Zwitterionic Ladder Stilbenes with Phosphonium and Borate Bridges

ARTICLE pubs.acs.org/Organometallics

Zwitterionic Ladder Stilbenes with Phosphonium and Borate Bridges: Intramolecular Cascade Cyclization and StructurePhotophysical Properties Relationship Aiko Fukazawa,*,† Eriko Yamaguchi,† Emi Ito,† Hiroshi Yamada,† Jian Wang,‡ Stephan Irle,*,‡ and Shigehiro Yamaguchi*,†,§ †

Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya, 464-8602, Japan Institute for Advanced Research, Nagoya University, Furo, Chikusa, Nagoya, 464-8602, Japan § CREST, Japan Science and Technology Agency (JST), Furo, Chikusa, Nagoya, 464-8602, Japan ‡

bS Supporting Information ABSTRACT: The synthesis and properties of the phosphonium- and borate-bridged stilbenes are reported. The zwitterionic ladder stilbenes were synthesized by the intramolecular cascade cyclization from the phosphanyl- and boryl-substituted diphenylacetylenes 1. The study of the substituent effects of the phosphanyl and boryl groups revealed the significant dependence of the reactivity on both the nucleophilicity of the phosphanyl group and the electrophilicity of the boryl group. Theoretical calculations indicated that this reaction is initiated by the nucleophilic attack of the phosphanyl group, irrespective of the degree of the electrophilicity of the boryl group, in contrast to the analogous intermolecular reaction promoted by the frustrated Lewis pairs of R3P and B(C6F5)3. Moreover, even in the case of compound 1c, which does not undergo cascade cyclization under thermal conditions, photoexcitation promoted the cyclization to produce the corresponding zwitterionic stilbene 2c, indicative of the potential use as a photoresponsive material. The photophysical properties of a series of zwitterionic stilbenes, 2ae, also display dependence on the substituents. The fluorescence quantum yields (ΦF) of the stilbenes 2d and 2e, with electron-withdrawing diarylboryl groups, were an order of magnitude higher than those of 2ac, with a dimesitylboryl group. Time-resolved fluorescence spectroscopy as well as the measurement of ΦF in the polymer matrices revealed that the electron-withdrawing diarylboryl groups significantly retarded the nonradiative decay process from the singlet excited state, resulting in a higher ΦF.

’ INTRODUCTION Polycyclic ladder π-conjugated structures are attractive as fundamental skeletons for novel optoelectronic materials. Their flat structures result in the effective extension of π conjugation and are beneficial to the formation of an effective π stacking in the solid state. In addition, the rigidity of the skeleton is crucial for attaining an intense luminescence. From these points of view, considerable efforts have been devoted to producing new ladder π-conjugated materials,1 some of which have indeed exhibited high performances in optoelectronic applications, such as light-emitting diodes,2 organic transistors,1e,3 and photovoltaic cells.2i,4 One key issue for the molecular design of ladder π-conjugated skeletons is the tuning of their electronic structures. A promising strategy is to incorporate main group elements other than C, N, and O. The exploitation of the electronic and structural features of the main group elements enables us to gain unusual electronic structures and thereby intriguing properties.5,6 The other issue for the design is the efficiency of the ladder skeleton construction. A simple and efficient synthetic method that is applicable to r 2011 American Chemical Society

large-scale synthesis is highly desirable for practical applications of the compounds in optoelectronic devices. In this context, an important class of reactions is the intramolecular 5-endo-dig cyclization of phenylacetylenes with heteroatom substituents at the ortho-position, which is a general methodology for the synthesis of benzoheterole skeletons. Importantly, extending this monocyclization to double cyclizations using o,o0 -disubstituted diphenylacetylenes as the key precursors enables the construction of heteroatom-bridged ladder stilbenes. On the basis of this synthetic strategy, a series of ladder stilbenes and analogous compounds with various main group elements, such as group 14 silicon,7,8 group 15 phosphorus,9 and group 16 sulfur and selenium,10 have been synthesized. As an example of the syntheses, we have recently reported the intramolecular cascade cyclization of phosphanyl- and borylsubstituted diphenylacetylenes 1, which produced phosphonium- and borate-bridged stilbenes 2, as shown in Figure 1a.11 Received: May 26, 2011 Published: June 22, 2011 3870

dx.doi.org/10.1021/om200453w | Organometallics 2011, 30, 3870–3879

Organometallics

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Scheme 1. Substituent Dependence of the Intramolecular Cyclization

Figure 1. Zwitterionic ladder stilbenes: (a) synthesis by intramolecular cascade cyclization, (b) KohnSham HOMO and LUMO of 2 (R = Me, Ar = Ph) calculated at the B3LYP/6-31G(d) level, and (c) an example of a π-extended derivative.

Among the various ladder compounds so far reported, the zwitterionic ladder stilbenes 2 are particularly unique in terms of their electronic structures. Thus, because of the zwitterionic structure, these compounds have significantly large dipole moments. For instance, the calculated dipole moment of stilbene 2 with R = Me and Ar = Ph is 11.8 D at the B3LYP/6-31G(d) level,11 which is larger even compared to that of 1-(4dimethylaminophenyl)-2-(4-nitrophenyl)ethene (10.6 D, same level of theory). In addition, the zwitterionic moieties significantly alter the electronic structure of the parent stilbene skeleton. Namely, the electron-donating borate moiety shifts the HOMO level up in energy, and the electron-withdrawing phosphonium moiety lowers the LUMO level (Figure 1b). Thus, the modification of the ladder skeleton with B and P atoms is suitable for attaining narrow HOMOLUMO gaps. These characteristic features make such compounds attractive for various optoelectronic applications, such as two-photon absorption materials.12 Indeed, we recently demonstrated that one of the π-extended derivatives (Figure 1c) shows a large two-photon absorption cross section upon excitation by near-infrared light. The other important aspect of this chemistry is the facile construction of the zwitterionic ladder structure. The incorporation of the Lewis basic phosphanyl group and Lewis acidic boryl group onto the appropriate positions on opposite sides of the diphenylacetylene moieties in the same molecule results in spontaneous cascade cyclization to produce the ladder structure. This reaction can be regarded as an intramolecular version of the reactions promoted by the frustrated Lewis pairs (FLPs) of bulky R3P and highly Lewis acidic B(C6F5)3.13 Recently, various fascinating reactions promoted by the FLP systems, such as the activations of H2,14,15 CO2,16 and alkenes,1719 have been reported. The activation of alkynes itself was also reported by

