Directional Control of Ï-Conjugation Enabled by Distortion of the

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Directional Control of #-Conjugation Enabled by Distortion of the Donor Plane in Diarylaminoanthracenes: A Photophysical Study Shunsuke Sasaki, Kengo Hattori, Kazunobu Igawa, and Gen-ichi Konishi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b03238 • Publication Date (Web): 27 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015

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The Journal of Physical Chemistry

Directional Control of π-Conjugation Enabled by Distortion of the Donor Plane in Diarylaminoanthracenes: A Photophysical Study Shunsuke Sasaki1, Kengo Hattori1, Kazunobu Igawa2, Gen-ichi Konishi1,* 1

Department of Organic and Polymeric Materials, PRESTO Japan Science and

Technology Agency (JST), Tokyo Institute of Technology, Tokyo 152-8552, Japan 2

Institute for Materials Chemistry and Engineering and Department of Molecular and

Material Sciences, Kyushu University, Fukuoka 816-8580, Japan

ABSTRACT We designed and synthesized a series of diarylaminoanthracenes in which the planarity of the diarylamine moiety is controlled by methylene- or ethylene- bridges. The X-ray crystallographic structures confirm that the methylene- and ethylene bridges gradually decrease the disorder of the diarylamine planes. To quantitatively analyse the photophysical properties and underlying electronic structures of these compounds, we carried out UV-Vis and fluorescence spectroscopy, fluorescence quantum yield, and fluorescence lifetime measurements. The results reveal that enhanced planarity of the diarylamine moiety optically forbids the charge-transfer transition between the diarylamine and anthracene moieties. Although it is generally accepted that a planar π-conjugated system favours electron delocalization, our results indicate that distortion of the donor plane induces interchromophoric conjugation rather than conjugation within the local structure. Density functional theory calculations further illustrate that the frontier orbitals of diarylamine and anthracene interpenetrate as the donor plane is distorted. Additionally, natural bonding orbital analyses reveal that distortion of the donor plane changes the directionality of the π-conjugation of the nitrogen n-orbital from intrachromophoric to interchromophoric.

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INTRODUCTION The applications of π-conjugated molecules in optics, electronics, and imaging techniques have become core topics in recent materials science investigations. For the design of π-conjugated functional molecules, a methodology that reinforces orbital coupling between π-systems is essential because strong π-conjugation can harness high luminescence quantum yields,1-8 increase two-photon absorption cross-sections,9-13 minimize reorganization energy of charge trasport,14-17 and improve various other properties. The dogmatic strategy to maximize π-conjugation is to increase the coplanarity of the π-skeleton.18 Consequently, the formation of rigid π-planes has been a long-standing problem for synthetic chemists, prompting various approaches such as carbon-bridge formation,19-26 extension of acene-type skeletons,27-30 exploitation of non-covalent bonding,31-33 and incorporation of main-group elements.34-37 These methodologies have successfully brought about excellent photophysical and optoelectronic properties. While the dense packing of these materials in the solid state can be advantageous for some electronic devices,34-35 this characteristic is often accompanied by poor solubility and the tendency to aggregate, which not only complicates handling during synthesis and purification procedures38-40 but also results in incompatibility with solution- or dispersion-based systems.41-44 Therefore, an alternative strategy to extend π-conjugation without rigidifying the π-plane is desired. Needless to say, the larger the dihedral angle becomes between the two p orbitals in a bond, the poorer the orbital overlap gets. Nevertheless, as shown in Figure 1, the distortion of a local π-plane in bichromophoric or higher-complexity systems can extend their π-conjugation. A distorted local chromophore is subjected to destabilization because of its inefficient local conjugation and, to compensate for the poor resonance stabilization, the locally distributed π-electrons (intrachromophoric conjugation) can be delocalized to an adjacent chromophore

(interchromophoric

conjugation).

