6430
J. Phys. Chem. 1994,98,6430-6435
Photoinduced Intramolecular Electron Transfer in pPolyphenylamines and 1- ( p(N,N-Dialky1amino)-p polyphenyl)naphthalenes Michael J. Foley and Lawrence A. Singer' Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744 Received: March 4, 1994;In Final Form: April 28, 1994'
Photoinduced intramolecular electron transfer in two series of aromatic amines of the type aryl-(C6H,),,-NR2, where aryl is phenyl and 1-naphthyl and n = 1-3, has been studied. From the solvatochromic behavior of the fluorescences from these compounds, excited-state dipole moments ranging from 10.9 to 29.9 D have been measured, suggesting that the emissions occur from charge-transfer states, except in 4-(N,N-diethylamino)biphenyl (aryl = phenyl; n = 1). The large quantum yields and short fluorescence lifetimes indicate efficient electronic communication between the donor and acceptor (ket> lo8s-'). Good agreement was found between the observed and calculated fluorescence energies in polar solvents using a simple electrostatic model. The results suggest that the phenyl rings in the extended systems act as part of the donor or acceptor moieties in electron transfer and are not spacer (or insulating) groups.
Introduction
1
An elucidation of the factors that influence the rate and energetics of photoinduced electron transfer is important for understanding fundamental biological processes such as photosynthesis. Model compounds which contain varying donoracceptor pairs held at a fixed distance have been used to study the effect on the thermodynamic driving force on the rate of electrontransfer.12 Oevering and co-w0rkers3*~ found fast electron transfer rates (108-10"J s-l) even at donor-acceptor separation distances up to 13.3 A in compounds which contain a 1,4dimethoxynaphthalene donor and a 1,l -dicyancethylene acceptor separated by rigid, nonconjugated bridges. Through-bond interaction involving a / u coupling between the bridge and the donor-acceptor pair was proposed to explain the high electrontransfer rates. Others2.5-8 have employed bis(porphyrin) and porphyrinquinone-typecompounds to study free energy, distance, and orientationeffects on biological electron transfer. In addition to the relevance of these studies for gaining a better understanding of photobiology, they provide molecular models for systems that may be useful in long-distance signal processing and t r a n ~ f e r . ~ In our work, we have prepared and studied the photophysics of two series of linear donor-acceptor systems: the p-polyphenylamines: 4-(N,N-diethylamino)biphenyl (l),4-(N,N-diethy1amino)-p-terphenyl (2), and 4-(N,N-diethylamino)-pquaterphenyl(3) and the naphthyl analogs, 1-(p-(N,N-diethylani1ino))naphthalene (4), 1-(p-(N,N-diethy1amino)biphenylyl)naphthalene (5), and 1-(p-(N,N-dibuty1amino)-p-terphenyly1)naphthalene (6). The structures are shown in Figure 1. In these compounds, u,u interactions are possible between the polyphenylene and the D and A groups in contrast to other recently studied systems6.7 where the phenylene group is orthogonal to the rest of the structure because of structural constraints. This paper describesa study of these compounds where phenyl rings have been introduced between the nominal donor-acceptor groups of previously studied directly-linked D-A compounds where D is N,N-diethylanilinoand A is aryl.10 The influence of this structural change on the properties and character of the charge-transfer state of these multichromophoriccompounds is examined. Experimental Section Solvents. In all spectroscopic measurements,the solvents were of spectrophotometricgrade and were used as received. For some
* Abstract published in Aduance ACS Abstracts, June 1, 1994.
5
Figure 1. Molecular structures of compounds 1-6.
