.77 (d, J = 9.4 Hz, lH), 1.86 (d, J = 10.2 Hz, lH), 1.88 (s, 2H), 1.92 (d, J = 11.8 Hz, 2H), 2.02-2.08 (m, 4H), 2.10 (s, 2H), 2.30 (s, 2H), 2.48 (d, J = 9.7 Hz, lH),2.84(~,6H),2.97(s,2H),3.21 (s, lH), 3.22 (s, 3H), 3.60 (s, 2H), 3.94 (s, 6H), 6.35 (dd, J = 8.0, 1.9 Hz, lH), 6.68 (d, J = 1.9 Hz, lH), 6.96 (d, J = 8.0 Hz, lH), 7.75 (s, 2H).
Electr~chemistry.Electrochemical measurements (cyclic voltammetry) were performed on a BAS 100 B electroanalyzer using a glassy carbon working electrode, a platinum auxiliary, and an Ag/AgCl/KCl reference electrode. The electroanalyzer was interfaced to a Bootstrap Dimension Penta 12PC. Measurements were done on 1 mM solutions of sample in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate as a supporting electrolyte, using a scan rate of 100 mV s-1 under a nitrogen atmosphere. Ferrocene/ferrocinium couple (E112 us SCE = 0.48 V) was used as an internal reference. The measured values (us SCE) are as follows: DMA[2], Eox= 0.73 V; [2]DMN[2], Ea = 1.01 V; DMN[4], Eox = 1.1 V;13' [4]DCV, E d = -1.7 V.13'. Electronic Absorption and Emission Spectra. Solutions were prepared in 1-cm path length quartz cuvettes in spectroscopically pure solvents to an absorbance of 0.1-0.2 at 290-300 nm for fluorescence measurements and to a maximum absorbance of 1.O for absorption spectra in the region of interest. Absorption spectra (0.5 nm resolution) were collected using a Hitachi Model 150-20 spectrophotometer/data processing system. Corrected emission spectra were recorded on a Hitachi Model F-4010 fluorescence spectrophotometer with a WD3 15 cutoff filter in the emission path to exclude second-order diffraction. The solutions were thoroughly degassed by repeated freezepumpthaw cycles prior to use. Time-ResolvedFluorescence Spectroscopy. Fluorescencedecay curves were collected using the picosecond laser excitation/timecorrelated single-photon counting method22 as previously described.23 Excitation pulses (4 ps fwhm, 292 nm) were provided by a cavity-dumped Spectra Physics Model 3500 picosecond dye laser system (repetition rate 4 MHz) synchronously pumped by a mode-locked Spectra Physics Model 2030 argon ion laser. The total instrument response of thedetection system was 85 ps fwhm. The fluorescence decays, recorded at two observation wavelengths, and over a range of time scales, were analyzed using iterative reconvolution routines.23 While the fluorescence decay from [2]DMN[2] was found to fit adequately to a single exponential function, the polychromophoric systems were better fit to the sum of two exponentials. On the short time range employed (1.5 ns in 5 12 channels) typical decays for the strongly quenched linked systems could be fit with a major, short (approximately 30 ps) and a minor, longer time constant. The longer lifetime component, revealed over an extended time range, was found to match closely that of the isolated DMN chromophore in [2]DMN[2]. Time-Resolved MicrowaveConductivityMeasurements. Dilute (ca. 10-4M) solutions contained within a microwave cavity cell were flash-photolyzed using single ca. 7 ns fwhm pulses of 308nm light from a Lumonics HyperEX 400 excimer laser. Any transient change occurring in the microwave conductivity, ACT(@), of the solution was monitored as a change in the microwave power reflected by thecell. The TRMC technique and themethod of data reduction have been described fully elsewhere.'!) The TRMC transients shown in Figure 7 are an average of four single laser shots. No degradation in signal size or decay kinetics was observed during the course of the measurements. Results and Discussion Electronic Absorption Spectra. The absorption spectrum of the DCV chromophore has been reported elsewhere and features
Lawson et al.
13102 The Journal of Physical Chemistry, Vol. 97,No. 50, 1993
CHART 111
CN
[2]DMN[B]DCV
-e
0.5
"
"
"
"
"
"
"
"
Me2N
\
Me0
"
Me2NA
\cN
Me2N
antLDMA[4]DMN[2]
OMe
sy*DMA[4]DMN[2] a
"
-
( acetonitrile ,E47)
u syn
A'-N-v
If
.-
I\
%i F
c
I
\
3.
A+-N'-V
A-N'-v
d'
A+-N-V '
0 2-
A-N+-V.