Stephan20 and Erker.21 The point of these intermolecular reactions is the use of the highly Lewis acidic B(C6F5)3. In contrast, the present intramolecular cyclization can be promoted even using the less Lewis acidic boryl (Ar2B) group. Elucidating this inherent difference and clearly demonstrating the features of the intramolecular reaction would allow us to synthesize a broader scope of ladder materials with intriguing properties and functions. In this article, we report the comprehensive study of the phosphonium- and borate-bridged zwitterionic ladder stilbenes from experimental and theoretical points of view. The study of the substituent effects of the phosphanyl and boryl groups on the reactivity enables a better understanding of the mechanism of the intramolecular cyclization. In addition, a new photochemical reaction of the phosphanyl- and boryl-substituted alkynes is disclosed. The photophysical properties of the zwitterionic ladder compounds are greatly influenced by the substituents, and these effects are also discussed in the present study.

’ RESULTS AND DISCUSSION 1. Thermal Intramolecular Cascade Cyclization. 1.1. Substituent Effects on the Reactivity. In our previous report, we found

that the reactivity of the phosphanyl- and boryl-substituted diphenylacetylenes 1 in the intramolecular cascade cyclization depends on the electronic effect of the substituents on the phosphorus atom.11 The reactions we examined are shown in Scheme 1. Starting from the Mes2B- and bromo-substituted diphenylacetylene 3, the lithiation with n-BuLi followed by the treatment with several kinds of R2PCl generated the corresponding phosphanyl- and boryl-substituted 1 as a key intermediate. While the highly nucleophilic (t-Bu)2P- or Cy2P-substituted diphenylacetylenes 1a and 1b spontaneously underwent cyclization at ambient temperature to produce the zwitterionic stilbenes 2a and 2b, respectively, the less nucleophilic Ph2P-substituted 1c remained intact even upon heating at 80 °C (Scheme 1). These results clearly demonstrated that the nucleophilicity of the R2P groups is crucial for the promotion of the cyclization. This is in contrast to the intermolecular alkyne activation with the frustrated Lewis pairs of B(C6F5)3/PR3,20,21 where highly electrophilic B(C6F5)3 obviously plays an important role in promoting the reaction. This difference prompted us to study how the electrophilic boryl group contributes to the promotion of the cascade cyclization. In order to explore the effect of the electrophilicity of the boryl groups, we chose Ph2P-substituted diphenylacetylenes with 3871

dx.doi.org/10.1021/om200453w |Organometallics 2011, 30, 3870–3879

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Scheme 2. Intramolecular Cascade Cyclization of Ph2P-Substituted Diphenylacetylenes 1d and 1e

several Ar2B groups as the precursors. Because the Ph2P group does not have sufficient nucleophilicity to promote the cyclization, we envisioned that we could assess the effect of the boryl group on the cyclization. As the aryl substituent in the Ar2B groups we employed 2,6-dimethylphenyl groups with chlorine (1d) or a perfluorobutyl group (1e) at the 4-position. To generate these precursors, we first synthesized a Ph2P- and bromo-substituted diphenylacetylene 4 and then introduced the Ar2B groups, as outlined in Scheme 2. Thus, the selective monolithiation of bis(bromophenyl)acetylene with n-BuLi in THF followed by treatment with Ph2PCl produced 4 in 81% yield. After the lithiation with t-BuLi, the phosphanyl- and borylsubstituted diphenylacetylene 1d and 1e were in situ generated by treatment with the corresponding Ar2BF, which was also prepared prior to the reaction and used without purification.22 Notably, whereas both compounds 1d and 1e were inert at ambient temperature, they underwent cyclization upon heating to 60 °C. The reactions were complete within 4 and 2.5 h and gave the zwitterionic stilbenes 2d and 2e in 44% and 49% yields, respectively. These results demonstrate that the electrophilic boryl group also facilitates the cyclization as the nucleophilic R2P group does. The cyclized products 2d and 2e were sufficiently stable toward moisture and air. 1.2. Mechanistic Insights. As demonstrated in Schemes 1 and 2, the reactivity of the diphenylacetylenes 1 toward the cascade cyclization is highly dependent on the substituents on both the phosphorus and boron atoms. The acetylenes 1 can be classified into three types: (I) compounds with a highly nucleophilic R2P group, which spontaneously undergo cyclization at room temperature (1a and 1b), (II) compounds with a highly electrophilic Ar2B group, which undergo cyclization upon heating (1d and 1e), and (III) the compound with a weakly nucleophilic Ph2P group and a weakly electrophilic Mes2B group, which does not undergo cyclization under the thermal conditions (1c). To gain insight into the differences in the reactivity of these three types of compounds, we conducted theoretical calculations for the reactions of 1a, 1c, and 1e, as representative examples for each type of compound. The geometries of the starting acetylenes 1, the transition states (TSs), and the cyclized products 2 were optimized at the B3LYP/def2-SV(P) level of theory using the TURBOMOLE program.23 Three possible mechanisms were examined, as shown in Figure 2: (1) nucleophilic activation of the acetylene through a

Figure 2. Three possible types of transition states (a) and the geometries of TSP for the reactions of 1a (b) and 1e (c), optimized at the B3LYP/def2-SV(P) level. Hydrogen atoms are omitted for clarity.

