Scholarly article on topic 'Extended π-Conjugated Pyrene Derivatives: Structural, Photophysical and Electrochemical Properties'

Extended π-Conjugated Pyrene Derivatives: Structural, Photophysical and Electrochemical Properties Academic research paper on "Chemical sciences"

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Academic research paper on topic "Extended π-Conjugated Pyrene Derivatives: Structural, Photophysical and Electrochemical Properties"

DOI: 10.1002/slct.201600598

I Organic & Supramolecular Chemistry

Extended p-Conjugated Pyrene Derivatives: Structural, Photophysical and Electrochemical Properties

Xing Feng,*[a,bl Nobuyuki Seto,[al Chuan-Zeng Wang,[a] Taisuke Matsumoto,[cl Junji Tanaka,[c] Xian-Fu Wei,[bl Mark R. J. Elsegood,[dl Lynne Horsburgh,[dl Carl Redshaw,[el and Takehiko Yamato*[al

This article presents a set of extended p-conjugated pyrene derivatives, namely 1,3-di(arylethynyl)-7-tert-butylpyrenes, which were synthesized by a Pd-catalyzed Sonogashira coupling reaction of 1,3-dibromo-7-tert-butylpyrenes with the corresponding arylethynyl group in good yields. Despite the presence of the tert-butyl group located at the 7-position of pyrene, X-ray crystallographic analyses show that the planarity of the Y-shaped molecules still exhibits strong face-to-face p-p stacking in the solid state; all of the compounds exhibit blue or green

emission with high quantum yields (QYs) in dichloromethane. DFT calculations and electrochemistry revealed that this category of compound possesses hole-transporting characteristics. In addition, with strong electron-donating (-N(CH3)2) or electron-withdrawing (-CHO) groups in 2d or 2f, these molecules displayed efficient intramolecular charge-transfer (ICT) emissions with solvatochromic shifts from blue to yellow (green) on increasing the solvent polarity. Furthermore, the compounds 2d and 2f possess strong CTcharacteristics.

Introduction

Large p-conjugated organic materials have attracted increasing attention in recent years, due to facile band-gap and colour control by structural modification, which makes them suitable for potential application in high performance organic elec-tronics,[1l such as organic light emitting diodes (OLEDs),[2l liquid-crystal displays,131 organic field effect transistors (OFETs)14,51 and organic photovoltaic cells (OPVs),161 as well as optical storage devices.171 Extension of the p-conjugation chromophores contributes to a lowering of the HOMO-LUMO gap, leading to a red-shift in absorption and emission spectra with increasing

[al Dr. X. Feng, N. Seto, C.-Z. Wang, Prof.Dr. T. Yamato

Department of Applied Chemistry, Faculty of Science and Engineering Saga University

Honjo-machi 1, Saga 840-8502 Japan E-mail: hyxhn@sina.com

yamatot@cc.saga-u.ac.jp [bl Dr. X. Feng, Prof. X.-F. Wei

School of Printing and Packaging Engineering Beijing Institute of Graphic Communication

1 Xinghua Avenue (Band Two), Daxing, Beijing, 102600, P. R. China E-mail: hyxhn@sina.com [cl Dr. T. Matsumoto, Dr. J. Tanaka

Institute of Materials Chemistry and Engineering Kyushu University

6-1, Kasugakoen, Kasuga 816-8580, Japan [dl Dr. M. R. J. Elsegood, Dr. L. Horsburgh Chemistry Department Loughborough University Loughborough, LE11 3TU, UK [el Prof.Dr. C. Redshaw

Department of Chemistry The University of Hull

Cottingham Road, Hull, Yorkshire HU6 7RX, UK □ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201600598

fluorescence quantum yields. In addition, molecules which possess p-conjugation with a planar structure would enhance charge carrier transport in optoelectronic applications by self-assembly via intermolecular p-p stacking.181

