Iron ( III ) bromide catalyzed bromination of 2-tert-butylpyrene and corresponding positions-dependent aryl-functionalized pyrene derivatives

The present work probes the bromination mechanism of 2-tert-butylpyrene (1), which regioselectively affords mono-, di-, tri-, and tetra-bromopyrenes, by theoretical calculation and detailed experimental methods. Bromine atom may directed to the K-region (positions 5and 9-) instead of the more reactive positions 6and 8-position in the presence of iron powder. In this process, FeBr3 plays a significant role to release of steric hindrance or lowing the activation energy of the rearrangement. The intermediate bromopyrene derivatives were isolated and confirmed by H NMR spectra, mass spectroscopy and elemental analysis. Further evidence on substituent positions originated from a series of aryl substituted pyrene derivatives, which were obtained from the corresponding bromopyrenes on reaction with 4methoxyphenylboronic acid by a Suzuki-Miyaura cross-coupling reaction. All of Positionsdependent aryl-functionalized pyrene derivatives are characterized by single X-ray diffraction, H/C NMR, FT-IR and MS, and offered a straightforward proof to support our conclusion. Furthermore, the photophysical properties of series of compounds were confirmed by fluorescence and absorption, as well as by fluorescence lifetime measurements.


Introduction
Pyrene and its derivatives 1 belong to a classical family of polycyclic aromatic hydrocarbons (PHAs) that have been extensively investigated for light-emitting device applications over recent years.This interest stems from their inherent chemical and photochemical characteristics, in particular an excellent deep blue chromophore which exhibits great chemical stability and high charge carrier mobility.However, owing to its planar structure, pyrene has a strong tendency to form π-aggregates/excimers, which, in-turn, leads to an excimer emission band and the quenching of fluorescence in condensed media and a resulting low fluorescence quantum yield.With this in mind, enormous effort has been paid towards exploring new methods to functionalize the pyrene-core for developing molecular materials and applications thereof.
In general, due to the presence of nodal planes located at the 2and 7-positions in both the highest occupied molecular orbital's (HOMO) and the lowest unoccupied molecular orbital's (LUMO) of pyrene, substituting pyrene at the 2-and/or 2,7-positions is more difficult compared to other positions (such as 1-, 3-, 6-and 8positions (active site)). 2 Thus, there are few examples which focus on substituting at the 2-and 7-positions of pyrene by borylation, 3 bromination, 4 nitration, 5 oxidation 6 and tert-butylation. 7On the other hand, the active sites, namely the 1-, 3-, 6-, and 8-positions have been thoroughly examined and the products used in a variety of applications as optical materials.1a,1b Since we first reported 8 the oxidation of pyrene at the K-region (4-, 5-, 9-and 10-positions) in 1997 by stepwise synthetic methods, the K-region also has been explored as a convenient synthetic route to the ketone 9 and used for preparing pyrene-fused azaacene derivatives for application inorganic semiconductors. 10s previously mentioned, bromopyrenes are significant intermediate compounds which play an important role in modern organic chemistry, not only for synthetic methodology, but also for advanced optoelectronic materials.Commonly, the 1-, 3-, 6-, and 8positions of pyrene preferentially undergo electrophilic aromatic substitution (S E Ar) reactions.Therefore, mono-, bis-, tri-, and tetrakis-substituted pyrenes were synthesized for organic electronic devices 1 and fluorescence probes. 