J. Phys. Chem. B 2005, 109, 4893-4899
4893
Electronic Interactions in Tertiary Oligophenylureas Frederick D. Lewis,* Todd L. Kurth, and Grace B. Delos Santos Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113 ReceiVed: NoVember 12, 2004; In Final Form: January 17, 2005
The syntheses, structures, and spectroscopy of a series of oligomeric tertiary oligophenylureas possessing one to five phenyl rings are reported. A convergent synthetic method employing tertiary monoamine and diamine building blocks is employed. NMR and molecular modeling are indicative of folded structures for all of the oligophenylureas in which adjacent phenyl rings have a splayed face-to-face geometry. NMR chemical shifts, absorption and emission maxima, and electrochemical oxidation potentials are all dependent upon the number of phenyl rings. The addition of a first inner phenyl has a pronounced effect on the chemical shifts, while a second and third inner phenyl have diminished effects. The oxidation potentials of the oligophenylureas display an abrupt decrease upon the addition of the second inner phenyl. The absorption and emission spectra are relatively insensitive to the addition of one to three inner phenyl rings. The electronic structures of the oligophenylureas possessing one to eight rings have been analyzed using ZINDO calculations. The frontier orbitals of the ureas with one to three phenyl rings are localized on a single phenyl ring (the inner ring for the three-ring urea), whereas the frontier orbitals of the higher oligomers are delocalized over two phenyl rings. In all cases, urea-localized n,π* transitions are lower in energy than the phenyl-localized π,π* transitions. The changes in properties with added phenyl rings parallel those previously observed for multilayered cyclophanes; however, they are less pronounced because of weaker coupling between the phenyl rings of the oligophenylureas.
Introduction
CHART 1
Arrays of aromatic chromophores are commanding increasing attention as potential components in molecular electronic devices.1 These arrays can be divided into two major structural types: linear arrays and π-stacked arrays. The linear poligophenylenes have attracted the most attention as a consequence of the desirable electronic properties of both the neutrals and their radical ions and the relative ease of their synthetic modification with electron-donor or -acceptor substituents.2,3 The multilayered cyclophanes (Chart 1) provide the only example of rigorously π-stacked multiple aromatic rings.4 Modifications of the multilayered cyclophanes with electron-donating and -withdrawing substituents on the outside rings have been reported, but require lengthy synthetic procedures. The electronic structures of both the oligophenylenes and layered cyclophanes are strongly dependent upon the number of phenyl rings.2,5-7 Both systems display a large red-shift in the absorption and fluorescence maxima upon addition of the third phenyl and progressively smaller red-shifts with additional phenyls. In the case of the oligophenylenes, the large shift for terphenyl is accompanied by a change in the character of the lowest singlet state from 1Lb to 1La, which results in a pronounced increase in the fluorescence rate constant and quantum yield.5 Semiempirical MO calculations describe the lowest singlet states of both the oligophenylenes and the threeand four-layered cyclophanes as resulting from a HOMOLUMO transition in which both orbitals are largely localized on the internal rings.2,6-8 A degree of π-stacking can also be achieved for molecules which are less rigid than the cyclophanes. The 1,8-diarylnaph* Corresponding author. E-mail:
[email protected].
thalenes have aryl groups which are face-to-face, but have splayed geometries with the shortest distance between rings at the point of attachment to naphthalene.9 Therien et al. have reported the synthesis of π-stacked porphyrin-bridge-quinone systems with one or two phenyl bridging units.10 Recently, Nakano11,12 and Rathore13 have independently reported the synthesis of oligomeric oligofluorenes, in which the fluorene rings are connected by a methylene linker and self-organize into folded arrays with splayed fluorene rings (Chart 1). Oligomers with one to four fluorene chromophores display continuous redshifted absorption and decreased oxidation potentials. The absorption maxima approach a constant value with five or six fluorenes. Tertiary oligomeric diarylureas (Chart 2) also adopt folded structures with face-to-face splayed benzene rings. Yamaguchi et al.14 reported the crystal structure of a folded phenyl urea oligomer possessing five phenyl rings, and Krebs and Jensen15 recently reported the synthesis of an oligomeric urea possessing nine naphthalene rings. However, the nature of the electronic interactions in these systems has not been systematically investigated.