Pyrene[9] is made up of four fused aromatic rings with a large p-electron system, which exhibits good solution-process-able properties with an excellent blue emission spectrum (with a long excited-state lifetime, high fluorescence intensity and quantum yield). However, the development of pyrene as a host material in blue light-emitting diodes is scarce, due to its tendency to readily aggregate in most media. Several research groups including ours have reported many new types of pyr-ene derivatives, and various substituent groups have been attached to the pyrene core by -C-C-, -C-N- or -C = C- bonds for suppressing excitation emission.^0-151 Some ethynyl-substituted pyrene-based compounds displayed more intriguing fluorescence characteristics, for instance, Ziessel et al reported a greenish luminescent 1,3,6,8-tetra-(4-ethynylphenylamino-acyl))pyrene with stable liquid crystalline properties for the fabrication of OLED-like devices;[16] Kim and coworkers synthesized a series of 1,3,6,8-tetrakis(ethynyl)pyrenes functionalized with varying numbers of N,N-dimethyaniline and 1-(tri-fluoromethyl)benzene moieties as peripheral electron-donors and acceptors for chemiluminescent (ECL) active materials, that exhibited enhanced charge transfer compared with 1,3,6,8-tet-ra-N,N-dimethylaniline (or 1,3,6,8-tetra-tri-

fluoromethylbenzene).[17,18] Interestingly, Adachi et al have designed a number of inverted singlet-triplet (iST) pyrene-based derivatives, and the electronic structures, spin-orbit couplings, transition dipole moments, and vibronic couplings have been investigated by theoretical calculations.™

Generally, an acetylene group is one option among other (such as ethylene) choices to extend the p-conjugation of mo-

lecular skeletons, and the advantage over an ethylene linker is better stability, in addition, it's very convenient to prepare this kind of compounds by Pd-catalyzed coupling reaction. Previously, in our laboratory, we synthesized cruciform-shaped and hand-shaped architectures incorporating p-conjugated alky-nylpyrenes as highly efficient blue emissive materials by the So-nogashira cross-coupling reaction.120,211 The optical properties of both types of pyrene-based material exhibited pure blue fluorescence with good quantum yield, as well as similar crystal packing in the solid-state. Our recent report on the synthesis of 1,3-dibromo-7-ferf-butylpyrene (1)[22] prompted us to further explore 1,3-bis(arylphenyl)pyrenes as blue emissive materials. The arylphenyl groups located at the 1,3-positions were twisted by a considerable angle relative to the pyrene core, a feature that can play a crucial role in hindering intermolecular interactions in the solid-state. In this article, a series of Y-shaped, extended p-conjugated pyrene derivatives have been synthesized by a Pd-catalyzed Sonogashira cross-coupling reaction using 1,3-dibromo-7-ferf-butylpyrene and corresponding arylethynyl groups in reasonable yield. We anticipated that the extended p-conjugated pyrene derivatives would exhibit interesting topological structures, leading to attractive photophysical properties, as well being air-stable. Indeed, the designed p-con-jugated molecules showed interesting CT characteristics in polar solvents as expected. Furthermore, we investigated the effect of the various substituents and substitution positions of the (arylethynyl)pyrenes for optical properties and crystal packing.

Results and Discussion

Synthesis and characterization

The synthesis of the Y-shaped arylethynyl-substituted pyrenes 2 are shown in Scheme 1. The target arylethynyl pyrenes 2 were

obtained in 32-67% yield through a Sonogashira coupling reaction between 1 and the corresponding arylacetylenes. All of the new pyrene derivatives 2 have been characterized by 1H/13C NMR spectroscopy (Figure S1), HR-MS and elemental analysis. Y-shaped compounds 2 with arylethynyl groups display good air-stability and solubility properties in common solvents, such as dichloromethane (CH2Cl2), N,N-dimethylforma-mide (DMF), tetrahydrofuran (THF) and methanol (CH3OH). Moreover, the structures of 2c (two polymorphs), 2d and 2f were further confirmed by single crystal X-ray diffraction. Very recently, compounds 2a and 2 b have been prepared by Cooper et al as key intermediation.1231 Herein, the effect of the sub-stituents on the crystal packing, photophysical properties and electrochemistry has been further investigated in detail; for comparison, two position-dependent arylethynyl-functionalized pyrenes 3 and 4 are illustrated in Scheme 1. This work allows us to fully understand the effect of the various substituents and the relationship between molecular structure and optical properties.