11For example, Thummel et al. 12 discussed the crystal packing of 1,3-, 1,6-, 1,8-, and 2,7-bis(2- [1,10]phenanthrolinyl)pyrenes and their use as pyrene-bridging ligands in ruthenium(II) chemistry, as evidenced by 1 H NMR spectra and single-crystal X-ray crystallography.Sankararaman 13 and coworkers reported a pyrene octaaldehyde derivative from 1,3,6,8tetrabromopyrene, which can cause molecular aggregations in nonpolar solvents and in the solid-state through cooperativity of the intermolecular π-π stacking and C-H•••O interactions, which has potential application in the field of molecular optoelectronics.Chow 14 and co-workers synthesized sterically congested tetraarylpyrenes as efficient blue emitters that exhibited pure-blue electroluminescence and formed respectable organic light-emitting diodes (OLEDs).More recently, Konishi 15 and co-workers systematically alkylated the active sites, namely the 1-, 3-, 6-, and 8positions of pyrene, and investigated the effects of the number of alkyl substituents on the photophysical properties of the pyrene chromophores.Recently, we reported a new type of fluorescent sensor based on a pyrene-linked triazole-modified hexahomotrioxacalix [3]arene, which used the 1-pyrenyl moiety for selectively detecting Zn 2+ and H 2 ions in neutral solutions.16 Bromination of the pyrene not only occurred at the active sites of 1-, 3-, 6-, and 8-positions, but also substitution occurred at the Kregion, namely the 4-, 5-, 9-, and 10-positions.For instance, our group reported a series of pyrene-based cruciform/hand-shaped light-emitting monomers with highly emissive pure-blue fluorescence from tetrabromo/pentabromopyrene. 17 Very recently, we explored a new bromide precursor, 1,3,5,9-tetrabromo-7-tertbutylpyrene, 18 prepared via bromination of 2-tert-butylpyrene (1) in CH 2 Cl 2 at room temperature using iron powder as the catalyst.
About twenty years ago, we reported a FeBr 3 -catalyzed rearrangement of a pyrene-based material in which the bromine atom was transferred from the active site (1-position) to the K-region (4position), ie bromination of 2,7-di-tert-butylpyrene with 1.1 mole equiv. of bromine in the presence of iron powder to afford 1-bromo-2,7-di-tert-butylpyrene, which can be further brominated with excess bromine in the presence of FeBr 3 -catalyzed, and afforded 4,5,9,10tetrabromo-2,7-di-tert-butylpyrene (Scheme 1). 8However, the detailed bromination mechanism still remains unclear.Interestingly, we recently succeeded in developing the bromide precursor 1,3,5,9-tetrabromo-7-tert-butylpyrene from 2-tert-butylpyrene (1) using iron-powder catalysis. 18We speculated that the sequence of reactions involved a stepwise bromination process from the 1-, 3-, 5-, to the 9-position.In this case, it seemed easy to understand that the bromination reactions of the pyrene preferentially occurred at the active sites of the 1-and 3-positions owing to the tert-butyl group protecting the ring against electrophilic attack at the 6-and 8-positions. 22Following this, as expected, the next step would be to substitute regioselectively at the 5-and 9positions, which should be favoured given that the bromine atoms substituted at the 1-and 3-positions would sterically hinder the 4and 10-positions.However, further experimental results revealed that the reaction process was more complicated than our initial predictions.
It is well known that bromination reactions can be violent and complex processes, thus to investigate the bromination mechanism is far from simple, because it is difficult to capture the transition state or a transition state analogue by experimental methods.Given this, the mechanism of electrophilic substitution was generally investigated by kinetic and stereochemical studies, or by theoretical analyses. 23In additional, if the experimental conditions cannot be well controlled, then the final components will be complicated and hard to characterize. 24In this paper, we present the first example of the systematic exploration of the bromination rearrangement reaction of pyrene affording the mono-, di-, tri-, and tetra-bromopyrenes by detailed experimental procedures and theoretical calculation.And the corresponding positions-dependent arylpyrenes derivatives containing the 4-methoxyphenyl group were synthesized from the corresponding bromopyrenes via a Suzuki-Miyaura cross-coupling reaction, which was characterized by single crystal X-ray diffraction and 1 H NMR spectroscopy.The detailed results have strongly supported the bromo-substituted positions in the pyrene ring as previous assumption.Furthermore, the effects of the methoxyphenyl group (both number and the substitution pattern) on their photophysical properties, as well as for their molecular packing, were investigated.