9-(N,N-Diphenylamino)anthracene

(1Ph) and its analogues are good examples for our hypothesis. Whereas the diphenylamino-functionalized 1Ph45 and 2Ph46-47 exhibit bright fluorescence, compound 2Ac,45 with dihydroacridyl groups, was recently revealed to be nonfluorescent (Scheme 1). A comparison of these two results led us to expect that the π-conjugation system is rather more delocalized in the locally distorted 1Ph than in 1Ac. 2 ACS Paragon Plus Environment

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However, the photophysical properties of 1Ac and 1Ph have not been systematically studied, nor have those of 2Ph, which has been extensively used as a dopant in organic light-emitting diodes (OLEDs).46-47 Most diarylaminoanthracenes were difficult to synthesize before the recent development of powerful Pd catalysts such as Pd-PEPPSI-IPr,45 and all of the prior studies were principally concerned with synthetic methodology45 or device performance as OLEDs. Herein, we report the synthesis and systematic study of the photophysical properties of various diarylaminoanthracenes, and evaluate the electronic effects that result upon distortion of the diarylamine plane. In addition to the widely distorted diphenylamine and coplanar dihydroacridine, we synthesized novel diarylaminoanthracenes, 1Az and 2Az, bearing the dihydro-5′H-dibenzo[b,f]azepine subunit, which has a moderately distorted plane, to clearly delineate the trend. The results indicate that increased distortion induces a more-allowed intramolecular charge-transfer (ICT) transition between the diarylamine and anthracene, which indicates a larger donor-π orbital overlap. Furthermore, such donor-π conjugation triggered by distortion of the donor plane could be clearly visualized by X-ray single crystal structure analysis and density functional theory (DFT) calculations. These results reveal that functional molecules with delocalized π-conjugation and intense fluorescence can be designed not only by rigidifying a π-plane, but also by distorting the partial structure of a π-system.

Figure 1. The concept of our study.

METHODS Instruments All the 1H NMR and

13

C NMR spectra were recorded on a 400 MHz JEOL

LMN-EX400 instrument and 300 MHz BRUKER DPX300 with tetramethylsilane 3 ACS Paragon Plus Environment


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(TMS) as the internal standard. FT-IR spectra were recorded on a JASCO FT-IR 469 plus spectrometer. Melting points were obtained by Yanaco micro melting point apparatus MP-500P. MS spectra (FAB+) were obtained by JEOL JMS700 mass spectrometer. All photophysical measurements performed in solutions were carried out using dilute solutions with optical density (O.D.) around 0.1 at the maximum absorption wavelength in 1 cm path length quarz cells at room temperature (298 K). In addition, all samples solutions were deaerated by bubbling with algon gas for 15 min before the measurements. UV-Vis spectra were recorded with a JASCO V-670 UV-Vis Spectrophotometer. Fluorescence spectra were recorded on a JASCO FP-6500 Spectrofluorometer and corrected to remove instrumental response functions. The wavelengths obtained by fluorescence spectrometer were converted to wavenumber by using the equation I( ) = λ2I(λ).48 Absolute quantum yields were measured by a Hamamatsu Photonics Quantaurus QY. Fluorescence lifetimes were measured at the most intense peaks using a Hamamatsu Photonics OB 920 Fluorescence Lifetime Spectrometer equipped with LEDs lamp (343 nm).

Synthesis Until quite recently, the synthesis of a diarylaminoanthracene was a challenging issue because of the severe steric hindrance of the diarylamine moiety. The Buchwald– Hartwig reaction using a Pd(II) salt and a P(t-Bu)3 ligand has been the primary approach for combining a bulky haloarene and a diarylamine,49-50 and several studies have reported the preparation of 2Ph using this catalyst system.46-47 However, the Pd– P(t-Bu)3 system sometimes leads to unfavorable side reactions,49-50 and therefore, usable substrates are severely limited. Very recently, Yorimitsu et al. expanded the scope of C– N

coupling to

9-(9'-hydroacrid-10'-yl)anthracene

(1Ac)45 and 51-52

diarylaminoarenes using the Pd–PEPPSI catalyst system.

various bulky

Therefore, we were able to

use the Pd(II)–P(t-Bu)3 catalyst for the preparation of 2Ph and adopted the Pd–PEPPSI catalyst for the syntheses of 1Ph, 1Az, 1Ac, 2Az, and 2Ac. All compounds were characterized by 1H NMR, 13C NMR, FT-IR, melting point measurements, and HRMS (For detailed information, see supporting information).