studies, acetonitrile was dried over anhydrous magnesium sulfate and freshly distilled under nitrogen prior to use. Spectroscopic Measurements. The ultraviolet-visible absorption spectra were recorded on a double-beam Shimadzu UV-260 spectrophotometer. Steady-state fluorescence spectra were recorded on nondegassed samples on an American Instrument Co. (AMINCO) spectrofluorimeter using a 150-W Xe or XeHg lamp as the excitation source and a Hamamatsu R446 photomultiplier tube. Nocorrection was made for the instrument throughput and response. The fluorescence quantum yields, r$f, were obtained using a standard solution of quinone sulfate in 0.1 N sulfuric acid as the reference)&#t[ = 0.55].'* The estimated experimental error in the quantum yields is *lo%. The fluorescence decays and timedependent fluorescence spectrawere measured on degassed samples using a 100-kW nitrogen laser (337.1 nm, pulse duration 10 ns) in conjunction with a Princeton Applied Research Model 160 Boxcar Integrator as previously described.12 The data points of the emission decays (volts vs time) were input into the computer program "Cricket Graph" (version 1.2, Cricket Software) which generated a least-squares equation of the decay, from which the lifetime could be obtained,
0022-3654/94/2Q98-6430$04.50/0 0 1994 American Chemical Society
Photoinduced Intramolecular Electron Transfer with a typical correlation coefficient (R2) of 0.90 and an experimental error of about &lo%. X-ray CrystallographicAnalysis. The X-ray crystal structure of 4-(N,N-diethylamino)-p-terphenyl(2) was obtained using a Siemens P21 diffractometer with Mo Ka x-rays. The dimensions of the unit cell (a, b, c, and 8) and the space group of the crystal were obtained using the calculated values of the indices and the pattern of measured intensities. A Fortran program “TRACER” was used to confirm the result. Data collection was subsequently performed stepwise by the diffractometer within the range 4 I 28 I40°, starting from (h,k,l) = (O,O,O). Two quadrantsof data were collected, for a total of 5631 reflections. Cyclic Voltammetric Measurements. Oxidation potentials of Nfl-diethylaniline,4-(Nfl-diethylamino)biphenyl(l), and ( N f l diethy1amino)pterphenyl (2) were measured by cyclic voltammetry in purified acetonitrile using a platinum counter electrode and either a platinum or glassy carbon working electrode versus a silver/silver hexafluorophosphine reference electrode with tetrabutylammonium perchlorate as the supporting electrolyte. Measurementswere carried out under nitrogen on approximately 10-3M solutions of rigorously purified substrates. The electrodes werecoupled to a Princeton Applied Research Model 179 Digital Coulometer and a Princeton Applied Research Model 175 Universal Programmer and recorded on an X-Yrecorder. The results appear in Table 3. All reported redox potentials are referenced to the normal hydrogen electrode. Syntheses. (AU new compounds were characterized by mass spectrometry). 4-(N,N-Diethylumino)biphenyl(I). 4-Nitrobiphenyl (Aldrich) was reduced to 4-aminobiphenyl accordingto the literature.13 4-Aminobiphenyl(0.70g, 4.1 mmol) and 60 mL of glacial acetic acid were placed in a small round-bottom flask, equipped witha condenser,and cooled to 15-20 OC. Next, NaBH4 (1.6 g, 42 mmol) was added pelletwise. After the addition, the mixture was warmed to room temperature and another 42 mmol of NaBH4 was added in a similar fashion. To complete the reaction, the mixture was heated at 50-60 OC for about 30 min. The mixture was cooled to room temperature and neutralized in a dilute NaOH solution. The aqueous layer was extracted with dichloromethane,washed with water, and dried over anhydrous magnesium sulfate. The solvent was removed in vucuo to yield a golden residue. After recrystallization from dichloromethane, 0.74 g (80%) of 4-(N,N-diethylamino)biphenylwas obtained as beige solids: mp 57-58 OC; lH NMR (250 MHz, CDCl3) 6 1.11 (t, J = 7.0 Hz, 6H), 3.29 (q, J = 7.0 Hz, 4H), 6.67-7.54 (m, J = 7-9 Hz, 9H); ‘3C NMR (250 MHz, CDCl3) 6 12.52, 44.26, 111.88, 125.64, 125.97, 127.76, 128.51, 130.76, 141.18, 146.99. 4-(N,N-Diethylumino)-p-terphenyl(2). 4-Nitro-p-terphenyl, obtained as described by Allen and c o - w ~ r k e r swas , ~ ~reduced to 4-amino-p-terphenylaccording to the literature.15 In a 250-mL round-bottom flask, 4-amino-p-terphenyl(2.0 g, 8.2 mmol) was stirred in 100 mL of acetic acid. Sodium borohydride (3.5 g, 0.093 mol) was added pelletwise while the reaction was kept at 15-20 OC. The mixture was warmed to room temperature and stirred for 30 min. Sodium borohydride (0.093 mol) was again added as before, and the resulting pale yellow solution was stirred at 60 OC for 30 min. The mixture was cooled to room temperature and neutralized with 75 mL of 4 M NaOH, upon which white solids formed. The basic solution was extracted with dichloromethane (3 X 50 mL), and the combined organic layers were washed with water and dried over anhydrous magnesium sulfate. The solvent was removed in uucuo to yield off-white solids. Column chromatographythrough alumina using petroleum ether/ CH2C12as eluting solvents, followed by recrystallization from ethanol, yielded 0.50 g (20%) of 4-(N,N-diethylamino)-p-terphenyl: mp 143-144 OC (lit 147.5-148.5 OC,16 as white crystals; ‘H NMR (250 MHz, CDCl3) 6 1.20 (t, J = 7.0 Hz, 6H), 3.40 (9,J = 6.9 Hz, 4H), 6.74-7.65 (m, J = 7-9 Hz, 13H); 13C NMR