TSP, in which the phosphanyl group first approaches the acetylene moiety, (2) concerted activation through a TSPB, in which both the phosphanyl group and the boryl group simultaneously approach, and (3) electrophilic activation of the acetylene by the Lewis-acidic boryl group through TSB. In all cases of 1a, 1c, and 1e, the TSP and TSPB were obtained as stationary points on the ground-state potential energy surfaces with one imaginary frequency, whereas the TSB could not be found. The energies of these transition states as well as the cyclized products 2 relative to the starting acetylenes 1 are summarized in Table 1. According to the quantum chemical calculations, TSP is lower in energy compared to TSPB by 7.88.6 kcal mol1 in all cases, indicating that the concerted mechanism is energetically unfavorable relative to the nucleophilic attack of the phosphanyl group to the acetylene moiety. Solvent effects are expected to 3872


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Table 1. Energies of the Transition States for Intramolecular Cascade Cyclization and the Cyclized Products Relative to 1a reactant

TSP/kcal mol1b

TSPB/kcal mol1b

2/kcal mol1b

1a

+27.2

+35.3

3.9 (2a)

1c 1e

+30.4 +29.1

+39.0 +36.9

+3.0 (2c) 2.5 (2e)

a

Calculated at the B3LYP/def2-SV(P) level with zero-point corrections and entropy term corrections. b Energies relative to 1.

Figure 4. Energy profiles for the intramolecular thermal cyclization through TSP starting from 1a (R = t-Bu, Ar = Mes), 1c (R = Ph, Ar = Mes), and 1e (R = Ph, Ar = 4-C4F9-2,6-Me2C6H2) calculated at the B3LYP/def2-SV(P) level with zero-point corrections and entropy term corrections. All energy values are given relative to 1.

Figure 3. Reaction pathways along the IRC given as a two-dimensional map, where the interatomic distances between B and C2 are plotted against the P and C1 distances for the intramolecular cascade cyclizations of 1a (black), 1c (blue), and 1e (red).

lower the energies of polar TSs and zwitterionic products further, while relative trends should be preserved. In addition, as shown in Figure 2, the geometries of the TSPs for the reactions of 1a and 1e are similar to each other, irrespective of the nucleophilicity of the R2P groups as well as the electrophilicity of the Ar2B groups. The interatomic distances of P---C1 and B---C2 in the TSPs were 2.232 (1e) to 2.254 Å (1a) and 3.120 (1a) to 3.171 Å (1e). To elucidate the mechanism more closely, the reaction pathways were investigated using the intrinsic reaction coordinate (IRC) method24 at the B3LYP/6-31G(d) level as implemented in the Gaussian 03 package.25 For this purpose, transition-state geometries were reoptimized due to the different functional form of the B3LYP implementation in TURBOMOLE and Gaussian and due to the use of a different basis set. The transition-state geometries and relative energies were however very similar. The minimum energy paths were tracked from the TSP to the starting materials 1 and to the products 2, but no stationery point could be found, indicating that the TSPs obtained for the reactions of 1a, 1c, and 1e are the sole transition state for this cyclization process. The resulting reaction pathways are shown in Figure 3 as a two-dimensional map, where the B---C2 distance was plotted as a function of the P---C1 distance. In all cases, the phosphorus atom first approaches the acetylene moiety with only a slight shortening of the B---C2 distance. In the TSP, the PC1 bond is about to form. Then the B---C2 distance dramatically decreases,

leading to the formation of the BC2 covalent bond. Worth noting is that the slopes of the curves for 1a, 1c, and 1e are very similar to one another. These results clearly demonstrate that the cyclization proceeds through essentially the identical mechanism, irrespective of the substituents on the phosphorus and boron atoms. This might be due to the long PC4 bond distance in 1, which results in close proximity between the acetylene moiety and the phosphorus atom (vide infra). The calculated activation energies are in the order 1c > 1e > 1a (Figure 4). This trend is consistent with the experimental observations. The major factor contributing to this trend might be the stability of the products 2 relative to the reactants 1. While compounds 2a and 2e are more stable than the corresponding reactants 1a and 1e by 3.9 and 2.5 kcal mol1, respectively, only compound 2c is less stable than 1c, by 3.0 kcal mol1, and thereby the reaction becomes endothermic. These differences are obviously due to the electronic effects of the substituents on the phosphorus and boron atoms. Thus, the electron-donating t-Bu groups on the phosphorus atom can stabilize the tetracoordinated cationic phosphonium moiety. Besides, the electron-withdrawing 4-perfluorobutyl-2,6-dimethylphenyl groups on the boron atom also can stabilize the tetracoordinated anionic borate moiety. 2. Photochemical Cyclization. The intramolecular cascade cyclization from the phosphanyl- and boryl-substituted diphenylacetylenes 1 to the phosphonium- and borate-bridged stilbene 2 is a certain type of isomerization. We envisioned that this isomerization can also be promoted by light. To explore this possibility, we examined the photoreaction of 1c, which does not undergo cyclization under thermal conditions. 2.1. Structural Features of Ph2P- and Mes2B-Substituted Diphenylacetylene. Prior to the study of the photoreaction, the structure of 1c was determined by an X-ray crystallographic analysis, as shown in Figure 5. In this structure, the boron atom adopts a trigonal-planar geometry with the sum of the CBC angles of 359.97°. The distances between P1 and C1 and between B1 and C2 are 3.617 and 3.849 Å, respectively, 3873


Organometallics

Figure 5. X-ray crystal structure of 1c (50% probability thermal ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): C1C2, 1.202(3); C1C9, 1.433(3); C2C3, 1.434(3); C3C4, 1.407(3); C9C10, 1.416(3); P1C4, 1.8395(19); B1C10, 1.576(3); C2 C1C9, 173.3(2); C1C2C3, 177.9(2); C4P1C15, 102.19(8); C4P1C21, 100.82(8); C15P1C21, 101.46(9); C10B1C27, 122.41(17); C10B1C33, 114.70(16); C27B1C33, 122.86(17); P1C4C3C2, 0.9(2); C1C9C10B1, 179.7(2).