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Computational methodology The ground-state equilibrium structures of 1Ph, 1Az and 1Ac were fully optimized by self-consistent field (SCF) calculation using ωB97X-D as the hybrid density functional and 6-311G(d,p)53-57 as basis sets. Hybrid functionals of ωB97-family can address non-local exchange correlation and especially ωB97X-D takes dispersion effect into calculation.58-59 Therefore ωB97X-D is suitable for calculation of our compounds, where π-systems are largely twisted and severe steric repulsions are expected. The analytical frequencies were obtained to ensure that a local energy minimum has been located. All calculation were performed by using the Gaussian 09 program package.60 All of the results obtained from natural bonding orbital (NBO) analyses were confirmed to possess reasonable Lewis structure. When stabilization with delocalization of lone pair electrons on a nitrogen atom is estimated, delocalization not only to π* orbital but also to σ* and Rydberg orbitals are included because conjugation systems of such largely twisted molecules as 1Ph, 1Az and 1Ac may involve diffused orbitals and σ* orbitals. Detail of Frontier orbitals, atom coordinates, absolute energies, results of time-dependent SCF calculation and NBO analysis are listed in supporting information.

RESULTS AND DISCUSSION Structural analysis The X-ray crystallographic structure of 1Ph revealed that the diphenylamine plane is largely distorted: each phenyl ring of the diarylamine is twisted away from the other phenyl ring to minimize steric repulsion between the two phenyl rings (Figure 2a). The distorted structure of 1Ph considerably differs from the crystallographic structure of 5 ACS Paragon Plus Environment


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1Ac reported by Yorimitsu et al.,45 where the phenyl rings in the dihydroacridine group are almost coplanar. The torsion angle θ between the two phenyl rings of the diarylamine moiety in 1Az (Figure 2b) is 34°, which is intermediate in value between those of 1Ph (θ = 44°) and 1Ac. Also, the optimized structures calculated by DFT (ωB97X-D/6-311G(d,p)) indicate that the angles θ are gradually reduced by the incorporation of methylene or ethylene bridges (θ = 58°, 35°, and 21° for 1Ph, 1Az, and 1Ac, respectively). Therefore, the series of compounds used in this study is suitable for examining whether partial distortion prompts interchromophoric conjugation between the anthracene and diarylamine moieties.

Figure 2. X-ray crystallographic structure of (a) 1Ph, (b) 1Az, and optimized conformations

of

(c)

1Ph,

(d)

1Az

and

(e)

1Ac

calculated

by

DFT

(ωB97X-D/6-311G(d,p)).

Absorption spectra A comparison of absorption spectra demonstrates that the distortion of the diphenylamine plane induces the optically allowed S0→1CT transition from a diphenylamine to an anthracene group. Absorption spectra measured in toluene solution are shown in Figure 3. For all the diarylaminoanthracenes, the spectra consist of two types of absorption bands. Vibronic bands in the shorter wavelength region represent the S0→1La transition, which is localized on anthracene and common to various 9- and 6 ACS Paragon Plus Environment

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9,10-substituted anthracenes.61-63 Broad and red-shifted bands have been ascribed to the S0→1CT transition in several studies on dialkylaminoanthracenes61-62 and 2Ph.63 We also recorded absorption spectra in THF and and N,N-dimethylformamide (DMF), and slight hypsochromic shifts were observed in the S0→1CT bands with increasing solvent polarity (Figure S17–S22). Kapturkiewicz et al. attributed the hypsochromic shift of the S0→1CT band to an eccentric ground-state dipole moment,64 which usually appears on bichromophoric systems such as diarylaminoanthracenes. However, the effect of solvent on spectral shape was insignificant compared with the effect of a restriction by an alkyl constraint. Figure 3 indicates that the molar absorption coefficient of the S0→1CT band drastically decreases as the diphenylamine moiety becomes increasingly planar because of the steric constraint imposed by the alkyl bridge. To quantify the extent of the allowance of the S0→1CT transition for each compound, the transition dipole moment Mabs was determined using the following equation:65

| | =

3ln10 ℎ 1 8

where [M–1 cm–1] is the molar absorption coefficient in the wavenumber scale;

[cm–1] is the absorption maximum of the S0→1CT band; h [erg s], c [cm s–1], n,

and NA [mol–1] are the Planck constant, the speed of light, the refractive index66 of the solvent, and the Avogadro constant, respectively. The obtained transition dipole moments, listed in Table 1, indicate that the S0→1CT transitions become more forbidden as the diarylamine plane is planarized because of the steric constraints imposed by the alkyl bridges. The transition dipole moments of 1Az and 1Ac amount only to 69% and 36%, respectively, of that of 1Ph. Similarly, the transition dipole moments of 2Az and 2Ac amount to 65% and 41%, respectively, of that of 2Ph.