Single crystal X-ray crystallography

Single crystals of the extended p-conjugated Y-shaped pyrene derivatives 2 c were obtained from MeOH/CHCl3 or hexane/CH2 Cl2, 2d-CHCl3 from CHCl3 and 2f from CH2Cl2/benzene by slow evaporation at room temperature. All of the compounds crystallize in the triclinic crystal system with space group P1. Compound 2 c was obtained as two different polymorphs. A number of reports showed that pyrene derivatives with terminal methoxyphenyl moieties trends more easily to form good quality single crystals for X-ray crystallography. So this experience allows us to design pyrenes for insight into investigating the relationship between position-dependent, molecular conformation and the optical properties by using X-ray diffraction equipment. The molecular structures are shown in Figure 1 and Figure S2 and the crystallographic data is listed in Table 1.

Scheme 1. The synthetic route to p-conjugated pyrene derivatives 2, and the cruciform-shaped and hand-shaped pyrenes 3 and 4 for comparison.

Figure 1. The molecular structures of 2cI, 2d-CHCl3 and 2f.

As shown in Figure 2, the structure of 2cI displays face-to-face p-stacking at a distance of 3.54 A, which involves 14 carbons in each pyrene molecule.1241 One ethynylphenyl substituent is coplanar with the pyrene core, whilst another forms a twist angle of 28.4°. A number of p-p stacking interactions between the phenyl rings and the neighbouring pyrene core were observed (3.32 A). In addition, another weak but important interaction involves the methoxyl group, which has a

Table 1. Summary of crystal data for the p-conjugated molecules 2cI, 2cII,

2dCHCl3 and 2f.

Complex 2cI 2cII 2d 2f

Empirical for- C38H30O2 C38H30O2 C40H36 C38H26O2

mula N2-CHCl3

Formula weight 518.62 518.62 664.07 514.59

Crystal system Triclinic Triclinic Triclinic Triclinic

Space group PT P1 P1 P1

a [A] 8.926(2) 8.902(2) 10.8101(12) 8.9354(5)

b[A] 12.408(3) 12.088(3) 11.1244(12) 11.8785(9)

c[A] 13.559(3) 27.070(8) 14.3383(16) 13.0754(6)

a[°] 99.758(9) 101.774(18) 91.4712(18) 97.029(5)

b[°] 100.102(9) 93.904(19) 91.6750(18) 102.787(4)

g[°] 98.809(8) 100.502(18) 101.8031(17) 100.349(5)

Volume[A3] 1430.9(6) 2787.1(13) 1686.2(3) 1312.22(14)

Z 2 4 2 2

l 0.71073 1.54184 0.71073 1.54184

Dcalcd[Mg/m3] 1.204 1.236 1.308 1.302

temperature [K] 293(2) 296(2) 150(2) 100(2)

measured reflns 12587 36576 19720 7853

unique reflns 4927 9802 9976 5075

obsd reflns 2470 4722 7491 3892

Parameters 362 731 552 462

R(int) 0.0337 0.0548 0.0184 0.0219

R[l > 2a(/)][aI 0.0774 0.0887 0.0457 0.0545

wR2[all data][b] 0.2342 0.2725 0.1344 0.1687

GOF on F2 1.007 1.010 1.059 1.052

a Conventional R on Fhkl: S | |Fo - I Fc | |/o | Fo | . b Weighted R on j Fhkl j 2:

S[w(Fo2 - Fc2)2l/S[w(Fo2)2]1/2

I Chemistry , SELECT V Full Papers

one molecule in the asymmetric unit. (See supporting information)

Figure 3 reveals that the asymmetric unit contains a molecule of 2d and a chloroform linked by a C-H-p interaction at a

Figure 2. X-ray crystal structure representations of 2cI, illustrating (a) and (b) the co-facial p-stacking structures and (c) the principal intermolecular packing interactions.

contact with the next layer of the ethynyl fragment of pyrene via a C-H-p interaction (2.42 A).