A*-N-V'
1 -
250
270
290 310 Wavelength (nm)
330
350
"
Figure 2. Electronic absorption spectra in acetonitrile of (a) [ZIDMN[2] and (b) DMA[2].
a singleabsorption band at 228 nm.13c The electronic absorption spectraof [2]DMN[2] and DMA[2] in acetonitrile aredisplayed in Figure 2. The absorption spectrum of the [2]DMN[2] donor (Figure 2a) is similar to that reported for a DMN[4] analogue13cand exhibits a shoulder in the 325-328-nm region. The DMN SO SI transition energy is approximately 3.8 eV. The DMA chromophore (Figure 2b) displays absorption bands in the region of 306 and 250-255 nm. The So S1transition energy for the DMA chromophore is slightly higher than that for the DMN chromophore, with an energy of ca. 4.0-4.1 eV. It is apparent from the absorption spectra of [2]DMN[2] and DMA[2] that excitation of the linked DMA[4]DMN[2] and DMA[4]DMN[8]DCV systems will lead to initial excitation of both DMA and DMN chromophores. However, a careful comparison of absorption and fluorescence excitation spectra of DMA [4]DMN [21 in n-hexaneshows that unavoidable excitation of DMA at 308 nm leads to rapid population of locally excited DMN uia singlet electronic energy transfer (EET). Energetics of the Electron-TransferProcesses. The energetics of ET processes are most conveniently discussed using the Weller treatment.Z4 The Gibb's free energy (AG) for photoinduced ET, as a function of solvent dielectric constant (e) and interchromophore center-to-center distance (R,) is given by
-
AG
A-N-V (benzene, &=2.3) o syn
5r
-.
E,,(D) - E,d(A) - E(S1) (14.45/riOn)(1/c- 1/37) - 14.45/(cRc) (2)
where E,,(D) is the oxidation potential of the donor and E,d(A) is the reduction potential of the acceptor (both measured in acetonitrile), E(S1 ) is the transition energy of the locally excited state, and ri,, refers to the average radius of the donor and acceptor ions. Previous studies on the analogous DMN[n]DCV systems have shown that eq 2 gives a reasonable approximate description of the solvent and distance dependence of AG.130 For the present systems we have calculated the free energies for the various CS states, using theelectrochemical data (see Experimental Section), spectral data, and R, values (obtained from the force-field optimized geometries) for the case of a polar solvent, such as
A-N-V
Figure 3. Calculated free energy levels (relativetoground state) of locally excited and CS states present in DMA[4]DMN(8]DCV triads, abbreviated by A-N-V (A = DMA, N = DMN, and V = DCV). The energies were calculatedusing eq 2 for the cases of (a) acetonitrile and (b) benzene. The locally excited states are denoted by an asterisk, and the syn and anri isomers are indicated by the clear and shaded bars, respectively.
acetonitrile (e = 37), and for a nonpolar solvent, such as benzene (e = 2.3).*5 The value for 6," was taken to be 4.5 A which has previously been shown to be satisfactory for these types of chromophores.13c The results for acetonitrile and benzene are displayed in Figure 3a and b, respectively. From Figure 3a, it can be seen that, in polar solvents, photoinduced ET is exergonic for both model bichromophoric systems, DMA[4]DMN[2] and [2]DMN[8]DCV, by 4 . 4 and -1.1 eV, respectively. For both syn- and anti-DMA[4]DMN[8]DCV triads, the calculated ordering of levels indicates that eventual population of the giant CS state, DMA+[4]DMN[8]DCV-, would be thermodynamically feasible. In nonpolar solvents (Figure 3b), the energies of the various CS states are raised relative to those in acetonitrile. This results in the DMA+[4]DMN-[8]DCV CS state lying within 0.1 eV of both the locally excited DMN* and DMA* states. In addition, the syn- and anti-DMA+[4]DMN[8]DCV- CS states and the DMA[4]DMN+[8]DCV- CS state are calculated to have comparable energies in nonpolar solvents. The syn- and antiDMA+[4]DMN[I]DCV- CS states lie 0.16 and 0.08 eV below the DMA[4]DMN+[8]DCV- CS state, respectively.
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13103
Completely Rigid Covalently Linked Triads Electronic Energy Transfer
Primary Electron Transfer
Secondary Electron Transfer
60 A-N-V
I'
k l (1)
v
0 0 A-N-V
kf(l,
A-N-V
1'
k C r (3,
Figure4 Reaction schemedepicting the possible fate of initially excited DMA[4]DMN[8]DCV triads, abbreviatedby A-N-V (- = DMA, N = DMN, and V = DCV). Subscripts refer to fluorescence(kr(l),kf(z),etc.), electronic energy transfer (kwt),charge separation (kcl(l),ka(2), etc.), and charge recombination (km(l),kcr(2),etc.). Reverse reaction rata are denoted by k4(1), k4(2), etc. ~
100 I
I
TABLE I: Fluorescence Lifetimes (350-nm Region), T, and Calculated Rates of Electron Transfer, in Benzene DMA[4]DMN[2] [2]DMN[8]DCV L-and H-triads [2]DMN[2]
~(ns) 7.3' 20 0300
340
380
420
460
500
Wavelength (nm)
T
(ns) 4.5'~~
kcs
kcs
T
(107s-1)
(ps)
b
3oc
7
(107~4) (ps) 33od
kcs (io's-1)
3oc 3 3 W L ' 3oc 3 3 W H
a Uncertainty,hO.1 ns. TRMC measurements suggest that this may be emission from a highly polar state, rather than from locally excited [2]DMN*[2]. Uncertainty, h5 ps. Uncertainty, h0.5 X 1O1Os-l. a G triad contaminated with