The Journal of Physical Chemistry, Vol. 98,No. 26, 1994 6431 (250 MHz, CDCl3) 6 12.61,44.39,111.91,126.37,126.88,126.99, 127.33, 127.81, 128.36, 128.72, 138.50, 140.22, 140.94, 147.21. 4-Bromo-4’-(NJV-diethylumino)biphenyl, used in the synthesis of compounds 3 and 5, was obtained from 4-nitrobiphenyl (Aldrich) in three steps according to the literature.lOJ7.18 Compounds 3and 5 were prepared using a variation of Kumada’s nickel-phosphine-catalyzed cross-couplingreaction19as described below. The catalyst, (bis( 1.2-dipheny1phosphino)ethane)nickel(11) chloride, NiCh(dppe), was obtained from Alfa Products and purified with ethyl ether prior to use. I-(Nfl-Diethylumino)-p-quuterphenyl(3).4-Bromobiphenyl (Aldrich) (.692 g, 2.97 mmol), magnesium ribbon (0.36 g, 15 mmol), and a few crystals of iodine were placed in a 50-mL, 3-neck round-bottom flask, equipped with a reflux condenser. The system was purged with nitrogen, and about 5 mL of freshly distilled THF was added via a syringe. After a few minutes, the solvent began to boil, and 15 mL of additional THF was added. The resulting mixture was refluxed under nitrogen for 90 min. In another 50-mL flask, equipped with a condenser and addition funnel, were placed 4-bromo-4’-(N,N-diethylamino) biphenyl (0.715g, 2.35 mmol), theNiClz(dppe) catalyst (16.5 mg,O.O312 mmol), and a few milliliters of THF. The Grignard reagent was transferred while warm to the addition funnel via a syringe and slowly added over 15 min, upon which the reaction turned dark brown. After a 17.5-h reflux under nitrogen, the mixture was cooled to room temperature and poured into 50 mL of a dilute ammonium chloride solution. This was extracted with dichloromethane (3 X 50 mL), washed with water, and dried over anhydrousmagnesium sulfate. The solvent was removed in uucuo to yield a yellow residue, which was chromatographed on silica gel using petroleum ether/ether as the eluting solvent to yield yellow solids of 4-(N,N-diethylamino)-p-quaterphenyl. These were recrystallized in absolute ethanol to afford 0.109 g (12.3%) of solid product: mp 275-280 OC (decomposes); 1H NMR (250 MHz, CDCl3) 6 1.19 (t, J = 6.9 Hz, 6H), 3.39 (4, J 6.7 Hz, 4H), 6.74-7.74 (m, J = 7-9 Hz, 17H); 13C NMR (250 MHz, CDCl3) 6 12.64, 44.39, 111.89, 127.01, 127.09, 127.22, 127.36, 127.47, 127.54, 127.80, 128.79. Note the quaternary carbons could not be resolved above the instrument noise due to a lack of solubility of the compound in CDCl3. 1-@-(NJV-Diethylani1ino))naphthalene (4) was prepared using Kumada’s cross-coupling reaction19 by reacting 1-naphthylmagnesium bromide with 4-bromo-N,N-diethylaniline in the presence of NiClz(dppe) as previously described.I0 I -(p-(N,N-Diethy1umino)biphenylyl)nuphthulene (5). This compound was obtainedusing the NiCl2(dppe)-catalyzed reaction used to prepare compounds 3 and 4 by reacting 5.02 mmol of 1-naphthylmagnesium bromide with 4.0 mmol of 4-bromo-4’(N,N-diethy1amino)biphenyl in anhydrous THF for 19 h. Chromatography through silica gel, followed by recrystallization from ethanol, yielded 0.314 g (22%) of compound 5 as yellow crystals: mp 17Ck175 OC; lH NMR (250 MHz, CDClp) 6 1.26 (t, J = 7.0 Hz, 6H), 3.45 (q, J = 7.0 Hz, 4H), 6.84-8.11 (m, J = 7-9 Hz, 15H); 13C NMR (250 MHz, CDCl3) 6 12.55,44.56, 112.22, 125.39, 125.68, 125.84, 127.85, 129.52, 130.33,131.63, 133.80, 138.12, 140.09, 146.91. 4-Bromo-4’-(Nfl-dibutylamino)-p-terphenyl, used in the synthesis of compound 6, was obtained from p-terphenyl (Aldrich) in four steps, according to the literat~re.13J~J~*~O I-@-(N,N-Dibutylumino)-p-terphenyl)nuphthule~ (6). This compound was obtained with the same NiCl2(dppe)-catalyzed reaction used to prepare 3-5 by reacting 4.31 mmol of l-naphthylmagnesium bromide with 3.05 mmol of 4-bromo-4’-(N,Ndibuty1amino)-p-terphenyl in anhydrous THF for 20.5 h. Chromatography first through silica gel, and then through alumina, followed by recrystallization from ethanol, yielded 0.200 g (14%) ofcompound6as bright yellow solids: mp 100-102°C; ‘HNMR (360 MHz, CDC13) S 1.04 (t, J = 7.3 Hz, 6H), 1.44 (m, J = 7.3
6432 The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
Foley and Singer 60
4
Figure 2. ORTEP plot of 4-(N,N-diethylamino)-p-terphenyl (2). The atoms are represented as 50% probability thermal ellipsoids. The hydrogens have been omitted for clarity.