Similarly, the polymorphs of 2cII was crystallized from hex-ane/CH2Cl2 solution, The X-ray structure determinations revealed that the 2cII belongs to the same space group as 2cI. The crystal packing analysis shows that polymorph 2cII was arranged similar as 2cI. There is a fine distinction between both crystals namely the asymmetric unit cell of 2 cII contained two molecules, whereas the crystals of 2cI were found to contain

Figure 3. X-ray crystal structure representations of 2d-CHCl3, illustrating (a) and (b) the co-facial p-stacking structures and (c) CHCl3 molecules were captured in voids supported by C-H-p interactions.

distance of 2.41(2) A. Unlike the situation above for 2cI, where the p-methoxylphenylethynyl moieties and the pyrene ring are almost coplanar, here for 2d, the torsional angle between the pyrene unit and one of the the 4-(N,N-dimethylamino)-phenyl-ethynyl fragments is close to perpendicular with a twist angle of 87.87(3)°. Moreover, two neighbouring pyrene moieties are presented with displaced face-to-face patterns forming a star-shaped architecture in the solid-state; the p-p stacking interactions are approximately 3.37 A (Figure 3a). In addition, the terminal phenyl groups of neighbouring molecules were connected by p-p stacking interactions with a distace of 2.89 A (Figure 3b).

From the packing pattern, a two-dimensional supramolec-ular network was constructed by these complicated p-p stacking interactions, and the CHCl3 molecules were captured in the molecular voids1251 by C-H-p interactions.

The asymmetric unit of 2f contains one molecular with no solvent of crystallisation. As shown in Figure4, 2f displays a slightly curved core structure (torsional angle < 5o) with shallow twist angles of 17.57(9) and 3.03(8)° between the pyrene core and the terminal phenyl groups. Obviously, the planar molecular structure would tend to form strong co-facial p-p stacking, and the pairs of pyrene units arrange in head-to-tail stacking via a centre of symmetry by p-p interactions (3.48 A). In addition, the pyrene units and the adjacent phenyl rings display face-to-face p-p stacking at a distance of 3.32 A. Interest-

Figure 4. X-ray crystal structure representations of 2f, illustrating (a) and (b) the detail of the cofacial p-stacking structures and (c) the principal intermolecular packing interactions.

ingly, from the packing structure of 2f, there are two key p-p interactions leading to a three dimensional infinite supramolec-ular array.

Furthermore, in the packing structure of 3, the four phenyl rings are not coplanar with the central pyrene; the two unique dihedral angles are 18.3 and 23.6°. As shown in Figure 5, two

47.3°, and two neighbouring pyrene moieties adopt a slipped face-to-face motif with off-set head-to-tail stacking with a cent-roid-to-centroid distance of 4.65 A. Each molecule of 4 displays 24-point p-p stacking with molecules above and below using both the pyrenyl and ethynyl carbon with intermolecular distances of 3.42-3.58 A.

As mentioned above, the crystal packing is markedly different for the pyrenes 2, 3, and 4. With the substituted group number increasing, the crystal packing has been transformed from face-to-face p-p stacking with off-set head-to-tail stacking to a herringbone arrangement on going from 2, 3 to 4. On the other hand, bulkier ferf-butyl groups located at the 7-posi-tion of pyrene can play a more crucial role in hindering the p-p stacking versus the arylethynyl-substituted group. In compound 4, due to the nodal planes passing through the 7-posi-tion of the pyrene, the phenyl moiety attached at this position would be of limited impact for the crystal arrangement, as well as the electronic interactions.™ The planar molecular structure of the pyrene derivatives 2, 3 and 4 is beneficial for extending p-conjugation and improving the optical density, which can lead to special photophysical properties both in solution and in the solid state.

Photophysical properties

The absorption spectra of the title compounds 2 in dilute di-chloromethane are presented in Figure 6 and the optical data

Figure 5. The packing structure (a) side view and (b) top view for 3 and (c) side view and (d) top view for 4.

neighbouring pyrene moieties possess a centroid-to-centroid distance of 5.99 A. No significant p-stacking interactions between the pyrene rings were observed and molecules adopt a herringbone packing motif. Non-covalent interactions play an important role in the stacking of the structures.