10.1021/jp0448090 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005
4894 J. Phys. Chem. B, Vol. 109, No. 11, 2005 CHART 2
Our interest in the electronic interactions between the π-stacked nucleobases in the well-defined base-pair domains of synthetic DNA hairpins led us to seek synthetically accessible organic systems possessing π-stacked aromatic rings.16 Oligoaryl ureas were chosen for these studies on the basis of the ease of synthesis of dimers and oligomers possessing either a single type of arene or mixtures of arenes, including donor-bridgeacceptor systems. We have previously reported the results of our studies of the electronic interactions between aryl groups in tertiary diarylureas17-19 and of the reductive elimination of diarylureas and several oligomeric arylureas.20,21 We report here the results of an investigation of the electronic interactions in a family of tertiary oligophenylureas 1-5 possessing one to five phenyl rings (Chart 1). Weak coupling between the π-stacked phenyl groups is responsible for the unique properties of these molecules. Experimental Section General. 1H NMR spectra were measured on an Inova 500 spectrometer. UV-vis spectra were measured on a HewlettPackard 8452A diode array spectrometer using a 1-cm path length quartz cell. Total emission spectra were measured on a SPEX Fluoromax spectrometer. Low-temperature spectra were measured in a Suprasil quartz “EPR” tube (i.d. ) 3.3 mm) using a quartz liquid nitrogen coldfinger dewar at 77 K. Oxidation potentials were estimated from the irreversible oxidation waves of cyclic voltametric measurements performed in CH2Cl2 containing 5 × 10-4 M TMAPF6 with a scanning rate of 0.1 V/s using a Pt working electrode. Reduction waves were not observed within the operating range of the solvent and electrolyte. INDO/S-CIS-SCF (ZINDO) calculations (26 occupied and 26 unoccupied frontier orbitals) were performed on a PC with the ZINDO Hamiltonian as implemented by CAChe Release 6.0.22 All molecular models used in the semiempirical calculations were based on AM1 optimized ground-state geometries by using the MOPAC suite of programs as implemented under CAChe Release 6.0. All data-fitting procedures were carried out by using Origin (version 6.1).23 Materials. Aniline, N-methylaniline, p-phenylenediamine, and N,N′-dimethyl-N,N′-diphenylurea (2) were commercial materials. N,N,N′-trimethyl-N′-phenylurea (1) was prepared by the method of Lapore.24 All solvents were spectrograde. N,N′-Dimethyl-p-phenylenediamine was prepared in approximately 60% yield from p-phenylenediamine, by the method of Krishnamurthy.25 1H NMR (CDCl3, 500 MHz): δ 6.56 (4H, s), 3.20 (2H, s), 2.79 (6H, s). MS m/e 136 (M+).
Lewis et al. N,N′-Dimethyl-N,N′-p-phenylenedicarbamoyl Chloride. In a well-ventilated hood, N,N′-dimethylphenylenediamine was placed in a round-bottom flask with 5% excess phosgene in toluene (20% in toluene, Fluka). Excess triethylamine was then added, and the solution was refluxed for 1 h. The solution was purged with N2 for one-half hour to displace residual phosgene. The solvent was removed under vacuum yielding a yellow solid. 80% typical yield. NMR shows pure dicarbamoyl chloride product: 1H NMR (CDCl3, 400 MHz): δ 7.33 (4H, s), 36.0 (6H, s). MS m/z 260 (M+). N,N′′-1,4-Phenylenebis[N,N′-dimethyl-N′-phenyl]urea (3). The triphenylurea was synthesized by adding two equivalents of N-methylaniline to a toluene solution of the p-phenylenedicarbamoyl chloride with excess triethylamine and refluxing for 3 h. The solution was rotovapped to dryness and then dissolved in methylene chloride and washed with water. An alumina column (hexane/acetone) was run, resulting in several pure fractions of product. 80% yield. 3: mp 197-199 °C. 1H NMR (CDCl3, 500 MHz): δ 7.01 (4H, t, 8 Hz), 6.90 (2H, t, 8 Hz), 6.74 (4H, s), 6.49 (4H, s), 3.11 (6H, s), 3.06 (6H, s). MS calcd for C24H26N4O2 m/e 402.6; found, 403.1. N,N′-Dimethyl-N,N′-bis[4-[methyl[(methylphenylamino)carbonyl]amino]-phenyl]urea (4). The tetraphenylurea was synthesized via the self-coupling of the product of reaction, as described for 2, between the N-methyl-phenylcarbamoyl chloride and N,N′-dimethylphenylenediamine yielding N,N′-dimethyl-N(4-methylaminophenyl)-N′-phenylurea (D), 70% typical yield: 1H NMR (CDCl , 500 MHz): δ 7.07 (3H, t, 8 Hz), 6.94 (1H, 3 t, 8 Hz), 6.80 (2H, d, 8 Hz), 6.57 (2H, d, 8 Hz), 6.27 (2H, d, 8 Hz), 3.60 (1H, s), 3.14 (3H, s), 3.12 (3H, s), 2.74 (3H, s). Addition of this compound to a half equivalent solution of phosgene yields the tetraphenylurea in 50% yield. 