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Figure 3. Absorption spectra of diarylamine-substituted anthracenes in toluene solution.

Table 1. Absorption maxima λabs of S0→1La bands and S0→1CT bands and transition dipole moments Mabs of S0→1CT transitions in toluene solution. Entry λabs (S0→1La) [nm] λabs (S0→1CT) [nm]a 353 428 1Ph 366 426 1Az 367 416 1Ac 356 461 2Ph 372 456 2Az 373 440 2Ac a These maxima and area of S0-1CT band were evaluated by gaussian-fitting.


Mabs [D]a 1.4 0.94 0.49 1.7 1.1 0.71

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Fluorescence spectra Fluorescence spectra also reveal that the planarity of the diarylamine moiety dramatically forbids the S0←1CT transition because of severe orbital decoupling. As exemplified in Figure 4 for 1Ph, the fluorescence spectra of all diarylaminoanthracenes exhibit large positive solvatochromic shifts. The red-shifts in polar environments are accompanied by a decrease in radiative transition rates, which results in lower fluorescence quantum yields. These trends are characteristic of the S0←1CT fluorescence behaviors of D-π-65,67-69 and D-π-A-type65,70-71 aromatic amines, wherein the polar environment and a strong donor–acceptor pair decouple donor and acceptor orbitals from each other (charge separation), leading to the forbidden S0←1CT transition. Only 1Ac showed another small fluorescence band in the short wavelength region, which remained regardless of repeated purification and concentration (1.0 × 10−4 M, Figure S27). Though we could not confirm the origin of this band, it is likely to be due to emission from an excited state localized on either the diarylamine or the anthracene moiety. Other fluorescence spectra and excitation spectra are compiled in Figure S23-S28 and S30-31. Table 2 summarizes the fluorescence maxima λfl, quantum yields Φfl, lifetimes τfl, radiative rate constants kr, non-radiative rate constants knr, and transition dipole moments Mfl of the S0←1CT fluorescence measured in toluene solution. As the ethylene (1Az and 2Az) and methylene bridges (1Ac and 2Ac) increase the planarity of the diarylamine, the fluorescence quantum yields decrease to less than one-tenth of those of 1Ph and 2Ph. The fluorescence transition dipole moment was calculated as follows:65

| | =

3ℎ!" 2 64 %

where is the fluorescence maximum in the wavenumber scale. Mfl indicates that the alkyl bridge constraints on the diarylamino planes (1Az and 2Az) make the S0←1CT transition forbidden. Numerous studies have pursued the origin of the forbidden character of the S0←1CT transition,65,67-72 and now it is undoubtedly accepted that a decoupling between the donor and acceptor orbitals (Franck–Condon forbidden transition) is the main reason for this forbidden character, especially in case of largely twisted and bichromophoric D-π and D-π-A systems. Additionally, from Lippert– Mataga plots (Figure S29), the increment in dipole moment upon excitation was 9 ACS Paragon Plus Environment