However, in compound 4, when a p-methoxyphenyl moiety is located at the 7-position of pyrene, it is almost coplanar with the pyrene ring. The other four phenyl rings at the 4,5,9,10-po-sitions of the pyrene form dihedral angles of between 6.1 and

Figure 6. (a) Normalized UV-vis absorption and (b) fluorescence emission spectra of compounds 2 recorded in dichloromethane solutions at ~ 10"5-10"6 M at 25°C.

is summarized in Table 2. The maximum absorption wavelength of the ethynylpyrenes 2 is exhibited at least ca. 68 nm red-shifted compared with 2-ferf-butylpyrene (338 nm).[15] Notably, the photophysical properties are highly dependent on the substituent units present. Y-shaped 2a-c and 2e show similar absorption behaviour and exhibit two prominent absorption bands in the regions 300-350 nm and 375-425 nm, respectively. For 2d and 2f, the maximum absorption wavelength has obviously red-shifted (~ 19 nm) relative to 2a, arising from the intramolecular charge transfer (ICT) increase. Indeed, both 2d and 2f feature a broader and intense absorption in the long wavelength region 475-525 nm, indicating that the molecules tend to be more coplanar between the pyrene core and terminal ethynylphenyl groups by extending the p-

Table 2. The photophysical and electrochemical properties of compounds 2.

R l a Amax abs a Amax PL Log e F LUMO (eV) HOMO Energy gap i Tmf t g 1 d

nm (nm) (eV) (eV) ns

2a 406 421 (325) 4.87 0.93 -2.01b (-2.41)c -5.01b (-5.34)d 2.99b (2.93)e 2.89a 198 479

2b 409 423 (330) 4.88 0.93 -1.93 (-2.39) -4.93 (-5.31) 2.99 (2.92) 2.93 288 469

2c 412 427 (333) 4.82 0.94 -1.85 (-2.37) -4.79 (-5.27) 2.94 (2.90) 2.94 195 449

2d 431 517 (380) 4.74 0.86 -1.63 (-2.61) -4.46 (-5.27) 2.83 (2.67) 3.51 237 440

2e 411 427 (313) 4.80 0.91 -2.37 (-2.42) -5.33 (-5.32) 2.97 (2.90) 2.48 220 469

2f 425 485 (363) 4.77 0.80 -2.56 (-) -5.39 (-) 2.83 (2.77) nd nd nd

a Measured in dichloromethane at room temperature. b DFT/B3LYP/6-31G* using Gaussian. c Calculated from the empirical formulae HOMO = -(Eox + 4.8), d LUMO=HOMO + Eg,e Calculated from Aedge,f Melting temperature (Tm) obtained from differential scanning calorimetry (DSC) measurement.g Decomposition temperature (Td) obtained from thermogravimetric analysis (TGA), nd: not measured.

conjugation, which is consistent with the crystallographic results. With an increased length of p-conjugation, the molar absorption coefficients (e) of 2d and 2f are lower than the others observed.

In particular, the 1,3-bis(4-methoxylphenylethynyl)pyrene 2c exhibits a more planar structure than 7-tert-butyl-1,3-bis(4-methoxyl-phenyl)pyrene,[22] which is beneficial to extend the conjugated pathway and increase the absorption cross-section. Indeed, the molar absorption coefficient of 2c (log e = 4.82) is higher than 1,3-bis(4-methoxylphenyl)pyrene (log e = 4.51). According to the literature/271 due to the special electronic structure of pyrene, substituents at different position would cause significant difference of electron distribution. For example, the absorption of 2a-c exhibited a great differences when compared to 3. Clearly, substitution at the K-region (4,5,9,10-posi-tions) exerts a greater influence on the S2<—So excitation than substitution at the active sites of the 1,3-position. The compounds 2a-c with similar S2^S0 excitation coefficients (log e = 4.87 for 2 a, 4.88 for 2 b and 4.82 for 2 c) are much larger than 3 (log e = 4.66).[20]