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Figure 6. UVvis absorption (solid line) and fluorescence spectra (broken line) of 1c (black) and its cyclized product 2c (green) in THF. Photographs of 1c and 2c under irradiation with black light at 365 nm are shown in the inset.

Table 2. Characterization of the Longest Absorption Band of 1ca excited

transition energy/eV

oscillator

assignments

state

(wavelength/nm)

strength f

(CI coefficient)

S1

3.41 (363)

0.0825

HOMO f LUMO (0.64) HOMO1 f LUMO (0.17) HOMO2 f LUMO (0.19)

Scheme 3. Intramolecular Photocyclization of 1c S2

3.50 (354)

0.0801

HOMO f LUMO (0.17) HOMO1 f LUMO (0.65) HOMO3 f LUMO (0.14)

a

demonstrating that the phosphorus atom is located closer to the acetylene moiety than the boron atom. This is presumably due to the longer P1C4 bond than the B1C10 bond. It is also interesting to note that the acetylene moiety is slightly bent toward the phosphorus atom with the C2C1C9 and C1C2C3 bond angles of 173.3(2)° and 177.9(2)°, respectively. 2.2. Photoreactions. Upon photoirradiation with black light, a colorless solution of 1c in CH2Cl2 indeed turned into a bright yellow solution, which indicated the formation of the cyclized product. We therefore optimized the reaction conditions. The best result was obtained when the irradiation of the light (>320 nm), using a high-pressure mercury lamp with an aqueous KNO3 solution (0.4 M) as a solution filter,26 was performed for 1 h on a 1 mM solution of 1c in CH2Cl2. The purification by reversed-phase silica gel column chromatography gave the cyclized product 2c in 61% yield as a yellow solid (Scheme 3).27 Notably, the 5-endo-dig mode of cyclization is unprecedented for the photoinduced intramolecular cyclization of ortho-substituted arylacetylenes to the best of our knowledge.28,29 This photocyclization should be useful as an alternative method to convert the inert acetylene to the zwitterionic cyclized product. It is also noteworthy that the photophysical properties of the reaction mixture substantially changed before and after the photoirradiation. Figure 6 shows the UVvis absorption and fluorescence spectra of the reactant 1c and the cyclized product 2c. The absorption and fluorescence maxima are significantly

Calculated at the B3LYP/6-31G(d) level using the TD-DFT method.

red-shifted by 76 and 92 nm, respectively, upon cyclization (1c: λabs = 336 nm, λem = 452 nm; 2c: λabs = 412 nm, λem = 544 nm). In addition to the change in photophysical properties, the dipole moment of the present molecular system is significantly changed upon photoisomerization (1c: 1.05 D, 2c: 12.7 D, calculated at the B3LYP/6-31G(d) level using the Gaussian 03 program). This characteristic may increase the potential utility of this molecular system as a photoresponsive material. 2.3. Theoretical Calculations. To gain mechanistic insights into the photocyclization, time-dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-31G(d) level using the Gaussian 03 program.25 The vertical excitation calculations suggested that the longest-wavelength absorption band of 1c at 336 nm is ascribed to two superimposed electronic transitions to the lowest and second-lowest excited states, S1 and S2 (Table 2). Figure 7 shows KohnSham molecular orbitals of 1c relevant to these two excited states. The S0 f S1 transition mainly consists of the intramolecular charge transfer from the HOMO, which is mostly localized on the lone pair of the phosphanyl group, to the LUMO, which shows a significant contribution of a boron p orbital. On the other hand, the S0 f S2 transition is mainly attributable to the intramolecular charge transfer from HOMO1, which is mostly localized on the mesityl groups on the boron atom, to the LUMO. Among these excited states, the S1 should be relevant for the cyclization. We therefore next conducted the geometry optimization of the S1 local minimum for 1c at the TDB3LYP/def2-SV(P) level of theory. The calculations revealed that in the excited-state minimum energy structure (Figure 8), 3874


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Figure 8. Optimized geometry of S1 for 1c calculated at the B3LYP/ def2-SV(P) level. Hydrogen atoms are omitted for clarity.

Figure 7. Pictorial representation and the energy levels of the relevant KohnSham molecular orbitals of 1c, calculated at the B3LYP/ 6-31G(d) level.

while the acetylene moiety still maintains a linear geometry, the C1C2 and C2C3 bond distances are elongated and shortened, respectively, compared to those of its S0. In addition, the P1C4 and B1C10 bond distances become shorter than those in the S0. These structural features demonstrate the significant quinoidal character of the diphenylacetylene moiety including the phosphanyl and boryl moieties for the S1 minimum energy structure. Starting from this structure, we attempted to find a possible transition-state structure for the excited-state cyclization by bending the geometry of the acetylene moiety. However, all attempts resulted in the direct convergence into the cyclized product 2c, indicating that the cyclization likely proceeds with almost no energy barrier from the S1 minimum energy structure. 3. Substituent Effects on the Photophysical Properties. Notably, we found that the zwitterionic ladder stilbenes 2 showed significantly different fluorescence profiles from one another. Compounds 2d and 2e showed intense green fluorescences, whereas the other derivatives 2ac showed only faint fluorescences. We are interested in this difference and thus investigated the substituent effects on the photophysical properties. The data of the UVvis absorption and fluorescence spectra, fluorescence quantum yields, and the excited-state dynamics for 2ae are summarized in Table 3. In the UVvis absorption spectra, the zwitterionic stilbenes 2ce, with phenyl groups on the phosphorus atom, exhibited