Stoichiometric bromination of 2-tert-butylpyrene (1)
A three-neck round bottom flask fitted with a dropping funnel and a CaCl 2 drying tube, was filled with 1 (200 mg, 0.78 mmol) in 20 mL of CH 2 Cl 2 and was stirred for 30 min.at 0 °C.A solution of Br 2 (depending on stoichiometric ratio) in 5 mL of CH 2 Cl 2 was added drop-wise.After the addition of bromine was completed, the mixture was warmed to room temperature (28 °C) and stirred for 5 h.The crude product was washed with hot hexane and the yields of products are compiled in Table 1.The synthesis of the key intermediate bromopyrenes is shown in Scheme 2. 1-Bromo-7-tert-butylpyrene (2a) and 1,3-dibromo-7-tertbutylpyrene (2b) have been synthesized with two kinds of bromination reagents and used to achieve the chemical modifications of the pyrenes for the required products. 22Firstly, 1) a mixture of 1 and 1.0 equiv.bromine in CH 2 Cl 2 at 28 °C in the presence of an iron powder catalyst, afforded 2a in 83% yield; in the other hand, in the absence of iron powder, mixture of 1 and 1.0 equiv.bromine in CH 2 Cl 2 at -78 °C afforded 2a in 75% yield 22b ; 2) A mixture of 1 and BTMABr 3 (1.0 equiv.) in CH 2 Cl 2 at 28 °C afforded the desired product 2a in 84% yield.3) According to reported, 22b a mixture of Br 2 and 1 in anhydrous CH 2 Cl 2 at -78 °C afforded 2a in 89% yield in the absence of iron powder; however, in the presence of iron powder, a mixture of 1 and 2.0 equiv. of bromine at 28 °C afforded a mixture 2a in 50% yield and 2b in 35% yield.4) A mixture of 1 and BTMABr 3 (3.5 equiv.) in CH 2 Cl 2 at 28 °C afforded the desired product 2b in 76% yield.Similarly, 5) for comparison, we synthesized 1,3,6-tribromo-7-tert-butylpyrene (2c) in the absence of iron powder in 65% yield according to the reported procedure. 256) When 1 was mixed with stoichiometric Br 2 (1:3~5) under the same experimental conditions, the intermediate product 1,3,5-tribromo-7tert-butylpyrene (2d) was obtained with 2f, which could not be separated from the crude compound by column chromatography.We attempted to isolate both 2d and 2f in pure form by highperformance liquid chromatography (HPLC) but failed.7) When the same reaction was carried out with 6.0 equiv. of bromine in the presence of iron powder, the Lewis acid-catalyzed rearrangement of bromine was observed to give 1,3,5,9-tetrabromo-7-tert-butylpyrene 2f in 84% yield. 18It seems that compound 2f might be formed by the isomerization of compound 2c under FeBr 3 -catalyzed, which should be produced from bromine and iron powder present during the bromination.8) Reaction of 2c with Br 2 (1:2.5) was carried out in the presence of iron powder.A mixture of bromides 1,3,5,8-tetrabromo-7-tert-butylpyrene 2e and 1,3,5,9-tetrabromo-7-tert-butylpyrene 2f was obtained in the ratio of 7:3 (determined by their 1 H NMR spectra analysis); 9) when mixture of 2e and 2f was treated with 2.5 equiv. of bromine in the presence of iron powder in CH 2 Cl 2 solution at 28 o C for 8 h, the expected product, 2f was obtained in 75% yield.10) However, attempted isomerization of compound 2e to 2f with other Lewis acids, such as TiCl 4 , AlCl 3 or FeCl 3 , performed under the same reaction conditions failed; only the starting compound 2e was quantitatively recovered.Brominations of compound 1 in the presence of iron powder to afford the bromo-substituted pyrene derivatives 2 were carried out under various reaction conditions and the detailed results are summarized in Table 1.