The fluorescence spectra of 2a-c and 2e exhibit a sharp peak at 1em max=421, 423, 427 and 427 nm with a shoulder respectively. The emission spectra of 2d and 2f display a single broad peak at 517 nm and 485 nm, respectively, which indicates that the emission occurs from the lowest excited state with the largest oscillator strength. With the p-conjugation increasing, a gradual bathochromic shift in the 1em max is clearly observed in the order of 2a « 2b « 2c « 2e < 2f < 2d, implying that the energy gap between ground and excited states would decrease in this order. In this process, the ICT plays an important role in lowering the energy gap.[28] The optical properties are consisted with Kim's reports.[29] All of compounds show strong emission from a deep blue to green color with high PL quantum yields (QYs) in the 0.80-0.94 range. The quantum yields of the compounds herein are higher than those that we reported previously for a series of 7-tert-butyl-1,3-diary-lpyrenes.[22] This was thought to be due to the increasing p-conjugation and delocalization of the electron density can improve the optical density, leading to strong FL emission in the solid-state.

Additionally, the fluorescence lifetimes (ts) were measured in dichloromethane, and the results are listed in Table 2.

Furthermore, the effect on the optical properties of these compounds have been examined in solvents of various polarity, such as cyclohexane, 1,4-dioxane, tetrahydrofuran (THF), dichloromethane (DCM) and N,N-dimethylformamide (DMF). The solvent dependence of the absorption and fluorescene spectra of 2 are displayed in Figure 7 and in the supporting in-

Figure 7. (a) Emission spectra of 2d in cyclohexane, THF, CH2Cl2, CH3CN and DMF at 25°C. (b) the solvatochromic colour change from blue to yellow, (c) Lippert-Mataga plots for compound 2d (-N(CH3)2).

formation (Figure S3). The fluorescence spectra of 2d and 2f showed an obvious intermolecular charge transfer (ICT) emission band in different solvents with different polarity; for example, in 2d, there was a remarkable solvatochromic color change from blue (432 nm in cyclohexane) to yellow (560 nm in DMF), owing to the presence of the terminal electron-donating N,N-dimethylamino groups (-N(CH3)2) which lead to a large change in the singlet excited-state dipole moment,™ as the polarity of the solvents increasing, the singlet excited states would decrease. The solvent effect of the absorption and fluorescence spectra was further evaluated by the Lippert-Mataga plot (Eqs. (1)-(2)),[31] which is dependent on the Stokes shift (Avst) and the solvent parameter Af(e, n).[32]

Where Av=vabs-vem stands for Stokes shift, vabs is the wave-number of maximum absorption, vem is the wavenumber of maximum emission, Am = me - mg is the difference in the dipole moment of solute molecule between excited (me) and ground (mg) states, h is the Planck's constant, R is the radius of the sol-

vent cavity in which the fluorophore resides (Onsager cavity radius), and Df is the orientation polarizability given by (Eq. 2).

Where e is the static dielectric constant and n the refractive index of the solvent.

Similarly, the emission spectra of 2f was red shifted by as much as ca. 76 nm depending on the polarity of solvents (from 432 nm in cyclohexane to 497 nm in DMF) with obvious ICT characteristics. However, the emission maxima of 2a-c, with the electronic-donating groups, and 2e were were only slightly red shifted by up to 10 nm depending on the solvent polarity even in DMF (see supporting information). Although the compounds 2a-c and 2e possess weak CT characteristics, they display relatively high QYs compared with the others.

Electrochemical properties

The electrochemical properties of the title compounds 2a-e were investigated by cyclic voltammetry (CV) in dichloro-methane containing 0.1 M nBu4NPF6 as the supporting electrolyte, with a scan rate of 100 mVs-1 at room temperature. As shown in Figure 8, all of the Y-shaped extended p-conjugated

Figure 8. Cyclic voltammograms recorded for compounds 2a-e.