red-shifted absorption maxima (λabs: 2c, 412 nm; 2d, 419 nm; 2e, 416 nm) compared to the alkyl-substituted 2a and 2b (λabs: 2a, 395 nm; 2b, 396 nm). In the fluorescence spectra, their emission maximum wavelengths vary in the range 517544 nm, and their fluorescence quantum yields (ΦF) are significantly different between 2ac (2a, 0.03; 2b, 0.02; 2c, 0.03) and 2d and 2e (2d, 0.20; 2e, 0.42). It is noteworthy that the ΦF values are related to the extent of the Stokes shifts. Namely, while the Stokes shifts of 2ac are in the range 59006300 cm1, those of 2d and 2e (5100 and 5000 cm1, respectively) are about 1000 cm1 lower than those of 2ac. Thus, there is a trend that the ΦF becomes higher as the Stokes shift decreases. To elucidate the origin of this trend, we determined the fluorescence lifetimes (τs) by the time-resolved fluorescence spectroscopy. The more emissive 2d and 2e showed longer fluorescence lifetimes than the less emissive 2ac. According to the ΦF and τs values, we calculated the radiative (kr) and nonradiative (knr) decay rate constants from the singlet excited states, summarized in Figure 9. Whereas the kr values are comparable to one another for all the compounds ((0.872.2) 107 s1), the knr values of the more emissive 2d and 2e (8.2 107 and 3.1 107 s1 for 2d and 2e) are 1 order of magnitude lower compared to those of 2ac (3.5 108, 4.3 108, and 6.5 108 s1 for 2a, 2b, and 2c, respectively). Thus, the high ΦF values for compounds 2d and 2e are attributable to the suppression of the nonradiative decay processes. The fact that 2d and 2e have smaller Stokes shifts indicates that the degree of structural relaxation in the excited state for these compounds is smaller compared to those for 2ac. Both 2d and 2e have electron-withdrawing aryl groups on the boron atom, which should decrease the electron-donating character of the anionic borate moiety. This substituent effect may be responsible for the smaller structural relaxation in the excited states for 2d and 2e, leading to the retardation of the nonradiative decay process and thereby the high fluorescence quantum yields. We also evaluated the fluorescent properties in polymer matrices. All the compounds 2ae dispersed in a poly(methylmethacrylate) (PMMA) matrix displayed higher ΦF values than those in THF (ΦF in PMMA: 2a, 0.36, 2b, 0.33; 2c, 0.22; 2d, 0.53; 2e, 0.52). This enhancement of the ΦF values in the PMMA matrix is likely due to the suppression of the 3875


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Table 3. Photophysical Properties of 2ae in THF fluorescence

absorption compd

λabs/nma (νabs/cm1)

log ε

λem/nmb (νem/cm1)

ΦFc (ΦF in PMMA)e

τsd

Stokes shift/cm1

2a

395 (25 300)

3.77

526 (19 000)

0.03 (0.36)

2.8

6300

2b

396 (25 300)

3.73

517 (19 300)

0.02 (0.33)

2.3

6000

2c

412 (24 300)

3.54

544 (18 400)

0.03 (0.22)

1.5

5900

2d

419 (23 900)

3.55

531 (18 800)

0.20 (0.53)

9.8

5100

2e

416 (24 000)

3.66

525 (19 000)

0.42 (0.52)

19

5000

a

Only the longest maximum wavelengths are shown. b Emission maxima upon excitation at the absorption maximum wavelengths. c Absolute fluorescence quantum yields determined by a calibrated integrating sphere system within (3% error. d Fluorescence lifetime. e 1 wt % in PMMA.

the analogous intermolecular reactions by the frustrated Lewis pairs of bulky R3P and B(C6F5)3, in which the concerted activation by the phosphines and the borane is suggested by recent theoretical studies.3032 Second, the photocyclization also can proceed to produce zwitterionic ladder stilbenes. This is important as an alternative route to the ladder skeleton. Notably, the cyclization results in significant changes in the photophysical properties, indicative of the potential application as a photoresponsive material. Third, the fluorescence properties of the zwitterionic stilbenes are significantly influenced by the electronic nature of the substituents on the boryl group. The increase in the electron-withdrawing effect of the Ar2B group results in the significant enhancement of the fluorescence intensity. All these findings demonstrate the characteristic features of the zwitterionic ladder systems. The balance of the electronic nature of the phosphanyl and boryl groups is crucial for their synergy in terms of both the reactivity and properties. Fine modification of the peripheral substituents would further increase the potentials of the zwitterionic ladder systems as attractive optoelectronic materials. Figure 9. Comparison of radiative (kr) and nonradiative (knr) decay rate constants among the various zwitterionic stilbenes in THF. The rate constants were calculated using ΦF and τs according to the formulas kr = ΦF/τs and knr = (1 ΦF)/τs.

rotational vibration of the substituents on the phosphorus and boron atoms, which results in the retardation of the nonradiative decay process from the singlet excited state. Notably, the two compounds 2d and 2e still exhibited higher ΦF than 2a, 2b, and 2c even in the polymer matrix. This fact supports the importance of the electronic effect of the electron-withdrawing Ar2B groups in the higher ΦF values in both solution and polymer matrix.