4: mp 194196 °C. 1H NMR (CDCl3, 500 MHz): δ 7.01 (4H, t, 8 Hz), 6.91 (2H, t, 8 Hz), 6.73 (4H, d, 8 Hz), 6.47 (8H, s), 3.13 (6H, s), 3.04 (6H, s), 3.00 (3H, s). MS calcd for C33H36N6O3 m/e 564.7; found, 565.1. N,N′′-1,4-Phenylenebis[N,N′-dimethyl-N′-[4-[methyl[(methylphenylamino)-carbonyl]amino]phenyl]-urea (5). The pentaphenylurea was synthesized via the reaction, as described for 2, of N,N′-dimethyl-N-(4-methylaminophenyl)-N′-phenylurea (D) with the N,N′-p-phenylenedicarbamoyl chloride (B). 50% yield. 5: mp 227-229 °C. 1H NMR (CDCl3, 500 MHz): δ 6.99 (4H, t, 8 Hz), 6.94 (2H, t, 8 Hz), 6.72 (4H, d, 8 Hz), 6.57 (4H, d, 8 Hz), 6.27 (4H, d, 8 Hz), 3.60 (3H, s), 3.14 (3H, s), 3.12 (3H, s), 2.74 (3H, s). MS calcd for C42H46N8O4 m/e 726.8; found, 727.2. Results and Discussion Synthesis and Structure. The oligomeric oligophenylureas 3-5 were prepared using the building block approach outlined in Scheme 1 (see Experimental Section). The use of Nmethylated building blocks provides higher yields of the oligomers than those obtained by exhaustive methylation of the corresponding secondary ureas.17 The building block approach can also be readily adapted to the synthesis of structurally diverse oligoarylureas which incorporate different aryl units.21 Yamaguchi and co-workers have reported the crystal structures of 2 and 5, both of which adopt folded, face-to-face geometries in the solid state.14,26 The solution NMR and UV spectra of 2 and other tertiary diarylureas have been analyzed by Lepore et al. using NMR and UV spectra.27 They conclude that 2 adopts a folded structure in solution, as well as in the crystal. The preference for folded versus extended structures is steric in origin, the nonbonded repulsion of two methyls in the
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J. Phys. Chem. B, Vol. 109, No. 11, 2005 4895
Figure 1. Mimimized folded structure for the pentaphenylurea 5.
SCHEME 1: Convergent Synthetic Route to the Oligophenylureas
extended conformation being larger than that between two phenyls in the folded conformation. The gas-phase geometries of the oligophenylureas were optimized using the Hartree-Fock semiempirical Austin Model 1 (AM1) method, which provides reliable estimated geometries for organic molecules.28 Local minima with folded structures such as that shown for 5 in Figure 1 are typically 2-3 kcal/ mol more stable than extended (unfolded) structures, as previously observed for several tertiary diarylureas.17,19 Multiple local minima with different dihedral angles between the long axes of adjacent phenyl rings were encountered for the folded structures, indicative of shallow ground-state potential energy surfaces. The perfectly aligned structure of Figure 1 should be entropically disfavored compared to less symmetric folded structures in solution. The 1H NMR spectra of 2-5 in CDCl3 are shown in Figure 2. In the case of 2, the aromatic protons are shifted upfield when compared to that of the secondary diphenylurea or the tertiary monophenylurea 1 (Chart 1). For example, the ortho protons of 2 and the secondary diphenylurea appear as doublets at 6.78 δ and 7.42 δ, respectively. A further upfield shift, accompanied by increased line-broadening, is observed for the “outer” phenyl protons of 3-5. The “inner” phenyl protons of 3 appear as a singlet, consistent with either a single symmetric conformation or rapidly interconverting conformations. The inside protons of 4 display some splitting, plausibly reflecting a difference in chemical shift for the internal protons which are ortho with respect to the central versus outside urea linkages. Further splitting is observed for the inner phenyl protons of 5, in accord with the nonequivalence of the central phenyl and flanking internal phenyls. The chemical shifts of the ortho protons of 2-5 are reported in Table 1 along with those for the corresponding multilayer cyclophanes29 and oligofluorenes.12 The outer-ring protons for all three systems display a large upfield shift upon addition of
Figure 2. Aromatic region of the 1H NMR spectra of oligophenylureas 2-5 in CDCl3 solution, downfield signals from outer rings and upfield signals from inner rings (low field singlet from solvent).