Hz, 4H), 1.67 (m, J = 7.4 Hz, 4H), 3.37 (t, J = 7.5 Hz, 4H), 6.79-8.09 (m, J = 7-9 Hz, 19H); 13CNMR (360 MHz, CDCl3) 6 14.01, 20.34, 29.43, 50.77, 111.86, 125.38, 125.75, 126.02, 126.35, 126.65, 127.26, 127.67, 127.74, 130.45, 131.57, 139.39, 139.78, 139.86, 139.88, 140.28, 147.61.
'. I
200
Results and Discussion X-Ray Crystal Structure of 2. 4-(N,N-Diethylamino)-pterphenyl (2) crystallizes in the monoclinic space group P21/n (No. 14), a primitive lattice with a 2-fold screw axis (about axis b) and a glide plane orthogonal to it with a translation of ( a c)/2.21 The density of the crystal was found to be 1.17 g ~ m - ~ . The final agreement or reliability factor, R(F),is 0.0356, which is well below the value (0.06) considered reliable for a crystal structure. The ORTEP plot of the crystal structure of compound 2, with the hydrogens omitted for clarity, is shown in Figure 2. [In the following discussion, ring 1 is the phenyl ring made up of carbons 1-6, ring 2 is the phenyl ring made up of carbons 7-12, and ring 3 (closest to the amino group) is made up of carbons 13-18]. The most salient results are the bond lengths of the single bonds
+
connectingthestructuralunits,C4-C7,1.494A;ClO-C13,1.485
A; C16-Nl9,
1.385 A, and the dihedral angles, rings 1 and 2, 0 = 29.9O; rings 2 and 3, w = -23.3O; ring 3 and diethylamino, = 5.3O. These results indicate strong overlap of the nitrogen lone-pair electrons with the a-orbitals of the aromatic system, as expected for an aniline, and only a slight twisting around the single bonds connecting phenyl rings 1-3. Electronic Absorption Spectra. The absorption spectra of compounds 1-3 are similar to the absorptionspectra of the parent polyphenyls (biphenyl, p-terphenyl, p-quaterphenyl),22with the expected red shift in the absorption maxima due to conjugation of the amino group with the aromatic a-system. However, for all three compounds, there is only a small bathochromic shift in the longest wavelength band in going from cyclohexane to acetonitrile, which suggests that direct electronic excitation occurs into a state with relatively little charge-transfer character. In acid media, however, these long-wavelength bands are blueshifted, indicating that they are associated with the nitrogen nonbonding electron pair. The other absorption bands are unaffected by solvent polarity. The absorption spectra of compound 4 in cyclohexane and acetonitrile are dominated by the N,N-diethylaniline chromophore. No naphthalene or 1-phenylnaphthaleneabsorptions are observed. Spectral shifts of 1400 cm-1 for the 'A 1Lb (usually 260 nm) and 3800 cm-1 for the 'A 'La (usually 204 nm) aniline transitions22areobserved. These results suggest that 4 behaves as a p-substituted aniline. Similarly, the long-wavelength portion of the electronic absorption spectra of 5 is dominated by strong (N,N-diethy1amino)biphenyltransitions (e 35 000). The longest wavelength transition red-shifts from 321.4 nm in cyclohexane to 327.6 nm in acetonitrile and blue-shifts to 293.6 nm in the presence of acid, indicating involvement of the nitrogen nonbonding electron pair in this transition. A lower intensity shoulder near 280 nm in these spectra may be the 'A lLa, transversely polarized, naphthalenetransition.22 This transition, presumably, is obscured under a stronger aniline band that occurs in the same spectral region in 4.
-
-
-
-
-
-
I 300
I
400
Wavelength (nm)
Figure 3. Ultraviolet-visible absorption spectra of 1-@-(N,N-dibutylamino)-pterphenyly1)naphthalene (6) in cyclohexane (-), acetonitrile (- - -), and 5050 ethanokwater, pH = 1 (.e.).