Regioselective bromination mechanism of 2-tert-butylpyrene (1)
From previous experimental section, 2a, 2b, and 2c were prepared that is not depending on the presence of iron (III) bromine, instead, the stoichiometric ratio of bromine reagent.This process is classical electrophilic substitution reaction.This result is strongly attributed to the high reactivity of the 1-, 3-, 6-and 8-positions in the pyrene ring.However, in the following bromination reactions for 2d, 2e and the role played by the FeBr 3 -induced rearrangement to form the 2f is not clear.To disclose the bromination mechanism of 1, the deitnsity functional theory (DFT) calculations DFT-calculated (B3LYP/6-31G* basis set) were carried out using the Gaussian 03 software package for investigating the potential-energy surface of each bromopyrene derivative in Scheme 3. B3LYP functional was chosen.The geometric structures of bromo-substituted compounds 2 are optimized at 6-31G* level and their parameters were given in supporting information.The frequency and the electronic distribution of 2 were further tested, the results show that the vibration frequency of 2 are with positive value, without imaginary frequency, indicate the calculation of molecular structure energy is the minimum and stable.The relative free energy (∆G 298 ) is exergonic by 7.9 kcal/mol in the process of bromination of 1, leading to 2a, which can be further bromination to afford 2b and 2c with an exergonic reaction of 8.2 kcal/mol and 15.4 kcal/mol, respectively.when 1 with the bromine reagent leads to 2c with an exergonic reaction of 23.3 kcal/mol.The subsequent bromination step from 2c or 2d to 2e and 2f with an exergonic is ≈ 8.8 kcal/mol.when 1 with the bromine reagent leads to 2c with an exergonic reaction of 23.3 kcal/mol.The subsequent bromination step from 2c or 2d to 2e and 2f with an exergonic is ≈ 8.8 kcal/mol.
Comparison of the geometric structures of 2, that show that the angle of bromine atom in 6-position of 2c (∠Br3-C5-C6 = 115.5 o ) or 8-position of 2e (∠Br4-C8-C7 = 115.7 o ), which is less than in other bromo substituted-position (for example: ∠Br1-C1-C2 = 120.6 in 2a).And the tert-butyl group located at 7-position of pyrene core have slightly crowded in 2c and 2e with angle of 125.7 o ± 0.1, which is larger than those in 1 and 2a-b, 2d and 2f.Interesting thing is that 2c or 2d was generated from 2b with similar exergonic (7.2 kcal/mol for 2c and 7.7 kcal/mol for 2d).However, the 2d can not observed by bromination 2b with an excess of bromine without iron powder presenting in our experiment.In fact, in the presence of iron (III) bromide, 2a-c and 2e was synthesized by undergo Friedel-Craftstype reactions from 1 with bromine.From Scheme 3, the presence of iron (III) bromide might be contribute to lower the activation energy of bromination and induce intramolecular bromine rearrangement in the process of 2d and 2f, that bromine atom would shift from active site (6-or 8-position) to K-region (5-or 9-position).The possible bromination reaction pathway has been summarized in Scheme 4. Firstly, The relatively easy electrophilic substitution at the ortho-position to a tert-butyl group (6-or 8-position) on the pyrene ring is remarkable because usually the steric bulkiness of a tert-butyl group might be expected to direct the substitution towards other positions of the pyrene ring. 7,8This result is strongly attributable to the high reactivity of the 1-, 3-, 6-and 8-positions of the pyrene ring.Second, however, the pyrene exhibit special electronic structure that the Kekulé structure (I, II and III) show the greatest number of benzenoid rings should have the greatest weight in the superposition diagram, meanwhile, also show the greatest number of double bonds in the "exposed" position have the greatest weight (see supporting information). 26So, when the bromide attracted the 6-position of 2b, due to the steric strain involving the tert-butyl group and lowing the Gibbs free energy, the bromide would rearrange to 5-position under FeBr 3 -catalyzed and afford 2c.Similarly, the tetrabromide 2f was obtained by the same FeBr 3catalyzed rearrangement in the bromination of tribromopyrenes 2c and 2d in the presence of iron powder.The above results strongly suggest that compounds 2c, 2d and 2e were the intermediates for the formation of the 1,3,5,9-tetrabromo-7-tert-butylpyrene 2f.