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estimated to be 8.3, 8.6, and 9.3 D for 1Ph, 1Az and 1Ac, respectively, and spectral shifts of 1360, 2050, and 2790 cm−1 can be observed for 2Ph, 2Az, and 2Ac, (Figure S24, 26, 28), correspondingly, on increasing the solvent polarity from toluene to DMF. This enhanced solvatochromism further supports the enhanced charge separation induced by the increased planarity of the diarylamino moiety. The decoupling between donor and acceptor orbitals is clearly visualized by computational calculations, as will be discussed. Table 2 also reveals that with the increase in the planarity of the diarylamine moieties, the non-radiative transition rate increases, which is usually observed for measurements in other solvents (Table S8). We have previously pointed out73-74 that excited states with enhanced charge separations and far-red-shifted emissions are more likely to be quenched by internal conversion according to the energy gap rule.75 Also intersystem crossing should be considered as a deactivation path since anthracene undergoes rapid intersystem crossing due to narrow S1-T2 gap.76-77 9,10-substitution of anthracene suppress the S1-T2 intersystem crossing78-79 by lowering the S1 level and therefore this type of intersystem crossing is unlikely to happen on our system. Nevertheless intersystem crossing is possible path since spatially separated HOMO and LUMO lead narrow 1CT-3CT gap.80-82 In any case, the increased radiationless transition rate due to the constrained diarylamino plane can be ascribed to enhanced charge separation. In addition to their emission behaviors in the solution systems, those in an aggregated state were evaluated because such largely twisted π-systems would be expected to exhibit aggregation-induced emissions.83 All the diarylamines exhibited aggregation in a mixed solvent of 1:9 v/v THF:water at a solute concentration of 1.0 × 10−4 M, as confirmed by UV-Vis spectroscopy (Figure S17–S22).84-87 However, they did not exhibit any significant increase in the fluorescence quantum yield; indeed, 1Ph and 2Ph showed rather large decreases. Though the fluorescence decay function is biexponential, the weighted average lifetime was derived to obtain the “estimated value” of the radiative and non-radiative rate constants (Table S8). As represented by Figure 4, 1Ph and 2Ph undergo non-radiative transitions 50–100 times more efficiently in the aggregated state than in solution. The results imply that efficient quenching pathways are triggered by intermolecular interactions such as energy transfer. 10 ACS Paragon Plus Environment

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Figure 4. Fluorescence spectra of 1Ph (Excitation wavelength corresponds to absorption maxima of S0→1La band). Fluorescence spectra of the aggregates were measured in THF : H2O = 1 : 9 (v/v); H2O was added in order to obtain colloidal suspensions, and aggregate formation was confirmed by UV-Vis spectroscopy.84-87

Table 2. Fluorescence maxima λfl, quantum yields Φfl, lifetime τfl, radiative rate constant kr, non-radiative rate constant knr and transition dipole moment of fluorescence Mfl in toluene solution λfla τflb kr knr Φflb Mfl [D] [nm] [ns] [106 s-1] [106 s-1] 489 0.92 21 44 3.6 2.2 1Ph 507 0.68 41 16 7.9 1.5 1Az 535 0.06 1Ac 510 ≥0.99 13 75 0.15 3.1 2Ph 525 0.75 34 22 7.5 1.8 2Az 553 0.06 2Ac a Excitation wavelengths correspond to absorption maxima of S0→1La band. b Excitation wavelength was fixed to 343 nm, which is the wavelength of the light source of lifetime spectrometer. Entry



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Computational calculations DFT calculations (ωB97X-D/6-311G(d,p)) clearly revealed that the orbital interaction between the diarylamine and anthracene moieties is triggered by the distortion of the diarylamine plane. As already presented in Figure 2, the structures of 1Ph and 1Ac optimized by our DFT calculations correspond well to their X-ray crystallographic structures even though the structure optimizations were initiated from the minimum energy configurations obtained from MM2 calculations, where the torsions between the anthracenes and diarylamines were far lower than the results obtained from the DFT calculations. Therefore, the DFT calculations in this study can be considered valid for explaining the spectroscopic results. The atomic coordinates, absolute energies, frontier orbitals, and detailed calculation methods are compiled in the Supporting Information. Among the frontier orbitals obtained from the DFT calculations, a comparison of HOMO and HOMO-1 (Figure 5) is important in considering the orbital interactions between the diarylamine and anthracene moieties, because these orbitals are completely mixed with each other as the diarylamine plane becomes non-planar. By contrast, the population density of the LUMO (1b3u orbital of anthracene)88 does not show any deformation and remains localized completely on anthracene (Figure S14–S16). As shown in Figure 5, the HOMO and HOMO-1 of 1Ac are completely localized on the hydroacridine and anthracene moieties, respectively. The HOMO-1 is ascribed to the 1

b2g orbital of anthracene, which is originally the HOMO of anthracene.88 Consequently,

the HOMO and LUMO on 1Ac are completely separated, and the HOMO–LUMO transition is forbidden and leads to complete charge separation. Such a separated HOMO and LUMO orbitals were reported also for 9-(N-carbazolyl)-anthracene.99 However, on 1Az, where the diarylamine plane is moderately distorted, both the HOMO on the diarylamine and the HOMO-1 on anthracene begin to interpenetrate each other. This orbital interaction results in an additional splitting of the HOMO–HOMO-1 gap. Finally, the HOMO and HOMO-1 on 1Ph are completely intermixed and seemingly indistinguishable. They vary only in the manner of orbital interaction: while the proximal π-orbitals between the diarylamine and anthracene are in-phase on the HOMO-1, they are out-of-phase on the HOMO, and thereby, they take a roundabout path so as to connect in an in-phase manner. Thus, the π-orbitals on the diarylamine and 12 ACS Paragon Plus Environment