’ CONCLUSION The incorporation of the zwitterionic phosphonium and borate moieties with a ring-fused structure is a powerful way to electronically modify the stilbene skeletons. Advantageously, this unique ladder stilbene skeleton can be easily constructed by intramolecular cascade cyclization from the phosphanyl- and boryl-substituted diphenylacetylene precursors. In this chemistry, the present systematic study in terms of the substituent effects has provided three important findings. First, the cascade cyclization under thermal conditions is mainly promoted by the nucleophilic attack of the phosphanyl group, while the electrophilic boryl group also contributes to the lowering of the activation energy. This mechanism is in contrast to those of

’ EXPERIMENTAL SECTION General Procedures. Melting points (mp) were determined with a Stanford Research System OptiMelt MPA100 instrument. Decomposition temperatures were defined by 5% weight loss temperatures (Td5), which were determined by differential thermal gravimetric analysis using a Seiko SII EXSTAR TG/DTA 6200 instrument. 1H and 13C{1H} NMR spectra were recorded with a JEOL AL-400 (400 MHz for 1H and 100 MHz for 13C) spectrometer in CD2Cl2. 13C{1H,19F} NMR spectra were recorded with a JEOL ECA600 spectrometer (150 MHz for 13C). Chemical shifts are reported in δ ppm using CH2Cl2 (5.30 ppm) for 1H NMR and CD2Cl2 (53.52 ppm) for 13C NMR in CD2Cl2 as an internal standard. 11B{1H}, 19F, and 31P{1H} NMR spectra were recorded with a JEOL AL-400 spectrometer (128 MHz for 11B, 376 MHz for 19F, and 162 MHz for 31P) using BF3 3 OEt2 (0.0 ppm), CF3COOH (78.5 ppm for 19F), and H3PO4 (0.0 ppm for 31 P) as an external standard, respectively. Mass spectra were measured with a Bruker micrOTOF Focus spectrometer with the APCI ionization method. Thin layer chromatography was performed on plates coated with 0.25 mm thick silica gel 60 F254 and reversed-phase silica gel 60RP18 F254S (Merck). Column chromatography was performed using silica gel PSQ100B (Fuji Silysia Chemical) or Wakosil 40C18 reversed-phase silica gel (Wako). Recycling preparative gel permeation chromatography was performed using LC-9201 (Japan Analytical Industry) equipped with a polystyrene gel column (JAIGEL 1H and 2H, Japan Analytical Industry) and toluene as the eluent. A 450 W high-pressure mercury lamp, UM-452 (USHO Inc.), was used for photoirradiation. Anhydrous Et2O, THF, hexane, and CH2Cl2 were purchased from Kanto 3876


Organometallics Chemicals. Bis(2-bromophenyl)acetylene33 and [2-(dimesitylboryl)phenyl][2-(diphenylphosphanyl)phenyl]acetylene (1c)11 were prepared according to the literature methods. All reactions were performed under an argon atmosphere, unless stated otherwise.

(2-Bromophenyl)[2-(diphenylphosphanyl)phenyl]acetylene (4). To a solution of bis(2-bromophenyl)acetylene (10.0 g, 27.3 mmol) in anhydrous THF (97 mL) was added n-BuLi in hexane (1.64 M, 16.6 mL, 27.3 mmol) dropwise over 1.5 h at 78 °C. After stirring for 1 h, Ph2PCl (5.0 mL, 6.1 g, 28 mmol) was added over 10 min, and the resulting mixture was stirred for 2 h at the same temperature. The resulting mixture was then concentrated under reduced pressure. Toluene (deoxygenated by Ar bubbling for 30 min) was added, and the resulting suspension was filtered through a plug of Celite under an argon atmosphere and rinsed with toluene. After concentration of the filtrate, the resulting mixture was subjected to silica gel column chromatography (PSQ100B, toluene, Rf = 0.93), followed by further purification by washing with cyclohexane to afford 9.77 g (22.1 mmol) of 4 in 81% yield as colorless solids: mp 108.4109.2 °C. 1H NMR (400 MHz, CD2Cl2): δ 6.84 (dd, J = 3.2 Hz, 7.6 Hz, 1H), 7.127.34 (m, overlapped, 15H), 7.53 (d, J = 8.0 Hz, 1H), 7.63 (dd, J = 3.2 Hz, 7.0 Hz, 1H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 92.98 (d, C, 3JCP = 7.5 Hz), 94.10 (d, C, 4 JCP = 2.4 Hz), 125.08 (d, C, 2JCP = 17.3 Hz), 127.06 (s, CH), 127.32 (s, C), 127.61(s, C), 128.50 (s, CH), 128.61 (s, CH), 128.82 (s, CH), 128.82 (d, CH, 2JCP = 12.4 Hz), 129.67 (s, CH), 132.37 (s, CH), 132.68 (s, CH), 132.83 (d, CH, 3JCP = 3.3 Hz), 133.53 (s, CH), 134.14 (d, CH, 2 JCP = 19.7 Hz), 136.58 (d, C, 1JCP = 10.7 Hz), 140.82 (d, C, 1JCP = 14.0 Hz). 31P{1H} NMR (162 MHz, CD2Cl2): δ 8.93. HRMS (APCI): calcd for C26H19BrP ([M + H]+) 441.0408, found 441.0408. Compound 2d. To a mixture of Mg (turnings, 0.166 g, 6.83 mmol) and anhydrous Et2O (5.0 mL) was added 5-chloro-2-iodo-1,3-dimethylbenzene (1.33 g, 4.99 mmol). The resulting mixture was stirred at reflux temperature for 2 h. The prepared Grignard reagent in Et2O was added dropwise into BF3 3 OEt2 over 1 min at 0 °C. After stirring for 1 h, all volatiles were removed under reduced pressure, and anhydrous hexane (10 mL) was added. The resulting suspension was filtered under an argon atmosphere and rinsed with anhydrous hexane. After concentration of the filtrate, the resulting solids were dissolved in anhydrous THF (2.0 mL) to prepare a Ar2BF solution. Another brown-colored Schlenk flask was charged with 4 (0.696 g, 1.57 mmol) and anhydrous THF (10 mL). A pentane solution of t-BuLi (1.55 M, 2.0 mL, 3.2 mmol) was added to this mixture over 5 min at 78 °C. After stirring for 1 h, the aforementioned Ar2BF solution in THF was added over 10 min, and the resulting mixture was stirred at 78 °C for 3 h and then at 60 °C for 5 h. After removal of volatiles, the mixture was subjected to silica gel column chromatography (PSQ100B, CH2Cl2, Rf = 0.93), followed by further purification by washing with hexane to afford 0.457 g (0.702 mmol) of 2d in 44% yield as yellow solids. Mp > 267 °C (decomposition without melting was observed at 267 °C under N2 atmosphere). Td5: 267 °C. 1H NMR (400 MHz, CD2Cl2): δ 1.95 (s, 12H), 6.72 (s, 4H), 6.936.97 (m, 2H), 7.057.07 (m, 1H), 7.28 (dt, J = 4.80 Hz, 7.20 Hz, 1H), 7.527.67 (m, 7H), 7.747.83 (m, 7H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 25.60 (s, CH3), 118.80 (d, C, 1JCP = 82.4 Hz), 124.03 (s, CH), 125.57 (s, CH), 127.39 (s, CH), 127.75(d, CH, JCP = 11.6 Hz), 127.96 (d, CH, JCP = 9.1 Hz), 128.04 (d, C, 1JCP = 100.4 Hz), 128.15 (s, C), 129.83 (d, CH, JCP = 11.5 Hz), 130.42 (d, CH, JCP = 12.3 Hz), 131.68 (s, CH), 133.31 (d, CH, JCP = 11.5 Hz), 135.00 (d, CH, JCP = 3.3 Hz), 135.60 (d, CH, 4JCP = 1.6 Hz), 141.78 (d, C, JCP = 18.1 Hz), 143.18 (s, C), 150.89 (d, C, JCP = 26.4 Hz), signals of three carbons bound to the boron atom were not observed. 11B{1H} NMR (128 MHz, CD2Cl2): δ 5.78. 31P{1H} NMR (162 MHz, CD2Cl2): δ 13.36. HRMS (APCI): calcd for C42H34BCl2P ([M + H]+) 651.1946, found 651.1921. Compound 2e. This compound was prepared in a similar manner to that described for 2d, using 5-nonafluorobutyl-2-iodo-1,3-