The absorption spectra of 6, shown in Figure 3, are similar to the spectra of 2 and 5, except for a further red shift of the longest wavelength band, apparently associated with the (N,N-dibuty1amino)pterphenylchromophore. As expected, this band is blueshifted to 302.0 nm in acidic solution because of loss of the strong interaction between the nitrogen nonbonding electrons and the terphenyl a-system. The very modest red shifts observed in the absorption maxima of 4-6 with increasing solvent polarity suggest direct excitation into states with minimal charge transfer. Fluorescence Spectra, Lifetimes, and Quantum Yields. A summary of the fluorescence data at room temperature for compounds 1-6 is shown in Table 1. The emission maxima are sensitive to solvent polarity, with strongersolvatochromicbehavior observed for the more extended systems 3,5, and 6. For example, 6 (the terphenyl system) shows a fluorescence spectral shift from 403 to 501 nm in going from cyclohexane to acetonitrile while the directly linked 4 only shows a shift from 395 to 458 nm. As further analyzed and discussed below, the fluorescence spectral shifts with all these compounds, except 1, are consistent with highly polar CT states. The fluorescence of 1 red-shifts from 359 to 380 nm between cyclohexane and acetonitrile, corresponding to an energy decrease of 0.20 eV, much smaller than the observed energy changes for compounds 2-6. This modest solvent response leads to a relatively small excited-state dipole moment (pe = 6.1 D, discussed below), which suggests that this fluorescence does not originate from a state with strong charge-transfer character. Because of the typically large Stokes shift in the fluorescence of the biphenyls23 that would lower the energy of the locally excited a,a*state, the lowest excited singlet state in 1 could be of mixed CT and a,r* character. In 2-6, the more favorable redox energetics (see discussion below), presumably result in the CT state being well below the locally excited a,a* state in polar solvents, so this situation does not arise. The emission maxima of 2 and 3 occur 0.49 and 0.64 eV, respectively, lower in energy in acetonitrile than in cyclohexane, indicative of very polar charge-transfer fluorescent states. The fluorescence spectra are broad and structureless in all solvents, except for 3 in cyclohexane where a structuredemission is observed (390,409nm). In 50:50aqueousethanolatpH 1, thefluorescence maxima of 2 (342 nm) and 3 (379 nm) are blue-shifted significantly to wavelengths that correspond to emission for p-terphenyl and p-quaterphenyl, respectively.24 Accordingly, charge-transfer fluorescence occurs from 2 and 3 under neutral conditions in all solvents, with the possible exception of 3 in cyclohexane. Similar trends wereobserved for the 1-@-(N,N-dialky1amino)p-polypheny1)naphthalenes4-6. The emission maxima of 4 in
The Journal of Physical Chemistry, Vol. 98, No. 26,1994 6433
Photoinduced Intramolecular Electron Transfer
TABLE 1: Spectroscopic Dnta for Comwunds 1-6 hdnm)
(energy in electronvolts) solvent cyclohexane
2.02
Af 0.101
benzene
2.28
0.117
ethyl ether
4.34
0.256
THF
7.58
0.309
€
ethanol
24.5
0.380
acetonitrile
37.5
0.393
slope (10-4 cm-1)"
1 359 (3.46) 367 (3.37) 364 (3.41) 371 (3.35) 374 (3.31) 380 (3.26) -0.371 2.81 0.20 .17/.25 8.715.0
3 390 (3.17) 422 (2.94) 427 (2.90) 45 1 (2.75) 466 (2.67) 490 (2.53) -1.36 2.63 0.64 .66/.68 4.915.3
2 381 (3.25) 403 (3.08) 403 (3.08) 422 (2.94) 43 1 (2.88) 447 (2.78) -1.01 2.68 0.49 .74/.75 4.715.4
5 400 (3.10) 419 (2.96) 425 (2.91) 45 1 (2.75) 473 (2.62) 497 (2.49) -1.36 2.62 0.61 31.49 4.714.9
4
395 (3.14) 403
(3.08) 418 (2.97) 430 (2.88) 450 (2.76) 458 (2.71) -1.08 2.64 0.43 .27/.41 4.710.8
6 403 (3.08) 424 (2.93) 432 (2.86) 459 (2.70) 474 (2.62) 501 (2.48) -1.33 2.58 0.60 .76/.53 7.217.3