The performance of the organic compounds in optoelectronic devices strongly relies on the molecular packing and intra/intermolecular interactions in the solid-state.Therefore, investigating the effects of a structure-property relationship between the substituent groups on the pyrene-core using crystal structure and photophysical properties is significant for organic electroluminescence materials.Herein, we expected that the integration of poly-methoxylphenyl groups with the pyrene core in a molecular structure might influence the crystal packing, leading to favourable optical features and charge-transport properties that could be useful in optoelectronic devices.

Description of crystal structures
Previously, we reported that several Y-shaped aryl-substituted pyrenes with electron-donating/withdrawing groups at the paraposition of the C 6 H 4 rings inefficiently impact on the molecular packing in the solid-state.22a Konishi et al. have validated that alkyl groups located at the 1-, 3-, 6-, and/or 8-positions of the pyrene ring play significant roles in tuning the photophysical properties. 15Herein, this article presents a series of position-substituted pyrenes that not only take place of the active sites (1-, 3-, 6-and 8-positions) but also the K-region (4-and 9-positions) of pyrene, and allow us to shed light on the effect of multiple 4-methoxylphenyl groups on the molecular packing and optical physical properties.Crystals of 3 suitable for X-ray structure analysis were grown from mixed solvents via slow evaporation at room temperature.And the key crystallographic data are summarized in supporting information Table S2; the crystal structures of molecules for 3 are shown in Figure 1.

Figure 2. Crystal packing of 3a by C-H•••π interactions
Colourless crystals of compound 3a suitable for X-ray crystallographic analysis were obtained by crystallization from a mixture of dichloromethane and hexane (1:2, v/v).The single-crystal X-ray structure is depicted in Figure 2. It can be seen that the 4methoxyphenyl groups in this molecule form a torsion angle (65.43 (17)°) with the plane of the centeral pyrene ring to prevent face-to face π-stacking and steric clashes between ortho H atoms on the phenyl ring and those at the 3-and 9-positions.An interesting feature of the compound in the solid-state is that there is an intermolecular 88 Å) between neighbouring molecules.These interactions led to a comparatively large twist angle between the pyrene core and the methoxylphenyl fragment, and effectively suppress the formation of the π-π stacking.
On increasing the numbers of 4-methoxylphenyl groups and substituting at different positions in the pyrene derivatives 3a→f, the space groups (orthorhombic for 3a, monoclinic for 3b, 3c and 3e, triclinic for 3d and 3f) became more asymmetric.It can be seen that the torsional angles between the 4-methoxyphenyl group and the pyrene core decreased from 65.43 (17)° to 48.08 (18)°.With the number of substituted group increasing, the conformation molecules trend for increasing co-planarity. 27However, without the tert-butyl group located at the nodal planes involving the 2-/7-positions in 1,3,6,8-tetrakis(4-methoxyphenyl)pyrene (4), 18 the torsion angle is unexpectedly larger than 3 and is up to 76.1 (4)°.Colourless rod crystals of 3c suitable for X-ray diffraction were recrystallization from a mixture of dichloromethane and methanol by slow evaporation at room temperature.Figure 3 shows the crystal packing in 3c.The crystal structure revealed the novel asymmetric substitution of the pyrene core.As expected, the 4-methoxyphenyl group successfully substituted at the 6-position of the pyrene with anapproximately perpendicular torsion angle of 88.58(4)°.In the crowded region at the 6-and 7-positions two bulky moieties were introduced, which contact each other by steric interaction.This causes two-fold tert-butyl group disorder with occupancy ratio 0.634 (12):0.366(12)