TABLE 1: Chemical Shift Data for Oligophenylureas, Layered Cyclophanes, and Oligofluorenesa layers urea outer inner cyclophane outer inner oligofluorene outer inner
2
3
4
5
6.81
6.74 6.49
6.73 6.47
6.72 6.57, 6.27b
6.47
6.19 5.40
6.09 5.15
6.07 5.07, 4.92b
6.77
6.58 6.25
6.50 6.10
6.42 6.00, 5.90b
a Chemical shifts in CD Cl for ortho protons of oligophenylureas, 2 2 and in CDCl3 for the aromatic protons of layered cyclophanes (data from ref 29), and H1 protons of oligofluorenes (data from ref 12). b Chemical shift for central inner ring.
the third ring and more gradual changes for addition of a second and third inner ring. The inner rings display modest upfield shifts upon incremental addition of inner rings, the largest shifts being observed for the inner rings of 5-layered systems. The upfield shifts are attributed to shielding by the π-electrons of the adjacent aromatic rings. The enforced coplanar geometry of the layered cyclophanes results in larger upfield shifts than does the splayed geometry of the phenylureas or fluorenes. The larger shifts for the fluorenes versus phenylureas plausibly are a consequence of the larger aromatic rings or less perfect stacking, either of which would provide greater shielding. The inner protons of 3-layered arylurea with naphthyl outer rings have a chemical shift of 5.81 δ, substantially upfield from the value
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Figure 3. Normalized ultraviolet absorption spectra of the oligophenylureas 1-5 in methylcyclohexane solution.
TABLE 2: Absorption and Emission Spectral Data for the Oligophenylureasa urea
λabs, nm (MC)
Absorbance ×10-3
λabs, nm (MTHF)
λfl (MTHF)
λphos (MTHF)
1 2 3 4 5
253 249 258 260 260
8.4 9.4 3.7 2.8 b
246 263 270 275
339 341 349 351 354
407 420 428 429 430
a Absorption maxima in methylcyclohexane (MC) or methyltetrahydrofuran (MTHF) solution at room temperature and emission maxima in a MTHF glass at 77 K. b Limited solubility.
for 3.21 This analysis is consistent with the preference of 2-5 for folded structures obtained in AM1 calculations. Electronic Absorption Spectra. The normalized absorption spectra of 1-5 in methylcyclohexane (MC) are shown in Figure 3. Compared to the monophenylurea 1, the absorption maximum of 2 is blue-shifted and the absorption band is broadened. The maxima of 3-5 are red-shifted and their bands further broadened. The molar absorbance for 2 is similar to that of 1, whereas the absorbance of 2-4 decreases with each additional phenyl ring, indicative of substantial hypochromism. The absorption maxima in MC and methyltetrahydrofuran (MTHF) solutions are reported in Table 2. Except in the case of 2, the maxima are red-shifted in the more polar MTHF, suggestive of partial charge-transfer character for the transition(s) responsible for the absorption band. The absorption bands of 3-5 can be fit to the sum of two Gaussians: a short-wavelength band which is independent of the number of phenyl rings and a longwavelength band which is red-shifted as the number of rings increases. The absorption spectra of the oligophenylureas are less structured than those of the layered cyclophanes6 or oligofluorenes.12 The cyclophanes have a resolved long-wavelength band which undergoes a large red-shift upon addition of an inner ring and more modest red-shifts for additional inner rings. The intensity of this band is similar for cyclophanes with 3-5 phenyl rings. The oligofluorenes also have resolved bands: a weak long-wavelength band which undergoes a progressive red-shift with added fluorenes and a stronger short-wavelength band which does not shift appreciably. The oligofluorenes display significant hypochromism, approaching approximately 70% for higher oligomers. The percent hypochromism for both the
Figure 4. Selected frontier molecular orbitals for the oligophenylureas 1 (a) and 2 (b).