intercept (10-4 cm-1)" U(eV)b &(CH/MeCN)c T&2H/MeCN)din ns 0 From a plot of v,(cm-') vs AJ Slope = -2k2Af/hca3.25 * Energy change in emission between cyclohexane and acetonitrile. Fluorescence quantum yields in CH-cyclohexane and MeCN-acetonitrile. Fluorescence lifetimes in CH-cyclohexane and MeCN-acetonitrile. cyclohexane and acetonitrile (395 and 458 nm, respectively) correspond to an energy change of 0.43 eV. Larger red shifts are noted with 5 (400-497 nm) and 6 (403-501) which correspond toenergy changes of 0.61 and0.60 eV, respectively. These results indicate that fluorescence occurs from highly polar states. However, in these multichromophoric systems, there are several possible combinations of interacting donor and acceptor groups, as discussed below. The fluorescencequantum yields were measured in cyclohexane and acetonitrilein all systems. Thevalues of &show no significant dependenceon solvent polarity; however they do range from lows of 0.17 (cyclohexane)/0.25 (acetonitrile) for 1 to highs of 0.74 (cyclohexane)/0.75 (acetonitrile) for 2. The somewhat lower values of c#q with 1 are consistent with fluorescence quantum yields in other biphenyls24and may reflect the possible mixed character (CT and ?F,T*)of the emitting state in 1. Single-exponentialfluorescencedecays with calculated lifetimes in the range of -5-9 ns were observed with all systems. Timeresolved fluorescence spectra recorded on a nanosecond time scale at various time intervals following excitation are similar to one another and to steady-state spectra. Because of the highly polar nature of the emitting state in these systems,solvent-solute dipole interaction is expected to be an important stabilizing force. The absence of a time dependencein the fluorescencespectra suggests that solvent reorganization around the emitting state occurs prior to emission. Excited-State Dipole Moments. The excited-state dipole moments of the fluorescing state, p,, can be estimated using the following relationships:zsz8 Y,
= vfl(0)- 2p;Af/hca3
Af = ( E - 1)/(2e
+ 1) - (n2- 1)/(4n2 + 2)
(2)
where h is Planck's constant, c is the speed of light, v n is the fluorescencemaximum (in cm-l) in a solvent of dielectric constant c and refractive index n, vn(0) is the fluorescence maximum in thegas phase, Af isthesolvent polarityparameter,ais theeffective radius of the solvent shell around the molecule, and p , is the excited-state dipole moment. According to eq 1, a plot of v n (in cm-l) versus Af should be linear with a slope of 2 ~ ( 2 / h c a ~ .These ~ 5 plots for 1-6 are shown in Figure 4. From the measured slopes,the values of the constants h and c, and an estimate of a, values of p , may be obtained. The
m
1
0
2
3
0.15
0.05
0.35
0.25
Af
5
8 0
4 5
r
6
n
I
2oooo:
E lwooi . 0.05
'
.
I
'
.
.
0.15
I
0.25
.
'
.
+
.
0.35
Af
Figure 4. Plots of the fluorescence maximum (cm-I) of compounds 1 4 against the solvent polarity parameter, AJ The slopesof the best-fit lines
are equal to -2k2Af/hca3.25
value of a for each compound was approximated as 40% of the long axis of an ellipsoidaround the molecule,28based on structural parameters obtained from the three-dimensional modeling program "Biograf" (Biodesign) and the X-ray crystal structure of compound 2. The observed and calculated excited-state dipole moments of 1-6 are listed in Table 2. The deviations from linearity in the plots in Figure 4 indicate that eq 1 is an imperfect model. Othersz9 have noted the importance of the solvent polarityon the measured dipole moment due to solute excited-state polarizability, a,. Because of solventinduced electronicchanges in the solute, there is a solvent-induced component in Ne. Thus, themeasured excited-statedipolemoment is larger than the permanent electric dipole for the solvent-free molecule, p,O. Accordingly, the values of p , in Table 2 should be viewed as approximations and not as absolute values.
6434 The Journal of Physical Chemistry, Vol. 98, No. 26, I994
Foley and Singer
TABLE 2: Calculated and Observed I.Os and Parameters Used in the Energy-Correlation Calculations for Compounds 1-6 cc, (D) "pd a (A). r (A)b cald Ob 1 2 3 4 5 6
4.65 6.66 8.39 4.80 6.54 8.77
4.36 6.51 8.66 4.47 6.58 8.70
20.9 31.3 41.6 20.8 31.6 41.8
6.1 17.2 28.3 10.9 19.4 29.9
a The effective radius of the solvent shell around the molecule, taken as 40% of the largcst molecular dimension.** Estimation of "r", the length of the excited-statedipoleused to calculatek(ca1c). The expected dipole moment for full electron transfer, calculated using the equation, ~r, = er. The donor and acceptor were treated as point charges, with the distance of the resulting dipole being either from the center of one of the phenyl rings to the center of the arene system, or from the center of the (N,N-diethy1amino)biphenyl donor to the center of the arene ring.