pyrene derivatives displayed an irreversible oxidation peak. In contrast, previous studies on similar Y-shaped pyrene molecules with aryl-substituted moieties have shown a quasi-reversible oxidation wave,[22] which might be due to the terminal nature of the p-conjugated arylethynyl substitution effect on the electronic properties. For 2a-e, the CV peaks for the positive potentials range from 1.23 to 1.28 V. According to the CVs and the UV-vis absorption, the highest occupied molecular orbital (HOMO) were estimated from the onset values of the first oxidation, and the optical energy gap (Eg) was derived from the UV-vis data. The HOMO energy levels for 2a-e ranged from -5.28 to -5.34 eV, followed the empirical formula LUMO =

HOMO-Eg, the LUMO levels of 2a-e are located from -2.37 to -2.61 eV. Given the considerable HOMO and LUMO energy values of the compound, it might have potential use as a luminescent hole-transporting material in OLEDs.[33]

Quantum chemistry calculations

To gain further insight into the electronic structures of the Y-shaped p-conjugated compounds 2, DFTcalculations were performed at the B3LYP/6-31G(d) level of theory using the Gaussian 2003 program.

The 3D-structures and energy density of the HOMO and LUMO levels of each material are displayed in Figure 9 and in

Figure 9. Selected computed molecular orbital plots (B3LYP/6-31G*) of the compounds 2d and 2f.

the supporting information Figure S4. As the molecular geometries show, the substituent groups are almost coplanar with the pyrene core. Thus there is a close correlation between the quantum calculations and the single crystal X-ray diffraction results; minor differences of molecular geometry arise from the theoretical calculations being carried out in the "gas-phase". On the other hand, the various substituent groups play a significant role for the HOMOs and LUMOs, for example, as the electron-releasing ability increases from 2a to 2d, the destabili-zation of the HOMOs is greater than that of the LUMO, the HOMO levels are mostly located over the full molecular skeleton, whereas the LUMOs only spread in the center of pyrene rings; in contrast, with electron-withdrawing moieties, the LU-MOs of 2e and 2f are almost fully localized on the entire molecular framework, and the HOMOs destabilized in the pyrene ring and a fragment of the phenyl groups. In addition, from the calculation data, we observed the energy gap of 2f is close to 2d with 2.83 eV, so both compounds exhibit similar UV spectra with a broad and strong absorption band in the 370-450 nm region. It is noteworthy that the HOMO-LUMO energy gaps of

the title compounds 2 are lower than that previously reported for Y-shaped 1,3-bisaryl-functionalized pyrene species,[22] due to the effect of arylethynyl moieties on the energies of the molecular orbitals.

Conclusions

In summary, a series of Y-shaped arylethynyl-functionalized pyr-enes 2 were synthesized by a Pd-catalyzed Sonogashira coupling reaction in reasonable yield. The Y-shaped, extended p-conjugated pyrene derivatives display strong face-to-face p-stacking with off-set head-to-tail stacking as compared with a Y-shaped carbon-carbon single bond arylsubstituted pyrene.[22] Furthermore, the effects of the substituents and their position on the crystal packing structure and photophysical properties of the chromophores has been evaluated. Compounds 2a-c and 2e in dichloromethane exhibited deep blue fluorescence with high quantum yield, while the chromophores 2d and 2f exhibit intermolecular charge-transfer (ICT) leading to intense optical absorbances over a wide spectral range. Strong fluorescence spectra of 2d and 2f show the maximum emission at 517 nm and 485 nm, respectively, with remarkable sol-vatochromic effects in polar solvents. Theoretical computation and experimental CV studies on their electrochemistry verify that Y-shaped extended p-conjugated pyrenes 2 can be utilized in OLED devices as luminescent hole-transporting materials. In future, we will attempt to fabricate highly efficient OLED devices using these materials.

Acknowledgements

This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices (Institute for Materials Chemistry and Engineering, Kyushu University)". We also would like to thank the EPSRC (overseas travel grant to C.R.), The Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Fund Program for the Scientific Activities of Selected Returned Overseas Professionals of Beijing and The Scientific Research Common Program of Beijing Municipal Commission of Education for financial support (KM201510015003).

Keywords: Pyrenes ■ Intramolecular charge-transfer ■ Single crystals ■ Luminescence material ■ Structure-property relationship

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Submitted: May 24, 2016 Accepted: May 25, 2016