Yellow needles of 3d were obtained from a mixed solution of dichloromethane/hexane = 1:1; the asymmetric unit of compound 3d contains one molecule (Figure 4).The molecule exhibits a nonplanar and the central pyrene ring has a slight bend; possibly arising from the imbalance of the electrostatic potential on the molecular surface. 27The inter-planar angles between the central pyrene coreand the outer substituent phenyl rings range from 44.2(5)° to 57.7(5)°.The crystal structure was arranged in columns along the aaxis, through essentially parallel, or near-parallel interactions between translationally equivalent molecules.Each molecule is interlaced with adjacent columns along the a-axis by the formation of π-π stacking with a centroid-to-centroid distance of 4.07 Å and inter-planar angle of 0°.There are numerous C-H•••π interactions formed between phenyl hydrogens and neighbouring aromatic rings.Compound 3e was dissolved in CHCl 3 and kept in a CH 3 OH atmosphere at room temperature to afford orange single crystals suitable for X-ray diffraction.The crystal system of 3e is monoclinic with space group P2 1 /c and is shown in Figure 5.A similar arrangement pattern to 3d was observed.The molecules also adopt aslipped face-to-face and π-π stacking pattern along the b-axis with acentroid-to-centroid distance of 4.96 Å.This is longer than that observed in 3d, arising from an extra 4-methoxyphenyl moiety playing a role to prevent the neighbouring molecules getting too close to each other.Also, the 4-methoxyphenyl group and bulky tertbutyl group share a crowded space at the 1-and 2-positions of the pyrene, and these sterics result in the Csp 2 hybridization angle C4-C5-C17 changing to 124.07( 6  Comparison of tetra-substituted pyrenes 3e, 3f and 1,3,6,8tetrakis(4-methoxyphenyl)pyrene (4), the crystal packing in the solid-state is different attribute to the position-substituted diversification. 18,28For the packing structures of 3a to 3f, the X-ray diffraction revealed that the torsion angle decreased between the substituent group and the pyrene core as well as the molecular packing varied significantly from 3D (column structure) to 2D (planar structure) with the number of 4-methoxylpehnyl groups increased.As mentioned above, the number of substituent groups and their positions, as well as the bulky tert-butyl groups play an important role in arranging the molecular conformations.The structures became more planar, which in turn was beneficial for πconjugation and improved the optical density, leading to strong FL emission in the solid-state. 29In the next section, we discuss the resulting photophysical properties resulting from the presence of multiple 4-methoxylphenyl substituents.

Photophysical properties
The normalized UV-vis absorption and fluorescence spectra for 3 recorded in dichloromethane are shown in Figure 6, For comparision, 1,3,6,8-tetrakis(4-methoxyphenyl)pyrene (4) 18 and 2-tert-butyl-4,5,7,9,10-pentakis(4-methoxyphenyl)pyrene (5) 17a were also summarized in here for investigating the effect of the positionsdependent aryl-functionalized pyrene derivatives for the packing structures and photophysical properties, and the corresponding photophysical data is summarized in Table 2. Except for 3c and 5, all molecules exhibited very similar photophysical characteristics with well-resolved spectral bands in both the shortwavelength of 283−305 nm and long wavelength of 347-391 nm regions.The absorption spectra of 3→4 show a maximum band at 347 nm for 3a, 363 nm for 3b, 363 nm for 3d, 381 nm for 3e, 381nm for 3f and 391 nm for 4.However, 3c has two absorption peaks centered at 295 nm and 375 nm with a shoulder peak at 363 nm; similarly, 5 displays two maxiumn absorption peaks at 296 nm and 356 nm with a shoulder peak at 343 nm.Clearly, both the number of substituents and the substitution position (pathway) strongly influence the electronic absorption; 2 the absorption maximum of 3 revealed a remarkable red-shift.It is thought that such phenomena arise from the increasing number of peripheral arms in this series, which extend the πconjugation of the pyrenes.The longest absorption (λ max ) exhibited by 3e and 3f was approximately red shifted by ca.34 nm versus 3a, indicating the HOMO−LUMO energy gap of 3 decreased on increasing the conjugation length.Interestingly, for the hand-shaped compound 5, despite it having the most substituents, the absorption does not show a significant red-shift in comparison with those of the tetrasubstituted derivatives 3e and 3f, which is probably a result of the nodal planes passing through the 2,7-positions, leading to a lower electronic density by the rotation of 4-methoxyphenyl group located at 7-position of pyrene core.This suggests the 2,7substitutions have a small influence on the electronic interaction, 2 and the results have also been reinforced by DFT calculations (mentioned below).