oligophenylureas and oligofluorenes is significantly greater than that for the π-stacked bases of DNA.30 ZINDO Calculations. The electronic structures and spectra of the oligophenylureas were explored using the semiempirical Hartree-Fock intermediate neglect of differential overlap (INDO) method as parametrized by Zerner and co-workers (ZINDO)31 and implemented in CAChe 6.0.22 Selected frontier orbitals for 1 and 2 are shown in Figure 4 , and the HOMO and LUMO orbitals of 3-5 and the octaphenylurea are shown in Figure 5.32 The calculated wavelength, oscillator strength, and description of the selected transitions are reported in Table 3. The highest-energy occupied orbitals of 1 and 2 are localized either on the phenyl ring (aniline-like) or on the urea group (Figure 4). The lowest-energy unoccupied orbital are either phenyl-localized or delocalized over the phenyl and urea carbonyl groups. The weakly allowed lowest energy transition for both 1 and 2 has extensive configuration interaction involving single electron transitions from several urea-localized filled orbitals to both urea-localized and delocalized vacant orbitals. These states are described in Table 1 as having n,π* character. The next two transitions for 1 can be described as phenyl-localized π,π* 1Lb and 1La transitions by analogy to the two bands of substituted benzenes, including aniline. The former band has a low oscillator strength and extensive configuration interaction, whereas the latter transition is domination by the HOMO f LUMO single electron transition and has a much larger calculated oscillator strength than either S1 or S2. The latter band dominates the long-wavelength absorption of 1. In the case of 2, the presence of two phenyl rings gives rise to two weak π,π* transitions (S2 and S3) and two allowed (S4 and
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Figure 5. (a) LUMO and (b) HOMO orbitals of the oligophenylureas possessing 3-5 and 8 phenyl rings.
TABLE 3: Calculated Properties of Selected Singlet Statesa urea
state
λ, nm
A (1/mol/cm)
character
1
S1 S2 S3 S1 S2 S3 S4 S5 S1 S3 S5 S1 S4 S8 S1 S5 S10
317 279 251 314 281 278 265 241 314 293 271 313 293 274 313 291 272
311 1060 10900 365 620 890 14600 27300 343549 1540 22900 352 297 30430 332 400 41400
n,π* 1st π,π* HOMO-LUMO n,π* 1(π,π*) 2(π,π*) 3(π,π*) 4(π,π*) n,π* 1(π,π*) 3(π,π*) n,π* 1(π,π*) 5(π,π*) n,π* 1(π,π*) 6(π,π*)
2
3 4 5
a Singlet state energy, molar absorbance, and description based on ZINDO calculations. See text for description of transitions and Figure 4 for selected molecular orbitals.