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dimethylbenzene (2.24 g, 4.98 mmol) and 4 (0.876 g, 1.99 mmol) as starting materials. Purification by filtration with a pad of silica gel (PSQ100B, AcOEt, Rf = 0.93), followed by washing with hexane, afforded 0.998 g (0.980 mmol) of 2e in 49% yield as yellow solids. Mp > 275 °C (decomposition without melting was observed at 275 °C under N2 atmosphere). Td5: 275 °C. 1H NMR (400 MHz, CD2Cl2): δ 1.99 (s, 12H), 6.94 (s, overlapped, 4H), 6.947.00 (m, 2H), 7.10 (d, J = 6.4 Hz, 1H), 7.29 (dt, J = 7.4 Hz, 6.4 Hz, 1H), 7.507.69 (m, 7H), 7.737.82 (m, 7H). 13C{1H, 19F} NMR (150 MHz, CD2Cl2): δ 25.77 (s, CH3), 109.31 (s, C), 110.77 (s, C), 116.67 (s, C), 117.82 (s, C), 118.63 (d, C, JCP = 82.3 Hz), 120.39 (s, CH), 123.04 (t, C, JCF = 23.9 Hz), 124.40 (s, CH), 125.55 (s, CH), 125.80 (s, CH), 127.86 (d, CH, JCP = 8.2 Hz), 127.90 (d, C, JCP = 99.6 Hz), 127.96 (d, CH, JCP = 5.8 Hz), 129.19 (d, C, JCP = 144.0 Hz), 129.97 (d, CH, JCP = 10.7 Hz), 130.49 (d, CH, JCP = 13.1 Hz), 131.82 (s, CH), 133.35 (d, CH, JCP = 11.5 Hz), 135.12 (d, CH, JCP = 2.5 Hz), 135.66 (d, CH, JCP = 1.7 Hz), 141.78 (s, C), 142.04 (d, C, JCP = 18.2 Hz), 150.72 (d, C, JCP = 25.5 Hz). 11 B{1H} NMR (128 MHz, CD2Cl2): δ 5.03. 19F{1H} NMR (376 MHz, CDCl3): δ 125.40, 122.78, 110.17, 80.77. 31P{1H} NMR (162 MHz, CD2Cl2): δ 13.94. HRMS (APCI): calcd for C50H35BF18P ([M + H]+) 1019.2282, found 1019.2321. Compound 2c. A Schlenk flask was charged with 1c (0.101 g, 0.166 mmol) and CH2Cl2 (deoxygenated by Ar bubbling for 30 min, 136 mL) and was immersed in a solution filter (KNO3 0.4 M aqueous solution) with a thickness of ca. 5 cm. This mixture was irradiated with a highpressure mercury lamp for 1 h. After concentration under reduced pressure, the resulting mixture was subjected to reversed-phase silica gel column chromatography (Wakosil 40C18, CH3CN, Rf = 0.53) to afford 62.0 mg (0.102 mmol) of 2c as yellow solids. Mp > 147 °C (decomposition without melting was observed at 147 °C under N2 atmosphere). Td5: 147 °C. 1H NMR (400 MHz, CDCl3): δ 1.92 (s, 12H), 2.10 (s, 6H), 6.54 (s, 4H), 6.896.90 (m, 2H), 7.027.04 (m, 1H), 7.24 (dt, J = 4.8 Hz, 7.4 Hz, 1H), 7.487.71 (m, 7H), 7.727.79(m, 6H), 7.89 (dd, J = 3.4 Hz, 7.8 Hz, 1H). 13C{1H} NMR (100 MHz, CD2Cl2): δ 20.46 (s, CH3), 25.69 (s, CH3), 119.21 (d, C, 1 JCP = 82.3 Hz), 119.954 (s, CH), 123.48 (s, CH), 125.18 (s, CH), 126.54 (d, C, 1JCP = 127.2 Hz), 126.95 (d, C, 1JCP = 103.2 Hz), 127.47(d, CH, JCP = 10.7 Hz), 127.82 (s, C), 128.26 (d, CH, JCP = 9.1 Hz), 128.93 (s, C), 129.60 (d, CH, JCP = 10.7 Hz), 130.32 (d, CH, JCP = 13.2 Hz), 131.70 (s, CH), 133.37 (d, CH, JCP = 11.5 Hz), 134.82 (d, CH, JCP = 3.3 Hz), 135.42 (d, CH, 4JCP = 1.6 Hz), 141.04 (s, C), 141.81 (d, C, JCP = 18.2 Hz), 151.48 (d, C, JCP =25.6 Hz). 11B{1H} NMR (128 MHz, THFd8): δ 3.29. 31P{1H} NMR (162 MHz, THF-d8): δ 14.70. HRMS (APCI): calcd for C44H41BP ([M + H]+) 611.3039, found 611.3051. X-ray Crystallographic Analysis of 1c. Single crystals of 1c suitable for X-ray crystallographic analysis were obtained by recrystallization from cyclohexane. Intensity data were collected at 100 K with Mo KR radiation (λ = 0.71070 Å) and a graphite monochromator. A total of 24 686 reflections were measured at a maximum 2θ angle of 50.0°, of which 6602 were independent reflections (Rint = 0.0610). The structure was solved by the direct method (SIR97)34 and refined by full-matrix least-squares on F2 (SHELXL-97).35 All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were placed using AFIX instructions. The crystal data are as follows: formula C44H40BP, fw = 610.54, crystal size 0.20 mm 0.20 mm 0.15 mm, monoclinic, P21/c (#14), a = 17.177(3) Å, b = 12.4963(19) Å, c = 17.480(3) Å, β = 114.0966(19)°, V = 3425.1(10) Å3, Z = 4, Dcalcd = 1.184 g cm3, μ = 0.111 mm1, R1 = 0.0490 (I > 2σ(I)), wR2 = 0.1207 (all data), GOF = 1.089. Photophysical Data Measurements. UVvisible absorption spectra were recorded on a Shimadzu UV-3510 spectrometer with a resolution of 0.5 nm. Emission spectra of solution samples were measured with a Hitachi F-4500 spectrometer with a resolution of 1 nm. Dilute solutions in degassed spectral grade solvents in a 1 cm 3877