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anthracene interact in a bonding manner on the HOMO-1 and in an antibonding manner on the HOMO. Since the HOMO is highly delocalized on 1Ph, its HOMO–LUMO transition is expected to be the strongly allowed S0-1CT transition. In fact, our single-point time-dependent DFT (TD-DFT) calculations (ωB97X-D/6-311G(d,p)) clearly demonstrate that the oscillator strength of S0-1CT transitions are enhanced with the increase in diarylamine plane distortion (Table 3). As expected from molecular orbitals, the HOMO-LUMO transition of 1Ac leads to optically forbidden charge separation (S0-1ET), and only its HOMO-1-LUMO transition, which is assigned to the S0-1La transition of antracene, is allowed.61-63 These results are fully consistent with the experimental absorption spectra (Figure 3). Detailed results of the TD-DFT calculations are compiled in Table S4-S6. To summarize, the DFT calculations revealed that the distortion of the diarylamine plane induces an efficient orbital coupling between the diarylamine (HOMO) and anthracene (HOMO-1), which results in the strongly allowed S0-1CT absorption and emission.

Figure 5. HOMO and HOMO-1 orbital and their energy level of 1Ph, 1Az and 1Ac calculated by DFT (ωB97X-D/6-311G(d,p)).



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Table 3. Main transition orbitals, transition wavelengths λcalc and oscillator strengths f of singlet excited states calculated by TD-DFT (ωB97X-D/6-311G(d,p)). λcalc f [nm] S1 HOMO→LUMO S0→1CT 375 0.13 1Ph S2 HOMO-1→LUMO S0→1CT 322 0.03 S1 HOMO→LUMO S0→1CT (S0→1ET character) 368 0.07 1Az S2 HOMO-1→LUMO S0→1CT (S0→1La character) 340 0.08 S1 HOMO→LUMO S0→1ET 362 0 1Ac S2 HOMO-1→LUMO S0→1La 346 0.15 S0→1ET indicates complete charge separation (electron transfer) from diarylamine to Entry

a

State

Assignmenta

Main transition orbital

anthracene moieties.

To elucidate why the lone pair on the distorted diarylamine plane switches to interact with the 1b2g orbital of anthracene, natural bonding orbital (NBO)89-95 analysis was performed using NBO version 3.1.96 A NBO is expressed by the linear combination of two directed natural hybrid orbitals (NHOs), similar to the Lewis structure diagram.97 Therefore, a hybridization parameter (x of spx) for each atom can be estimated by the NBO analysis. As shown in Figure 6, the NHOs of nitrogen comprising three σ-bonding electrons and neighboring carbons have nearly sp2 character. The sp2 character of the σ-bonding electrons and the pure p character of the lone pair on the nitrogen atom are common to 1Ph, 1Az, and 1Ac. Therefore, regardless of the distortion of the diarylamine plane, the lone pair is used only for conjugation with other NBOs rather than for hybridization. However, second-order perturbation theory analysis in NBO basis reveals that the breakdown of conjugation of the nitrogen lone pair is changed by the distortion of the diarylamine plane. In NBO analysis, the delocalization of electrons is expressed by the donation of an electron pair from an electron-rich NBO '( (a bonding orbital of

NHOs or a lone pair) to an electron-deficient NBO ' (an antibonding orbital of

NHOs). The resonance stabilization energy due to delocalization ∆*+ ,- can be estimated by second-order perturbation theory:98 ∆*+ ,- D

2'3 456 4'7 8 → A = −2 3 7 − 3

where 56 is the effective orbital Hamiltonian, and 3 and 7 are the respective orbital

energies of donor and acceptor NBOs. Therefore, when the ∆*+ ,- values from the 14 ACS Paragon Plus Environment