S5) π,π* transitions (Table 2). This results in broadening of the long-wavelength absorption band and a blue-shift in its absorption maximum. The number of low-energy transitions n,π* and π,π* for the oligophenylureas increases with the addition of each additional ring. The frontier orbitals of 3 (Figure 5) are localized on the inner phenyl ring, whereas those of 4, 5, and 8 are localized on two of the inner phenyl rings. ZINDO calculations for higher oligomers, including the octamer (Figure 5), show no evidence for increased delocalization of the frontier orbitals. Fully delocalized filled and vacant orbitals and orbitals localized on the outside phenyl rings have lower or higher energies, respectively, compared to the frontier orbitals. Configuration interaction becomes more extensive with each additional phenyl ring, with no single configuration dominating the description of any of the lowest singlet states of 3-5. Oligomers with n phenyl rings have n - 1 nearly-degenerate urea-localized transitions and multiple π,π* transitions. The lowest-energy n,π*, π,π* and first-allowed π,π* transitions are reported in Table 3. The calculated energies of these transitions are relatively insensitive to the number of phenyl rings. Inversion of the 1La and 1Lb bands observed for the oligophenylenes33 does not occur for the oligophenylureas. The apparent broadening of the absorption spectra at longer wavelengths reflects an increase in the number of partially allowed n,π* and π,π* transitions between 250 and 300 nm as the number of phenyl rings increases, rather than lowering of the S1 energy. The decrease in absorbance with added phenyl rings
(Table 3) can be attributed to the loss in oscillator strength with increasing configuration interaction. The electronic spectra of the multilayered cyclophanes have previously been investigated using semiempirical calculations.6,7 The addition of a second and third phenyl layer results in large red-shifts of the weak long-wavelength absorption band; however, additional phenyl layers effect relatively minor changes in the absorption spectra. The S1 transition of the 3- and 4-layered cyclophanes is assigned to configuration interaction of transitions localized on the inner rings, as is the case of the oligophenylureas. All of the multilayered cyclophanes have low S1 oscillator strengths (f ∼ 0.005). The much larger shifts for the cyclophanes vs ureas is attributed to stronger electronic coupling in the former vs the latter. The spectra of the multilayered cyclophanes can be modeled by stacked planar benzene rings with π-stacking distances equal to those of the cyclophanes but lacking the connecting methylene bridges. In contrast, ZINDO calculations for two phenyl rings with the splayed geometry of 2 but lacking the urea linker display essentially no electronic interaction. Thus, the urea linkers turn on the weak interactions between adjacent phenyl rings. To our knowledge, the electronic spectra of the oligofluorenes have not been analyzed. However, the appearance of their absorption and fluorescence spectra (vide infra) suggest that electronic interactions between adjacent fluorenes are excitonic in nature.12 Emission Spectra. The total emission spectra of 1-5 obtained at 77 K in a MTHF glass display two bands assigned to fluorescence and phosphorescence of the oligoarylureas (Figure 6). The fluorescence and phosphorescence band maxima are reported in Table 2. We have previously reported that the quantum yields of fluorescence and phosphorescence for 2 are low at 77 K (0.003 and 0.008, respectively).17 The relative intensities for 3-5 are somewhat higher than those of 2; however, quantum yields remain low. Weak room-temperature fluorescence is observed for 5, but not for 2-4. The fluorescence of 1-5 is assigned to their urea-localized n,π* states. The minor red-shift observed upon addition of phenyl rings plausibly results from the presence of additional nearly-degenerate urea-localized states. The weakly structured phosphorescene is attributed to a phenyl-localized π,π* triplet state, in accord with the results of ZINDO calculations.17 The modest red-shift with increasing number of phenyl rings parallels the calculated energy of the lowest-energy singlet state (Table 3). A state-energy diagram for the oligophenylureas (Scheme 2) resembles that for nitrogen heterocycles.34 Rapid internal conversion of the π,π* singlet states populated upon longwavelength excitation should yield the lowest urea-localized n,π* singlet state, which can either undergo internal conversion or intersystem crossing to the lowest-energy π,π* triplet state.
4898 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Figure 6. Luminescence spectra of oligophenylureas 1-5 in a MTHF glass at 77 K.
Lewis et al.
Figure 7. Cyclic voltamograms of oligophenylureas 2-5 in CD2Cl2 solution.