For the emission spectra, all compounds exhibit intense emissions in the blue region (391-434 nm).The emission maxima of 3 and 4 are bathochromically shifted depending on the numbers of 4methoxylphenyl units, revealing an identical trend to their absorption spectra.No characteristic excimer fluorescence was observed in any of the spectra.It is worth noting that 1,3,5,8-functionalized pyrene 3e exhibits an emission with λ em values of 420 nm, which almost overlaps with the emission spectrum (421 nm) of 1,3,5,9functionalized pyrene (3f).In general, the substitution pattern of the pyrene moiety has a substantial effect on the fluorescence wavelength, and the effect of being substituted at the active 1-, 3-, 6and 8-positions for the S 1 ←S 0 excitations is more significant than at the K-region (4-, 5-, 9-and 10-positions). 2 In the case of 3e and 3f, smaller red-shifts in their electronic absorption profiles indicate that the structural changes and/or electronic distribution changes can cause an electronic communication missing in the pyrene cores. 30For 5, a deep-blue emission was observed with a maximum peak at 411 nm and a shoulder at 430 nm in solution.The UV-vis absorption and emission spectra of selected pyrenes in the solid-state are shown in Figure 7, and the optical data are summarized in Table 2. Compared with the corresponding solutions, the absorption for 3b, 3c, 3e, 4 and 5 films reveals a slight red-shift (about 10 nm) (Table 2).However, the absorption of 3f as a film shows a slight blue-shift in comparison with that in solution.This unusual blue-shift might be due to the different dielectric constant. 31he emission maxima of 3b, 3e and 3f as thin films exhibited redshifts of less than 48 nm relative to those in solution, and the compound 3c exhibits a red-shift of 65 nm (from 406 nm to 471 nm).(Table 2).On increasing the numbers of 4-methoxylphenyl moieties, the red-shift decreased in the following order: 5 (3 nm) < 3f (22 nm) ≈ 3e (21 nm) < 3b (48 nm) < 4 (54 nm) < 3c (65 nm), indicating the positions-substitution of aryl-functionalized would influence the electronic interaction.Owing to the strong intermolecular interactions in the thin film of 3c and 4, the emission maxima exhibited a greater red-shift as a thin film state with high noise level. 32ifferent to the 1,3,6,8-tetrakis(4-methoxyphenyl)pyrene 4, 18 that tends to exhibit a high noise level PL spectrum in the solid-state.the tetra-substituted pyrenes 3e and 3f exhibited clear and sharp emission peaks in the blue-region without extra excimer emissions in the solid-state owing to the bulky tert-butyl group located at 7position of the pyrene ring suppressing the aggregation.Compound 4 also presents a very high fluorescence quantum yield (Φ f c ) of the range of ~0.94 in solution.
For comparison, the quantum yields of 3b, 3c, 3e and 3f in the solid-state were also investigated (0.58 for 3b, 0.28 for 3c, 0.58 for 3e and 0.72 for 3f).However, for 5, low fluorescence quantum yields in both solution and the solid-state were obtained due to energy loss that is likely occurring during the exciton migration. 33he fluorescence lifetime of 3a-c, 3e, 3f, 4 and 5 are 8.6 ns, 8.9 ns, 5.2 ns, 2.2 ns, 5.8 ns, 1.9 ns, 18.8 ns, respectively.Excellent optical features were obtained in these compounds, which make them of potential use in new optoelectronic devices, such as blue emitters in OLEDs, or as models for further exploring a new generation of organic materials based on pyrene.