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delocalization of the nitrogen lone pair to the acceptor NBOs ' on the entire diarylamine plane are summed, the whole resonance stabilization of the lone pair within the diarylamine group can be estimated. Similarly, resonance stabilization energy due to conjugation between the nitrogen lone pair and the anthracene moiety was estimated. The results are summarized in Figure 6 (additional details are listed in Table S7), and indicate that more distorted diarylamine planes result in less resonance stabilization within the diarylamine moieties, whereas the resonance stabilization with the anthracene moiety is enhanced. Based on the NBO analysis results, the driving force for the orbital interaction between the anthracene and diarylamine moieties can be explained as follows: in 1Ac, the lone pair is sufficiently stabilized through conjugation within the diarylamine moiety because of its planar structure. However, in 1Az and 1Ph, the lone pair cannot fully conjugate with the phenyl rings because of the N–Ph torsion, and thereby, the lone pair on nitrogen is destabilized. To compensate for the inefficient conjugation within the diarylamine moiety, the nitrogen lone pair conjugates with the anthracene moiety, despite the severe steric repulsion involved. However, this is only a plausible mechanism for conjugation induced by partial distortion. We will explore other molecular systems that exhibit similar phenomena to gain deeper insight.

Figure 6. Resonance stabilization energy acquired by delocalization ∆Edeloc from lone pair (N) to each π-system (Ph2: diphenylamine, Az: 10,11-dihydro-dibenzo[b,f]azepine, Ac: 9-hydroacridine, An : anthracene) and hybridized parameters of nitrogen atoms on (a) 1Ph, (b) 1Az, (c) 1Ac calculated by NBO program version 3.1.96 15 ACS Paragon Plus Environment


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CONCLUSIONS We synthesized a series of mono- and bis-diarylaminoanthracenes with various steric constraints by the Buchwald–Hartwig reaction using the Pd(II)–P(t-Bu)3 and Pd-PEPPSI catalyst systems. X-ray crystallography and DFT calculations revealed that the diarylamine moieties in 1Ph, 1Az, and 1Ac have different planarities. Correlations between the distortion of the diarylamine planes and their photophysical properties were evaluated. The spectroscopic experiments revealed that the distorted diarylamine plane made the charge-transfer transition between the diarylamine and anthracene more optically allowed. DFT calculations were performed to elucidate why the distortion of the diarylamine moieties induced the optically allowed S0-1CT transition; the results indicated that the distortion promotes an interaction between the donor orbital of the diarylamine and the 1b2g orbital of anthracene. NBO analysis further indicated that inefficient conjugation within the diarylamine moiety coincides with stronger conjugation between the lone pair on the nitrogen atom and the anthracene moiety. The elucidation of the interchromophoric conjugation promoted by distortion of the partial structure will be quite useful for the design of strongly conjugated molecules with highly skewed structures. Our future work aims to develop a concrete design strategy for molecules in which conjugation is induced by partial distortion, as well as to develop novel optical and electronic materials that exploit these unique structures and switchable fluorescence properties in response to slight conformational deformations.

ASSOCIATED CONTENT Supporting Information Detailed

synthetic

procedures,

NMR

spectra,

crystallographic

information,

photophysical data, and quantum-chemical caluculations. This information is available free of charge via the Internet at http://pubs.acs.org

ACKNOWLEDGEMENT We appreciate Mr. Taisuke Matsumoto (Institute for Materials Chemistry and Engineering, Kyusyu University) for disorder analysis of X-ray crystallographic structure, and Mr. Mahiro Iwasaki (Tokyo Institute of Technology) for development of 16 ACS Paragon Plus Environment

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Java program useful for running and analysing the results of DFT calculations. We also thank Emeritus Prof. Katsumi Tokumaru (Tsukuba University) for helpful discussion.

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(99) Evers, F.; Girard, J. G.; Grimme, Manz, J.; Monte, C.; Oppel, M.; Rettig, W.; Saalfrank, P.; Zimmermann, P. Absorption and Fluorescence Excitation Spectra of 9-(N-Carbazolyl)-anthracene: Effects of Intramolecular Vibrational Redistribution and Diabatic Transitions Involving Electron Transfer. J. Phys. Chem. A 2001, 105, 2911-2924.


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