SCHEME 3: Reductive Elimination of the Urea Linker
SCHEME 2: State Diagram for the Oligophenylureas (ic ) internal conversion, isc ) intersystem crossing)
In view of the low phosphorescence quantum yields, internal conversion appears to be the dominant decay pathway for the n,π* singlet states of 2-5. The luminescence properties of 2-5 are, in some respects, similar to those of the multilayer cyclophanes possessing 2-5 phenyl rings.35 Neither the cyclophanes nor oligophenylureas 2-4 display fluorescence at room temperature. A large redshift in the fluorescence and phosphorescence spectra is observed for the 3-layer vs 2-layer cyclophanes at 77 K, but only modest changes in the fluorescence and no change in the phosphorescence are observed for the higher oligomers. Otsubo et al. reported a decrease in the phosphorescence lifetime from 3.5 s for paracyclophane to 0.45 ( 0.04 s for its higher oligomers.35 The luminescence spectra of the π-stacked oligofluorenes differ markedly from those of the oligophenylureas and multilayer cyclophanes.12 The oligoflourenes display strong excimer fluorescence at room temperature in solution, independent of the oligomer size. This behavior is characteristic of weak coupling between fluorene chromophores, as opposed to the highly delocalized singlet states of the multilayer cyclophanes. The rigid splayed geometry for the oligophenylureas may prevent formation of a sandwich geometry, necessary for the observation of intramolecular excimer formation between phenyl rings.36 Oxidation and Reduction. Cyclic voltamograms for 2-5 are shown in Figure 7. The half-wave oxidation potentials of 2 and 3 are similar (1.12 ( 0.01 V vs SCE) and substantially
larger than those for 4 and 5 (0.86 ( 0.01 V). It is interesting to note that the large change in Eox occurs upon addition of the second inner phenyl ring, whereas the second inner phenyl ring causes relatively minor changes in the NMR, absorption, and emission spectra. Inspection of the HOMO orbitals of 3 and 4 (Figure 5) provides one plausible explanation for this seeming anomaly. The HOMO of 3 is largely localized on the inner phenyl ring, whereas the HOMO of 4 (and the higher oligomers) is delocalized over both inner phenyl rings. No reductive waves are observed in the cyclic voltamograms of the oligophenylureas in CH2Cl2. We have previously reported that reduction of the oligophenylureas with potassium metal in HMPA solution results in reductive elimination of all of the urea linkers (Scheme 3).21 The resulting oligophenylene anion radicals were characterized by EPR spectroscopy. The delocalized character of the low-lying vacant MO’s of the polyarylureas 2-5 (Figures 4 and 5) may facilitate C1-C1′ bonding between adjacent arene rings. To our knowledge, the oxidation potentials of the layered cyclophanes have not been reported. In the case of the oligofluorenes, the decrease in oxidation potential versus oligomer number is approximately linear for 2-5 arenes, with smaller decreases for longer oligomers.12,13 Plausibly, hole delocalization is more effective in the oligofluorenes, which have weakly interacting, but equivalent, arene units. In contrast, the oligophenylurea HOMO is localized on the inner phenyl rings of 4 and the higher oligomers (Figure 5). Concluding Remarks. The nature of the electronic interactions within the oligophenylureas is influenced by an unusual folded geometry in which the phenyl rings adopt a splayed faceto-face relationship. This geometry is responsible for the shielding of phenyl protons by neighboring rings, which are stronger for inner versus outer rings. This behavior parallels that of the multilayered cyclophanes, which display stronger shielding as a consequence of having parallel phenyl rings with
Electronic Interactions in Tertiary Oligophenylureas shorter average π-stacking distances. The absorption and emission spectra of the oligophenylureas display only small shifts upon addition of inner rings. The lowest energy transition is urea-localized and has n,π* character, accounting for the absence of room-temperature emission. Phosphorescence is assigned to a phenyl-localized π,π* state. The electronic spectral shifts are less pronounced than those of the corresponding multilayered cyclophanes. Convergence of the spectral properties of the phenylureas requires only three phenyl rings, whereas convergence for the cyclophanes requires 5 or 6 layers. Convergence of the spectral properties of the linear polyphenylenes requires an even larger number of phenyl rings.2 We have previously reported that the oligophenylureas 2-5 undergo reductive elimination of their urea linkers upon reaction of potassium in HMPA. In the absence of experimental values for reduction potentials and reaction rates, it is not possible to compare the reactivity of the phenyl ureas anion radicals. The oxidation potentials of the oligophenylureas decrease abruptly upon incremental addition of a fourth phenyl ring (Figure 7). We attribute this change to charge delocalization involving the two inner rings of 4 and the higher oligomers, on the basis of the appearance of their HOMO orbitals (Figure 5). In contrast, the HOMO’s of 1-3 are localized on a single phenyl ring (Figures 4,5). Hole delocalization in 4 suggests that the oligophenylureas may serve as bridging elements for hole transport in molecular electronic devices. Studies of donorbridge-acceptor systems with oligophenylurea bridges are currently in progress in our laboratory. Acknowledgment. Funding for this project was provided by NSF grants CHE-0100596 and CHE-0400663 and by a Undergraduate Research Grant from Northwestern University to G.B.D. References and Notes (1) (a) Tour, J. M. Chem. ReV. 1996, 96, 537-554. (b) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (2) Zojer, E.; Shuai, Z.; Leising, G.; Bredas, J. L. J. Chem. Phys. 1999, 111, 1668-1675. (3) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577-5584. (4) Vo¨gtle, F. Cyclophane Chemistry; Wiley: Chichester, England, 1993.
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