The increasing dependence of vfl on solvent polarity through the series 1-3 (Table 1) is reflected in the relative magnitudes of themeasuredvalues for pc: 6.1,17.2, and 28.3 D, respectively. Such a trend is expected since, as the *-electronic system is extended,the distance separating the effective donor and acceptor groups increases. The lengths of the dipoles in 1-3 are assumed to extend from the middle of the N,N-diethylaniline ring to the center of the adjacent phenyl (1), biphenyl (2), and p-terphenyl (3) systems. A similar trend is noted with the naphthyl series 4-6 where the values of pc increase by approximately 10 D with each additional phenyl ring: 10.9, 19.4, and 29.9 D, respectively. The dipole of compound 4 is assumed to extend from the center of the N,Ndiethylaniline ring to the center of the naphthalene ring. In compound 5, the dipole is assumed to extend from the center of the (N,N-diethy1amino)biphenylylgroup to the center of the naphthalene ring, and in 6, from the middle of the (N,Ndibuty1amino)-p-terphenyl moiety to the center of the naphthalene ring. The calculated p i s were determined by using these lengths and assuming full charge separation. Compounds 3, 4-(N,Ndiethylamino)-p-quaterphenyl,and 6 , l -(p-(N,N-dibuty1amino)p-terphenylyl)naphthalene,have almost identical calculated and observed excited-state dipole moments, which may reflect their similar molecular dimensions. Interestingly, the dibutylamino group in 6 appears to have no effect on he,which suggests these larger alkyl groups (compared with the diethyl groups in 3) do not screen away the polar solvent molecules that are important in stabilizing the excited-state electric dipole. The observed excited-state dipole moments of 1-6 are 29%, 55%, 68%, 52%, 61%, and 72% of the theoretical values expected for full charge separation in a 90° twist geometry, where the donor and acceptor moieties are electronically uncoupled. A similar trend (65-80% of calculated values for full charge separation) was noted in the previously studied directly linked (N,N-diethylanilino)arenes.lO This less than total charge separation in the fluorescing state may be associated with emission from nonorthogonaf geometries where some degree of T,U overlap between the radical ions promotes the back-electron transfer required for radiative decay. These nonorthogonal geometries should have less than complete charge separation (because of T,?T overlap), leading to smaller dipole moments than those expected for the energetically more stable, orthogonal geometries where full charge separation is obtained. Energetics of the Charge-Transfer State in Polar Media. The energy of the charge-transfer fluorescence in polar solvents can be calculated using eq 3.30
According to this equation, the energy of the fluorescence from
Figure 5. Energetics of the charge-transferfluorescenceaccording to cq 3 where solv'
is the solvation sphere in the charge-transfer state.
the CT state is dependent on the one-electron redox potentials of the donor and acceptor, the solvent destabilization energy, Ed, = (pe- psa)zAf/a331 with Af defined as in eq 2, and the Coulombic energy, C = eZ/rt. Since the one-electron redox potentials are values measured in polar media, they already include the energy of solvation of the ions. The Eda term, which makes a significant contribution to the energetics (25-40%), results from radiative decay to a relatively nonpolar Franck-Condon ground state that is destabilizedbecause of a solvation sphere that reflects the highly polar excited state (soh*). In polar media such as acetonitrile (t = 3 7 3 , C makes only a very slight contribution ( ~ . j -where ~, Td is the fluorescence lifetime of the donor chromophore. This analysis
Photoinduced Intramolecular Electron Transfer
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6435
TABLE 3: Comparison of Observed and Calculated Energies for the Charge-Transfer Fluorescence in Acetonitrile According to L 3. All Values in eV E& acreptor+onor plin
E&
-3.15
WN(CzH32
-2.48 0.898 0.747 0.059 2.58 2.78
N(C2H5)2
-
-(c2w2
-
m
E& C calc obs 0.898 0.971 0.088 2.99 3.26 Eab
-2.20 0.898 0.658 0.044 2.40 2.53
or
(
N(C2H32
In summary, we conclude that intramolecular electron transfer in these polyphenylyl, multichromophoric systems is fast (> 108 s-l) and efficient (large 4;s). Further, the phenyl rings in these extended D-A systems are not just spacer groups; they have a functional role in the charge-transfer energetics.
-2.33 0.898 0.480 0.080 2.67 2.71
-2.13 0.898 0.803 0.058 2.16 2.49
References and Notes
data fmm ref lob) N(C2H5)2
1).
Acknowledgment. We thank Professor Robert Bau for help in carrying out the X-ray crystallographic study of 4-(N,Ndiethylamino)-p-terphenyl and Professor Gordon Miskelly for help with the cyclic voltammetric measurements.
&-2.48 0.865 0.663 0.044 2.64 2.53
W
single fluorescenceis observed, presumably from 9or its energetic equivalent where (N,N-dibuty1amino)-pterphenylis the donor and naphthyl is the acceptor (Table 3). No fluorescence is observed, using 2 as a model, that could be assigned to8, a chargetransfer state localized in the terphenyl chromophore (Scheme
-2.33 0.865 0.650 0.058 2.49 2.49
N(C$&
-2.33 0.822 0.575 0.044 2.54 2.47
N(C,H&
-2.128 0.865 0.567 0.044 2.38 2.47
a Values of E d for arene acceptor for ref 32. Values of E,, for amine donors measured by cyclic voltammetry in purified acetonitrile using tetrabutylammonium perchlorate as the supporting electrolyte.