Organometallics square quartz cell were used for the absorption and fluorescence measurements. Absolute fluorescence quantum yields were determined with a Hamamatsu C9920-02 calibrated integrating sphere system. Fluorescence lifetimes were measured with a Hamamatsu Picosecond fluorescence measurement system C4780. Computational Method. Geometry optimizations of 1a, 1c, 1e, 2a, 2c, 2e, and corresponding transition-state structures were performed with the B3LYP theory and Ahlrichs’ def2-SV(P) basis set, implemented in the TURBOMOLE program.23 All stationary points were optimized without any symmetry assumptions and characterized by normal coordinate analysis at the same level of theory (the number of imaginary frequencies, NIMAG, was 0 for minima and 1 for TSs). The imaginary frequency vibrational modes of the transition states were confirmed visually to correspond to the cyclization reaction coordinate. The reaction pathways were then investigated by means of the intrinsic reaction coordinate method with the B3LYP theory and 6-31G(d) basis set, using the Gaussian 03 program, after reoptimization of the transition states using this program.25 TD-DFT vertical excitation calculations of 1c were performed using the B3LYP theory and 6-31G(d) basis set as implemented in the Gaussian 03 program, whereas optimization of the 1c S1 minimum energy structure was performed at the TD-B3LYP/def2SV(P) level of theory as implemented in the TURBOMOLE code.

’ ASSOCIATED CONTENT

bS

Supporting Information. Spectral data for all new compounds, results of theoretical calculations, complete ref 24, and the crystallographic information file (CIF) of 1c. These materials are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected]. ac.jp; [email protected].

’ ACKNOWLEDGMENT This work was partly supported by Grants-in-Aid (Nos. 17069011, 19675001, and 20750029) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. H.Y. acknowledges the Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists. We thank Mr. Yutaka Maeda (Chemical Instrument Room, Research Center for Materials Science, Nagoya University) for his kind help with 1H- and 19F-decoupled 13C NMR measurements. ’ REFERENCES (1) (a) Scherf, U. J. Mater. Chem. 1999, 9, 1853–1864. (b) Watson, M. D.; Fechtenk€otter, A.; M€ullen, K. Chem. Rev. 2001, 101, 1267–1300. (c) Bendikov, M.; Wudl, F. Chem. Rev. 2004, 104, 4891–4945. (d) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. (e) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (2) (a) Gr€uner, J.; Wittmann, H. F.; Hamer, P. J.; Friend, R. H.; Huber, J.; Scherf, U.; M€ullen, K.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1994, 67, 181–185. (b) Tasch, S.; List, E. J. W.; Hochfilzed, C.; Leising, G.; Schlichting, P.; Rohr, U.; Geerts, Y.; Scherf, U.; M€ullen, K. Phys. Rev. B 1997, 56, 4479–4483. (c) Tasch, S.; List, E. J. W.; Ekstr€om, O.; Graupner, W.; Leising, G.; Schlichting, P.; Rohr, U.; Geerts, Y.; Scherf, U.; M€ullen, K. Appl. Phys. Lett. 1997, 71, 2883–2885. (d) Jacob, J.; Zhang, J.; Grimsdale, A.; M€ullen, K.; Gaal, M.; List, E. J. W. Macromolecules 2003, 36, 8240–8245. (e) Qiu, S.; Li, P.; Liu, X.; Shen, F.; Liu, L.; Ma, Y.; Shen, J. Macromolecules 2003, 36, 9823–9829. (f) Jacob, J.;

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Zwitterionic Ladder Stilbenes with Phosphonium and Borate Bridges

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