(1) McLendon, G.; Miller, J. J . Am. Chem. SOC.1985,107,7811. (2) Wasielewski, M.; Niemczyk, M.; Svec, W.; Pewitt, E. J. Am. Chem. Soc. 1985,107, 1080. (3) Oevering, H.; Paddon-Row, M.; Heppener, M.; Oliver, A.; Cotsaris, E.; Verhoeven, J.; Hush, N. J. Am. Chem. Soc. 1987,109,3258. (4) Hush, N.; Paddon-Row, M.; Cotsaris, E.; Oevering, H.; Verhoeven, J.; Heppener, M. Chem. Phys. Lett. 1985,117,8. (5) Wasielewski, M.;Niemczyk, M. J . Am. Chem.Soc. 1984,106,5043. (6) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992,114, 6227. (7) Helms, A,; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1991,113, 4325. (8) Gust, D.; Moore, T.; Moore, A.; Gao, F.; Luttrull, D.; DeGraziano, J.; Ma, X.; Makings, L.; Lee, S.;Trier, T.; Bittersmann, E.; Seely, G.;
Wo0dward.S.; Bensasson, R.; Rouge, M.; De Schryver, F.; Vander Auweraer, M.J. Am. Chem. Soc. 1991,113,3638. (9) Slama-Schwok, A.; Blanchard-Desce, M.; Lehn, J. J . Phys. Chem. 1990, 94,3894.
SCHEME 1 N(C44
,
(IO) (a) Tseng, J.; Singer, L. J . Phys. Chem. 1989,93,7092.(b) Tseng, J.: Huann. S.:Singer. L. Chem. Phvs. Lett. 1988.153.401. (1 1) Ealvert, 1;Pitts, J. Phot&hemistry; John Wiley and Sons: New York, 1966. (12) Brown, R.;Legg, K.; Wolf, M.; Singer, L. Anal. Chem. 1974,46, '
1690. (13) LeFevre, R.;Turner, E. J . Chem.&. 1926,2041. (14) Allen, C.; Burness, D. J. Org. Chem. 1949,14, 175.
I
Locally FxatedState
t 8
1
assumes that intramolecular electron transfer is the only new pathway operating in the multichromophoric system. In the multichromophoric systems where N,N-diethylanilino (7d = 2.4 ns)24 is (or may be) the donor (2-9, ket must be >4 X IO8 s-l. In those systems where (N,N-diethylamino)-pbiphenylyl (3, 5, a n d 6) or (N,N-dibuty1amino)-p-terphenylyl (6) are the donors, k,, again must be >-lo* 5-1, based on the fluorescence lifetimes for 1 and 2 in Table 1. Interestingly, in 6 where the sequential CT states 8 and 9 are possible, only a
(15) Apfel, M.; Finkelmann, H.; Janini, G.; Laub, R.; Luhmann, B.; Price, A.; Roberts, W.; Shaw, T.; Smith, C. Anal. Chem. 1985,57,651. (1 6) The Sadtler Standard Spectra Collection; Sadtler Research Laboratories, spectrum no. 11965,1980. (17) Case, F. J. Am. Chem. Soc. 1938,60,424. (18) Gribble, G.; Lord, P.; Skotnicki, J.; Dietz, S.;Eaton, J.; Johnson, J. J. Am. Chem. SOC.1974,96,7812. (19) Tamao, K.; Sumitani, K.; Kiso, Y.; Zembayashi, M.; Fujioka, A.; Kodama, S.;Nakajima, I.; Minato, A.; Kumada, M. Bull. Chem. Soc. Jpn.
1976.49, 1958. (20) Kalamar, J.; Antos, K.; Hrivnak, J.; B m n , F. Chem. Zuesti 1968, 22, 674. (21) Stout, G.; Jensen, L. X-Ray Structure Determination: A Pructicul Guide, 2nd ed.;John Wiley & Sons: New York, 1989;Chapters 3 and 9. (22) Jaffe, H.; Orchin, M. Theory and Applications of Ultrwiolet Spectroscopy; John Wiley and Sons: New York, 1962. (23) Lakowicz, J. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983;Chapter 1. (24) Berlman, I. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971. (25) Mataga,N. In TheExciplex;Gordon,M., Ware, W., Eds.;Academic Press: New York, 1975. (26) Lippert, V. Z.Naturforsch. 1955,loa, 541. (27) Lippert, V. Z.Elektrochem. 1957,61,962. (28) Beens, H.; Knibbe, H.; Weller, A. J. Chem. Phys. 1967,47,1183. (29) (a) Okada, T.; Fujita, T.; Kubota, M.; Masaki, S.;Mataga, N.; Ide, R.; Sakata, Y.; Misumi, S.Chem. Phys. Lett. 1972,14,563.(b) Letard, J.; Lapouyade, R.; Rettig, W. J. Am. Chem. SOC.1993,lI S , 2441 and references therein. (30) Grabowski, 2.;Dobkowski, J. Pure Appl. Chem. 1983,55,245and references therein. (31) Liptay, W. 2.Nuturforsch. 1965,200, 1441. (32) Meerholz, K.; Heinze, J. J . Am. Chem. Soc. 1989,111, 2325.