Quantum chemistry computation
To gain further insight into the effect of multi-substituents and pathways on the electronic structure and spectral properties of compounds 3, 4 and 5, quantum chemical calculations were calculated using DFT methods at the B3LYP/6-31G* level.The calculated energies of the frontier molecular orbitals are presented in Figure 8 and in the supporting information.Scrutiny of the electronic structures reveals that both HOMOs and LUMOs of 3 were primarily delocalized over the entire pyrene component, as well as slightly in the peripheral phenyl moiety, the only difference being in the energy of these frontier molecular orbitals, which in turn relied on the system architecture.For instance, on increasing of the numbers of the substituents from 3a to 3f, the HOMO values are more posotive varied from -5.06 eV (3a) to -4.76 (3f) eV, the increased positions-dependent substitution resulted in a lowering of both the HOMOs and LUMO by 0.3 eV and 0.05 eV, respectively.Obviously, The effects of multiple substituents is greater for the HOMOs than for the LUMOs with a sizable shrinking of the HOMO−LUMO gap by 0.28 eV with respect to 3a, which is in good agreement with the experimentally measured results in tendency, the slight different to those obtained by UV-vis absorption (∆E gap opt = 0.27 eV) owing to the DFT-calculation was performed in the gas phase.Compared with 3e/3f and 4, the presence of the tert-butyl group at the 7-position, could lead to a lower energy gap by lowing the molecular LUMOs.With the number of substituent groups increased, from the mono-substituted 3a to the tetra-substituted pyrenes 4 and 3e/3f, the energy gap of the representative molecules decreased.Especially, the speical electronic structure of penta-substituted pyrene 5 inhibits absorption spectra is different from others, due to the substituted group at the K-region (4,5,9,10-positions) in favour of blue-shift by the improving energy gap of the molecular structure; when as the 4-methoxyphenyl group located at nodal planes would weak electronic coupling with entire pyrene, [4] attributing to little influence on S 2 ←S 0 absorption but lager influence on S 1 ←S 0 absorption. 34So, These conclusions are also consistent with our quantum chemical calculations.

Electrochemistry
The electrochemical properties of selected compounds 3 were investigated in CH 2 Cl 2 solution by cyclic voltammetry (CV) in a three-electrode electrochemical cell with Bu 4 NClO 4 (0.1 M) and Ag/AgCl as electrolyte and reference electrode respectively, and using ferrocene (Fc/Fc + ) as the internal standard with a scan rate of 100 mVs −1 at room temperature.
As shown in Figure 9, compound 3b exhibits a quasi-reversible oxidation processin the positive potential region with a oxidation process around 1.0 V (vs.Ag/AgCl), and compounds 3c, 3e and 3f exhibit two reversible or quasi-reversible oxidation processes, respectively.On the basis of the absorption spectra and the CV, the corresponding HOMO and LUMO energy levels were confirmed and the results are displayed in Table 3 and in the supporting information (Table S3).The HOMO values were calculated from the oxidation potential by the empirical formulae HOMO = -(4.8+Eox onset ), where E ox is the onset of the oxidation potential.The HOMO values were -5.44 eV for 3b, -5.06 eV for 3c, -5.35 eV for 3e and -5.36 eV for 3f.
The LUMO levels were determined from the HOMO and energy gap.
For 3b and 3c containing increasing numbers of 4-methoxyphenyl moieties in different positions on the pyrene, similar energy gap (3.17 eV for 3b and 3.13 eV for 3c) with different HOMO and LUMO levels are achieved.Additionally, 3e and 3f have same numbers of substituents but different substituted position, that also showed similar HOMO (-5.36 eV for 3e and -5.37 eV for 3f, respectively) and LUMO (-2.33 eV for 3e and -2.34 eV for 3f, respectively).From both Figure 9 and Table S2, it can be seen that oxidation potential are shifted to more positive values when increasing the number of substituents.The half-wave potentials for compounds 3b (2 substituents), 3c (3 substituents), 3e (4 substituents) and 3f (4 substituents) are 1.45 eV, 1.51 eV, 1.64 eV and 1.67 eV, respectively.Furthermore, the reduction potentials was effected dependent on position, and substituted at active sites of 1,3,6,8-position would lower LUMO level and K-region of 4,5,9,10-position contributes to improve LUMO level.

Conclusions
In summary, the bromination mechanism of pyrene was explored by experimental methods.Clear evidence was observed for theformation of mono-to tetrakis(4-methoxyphenyl)-substituted pyrenes (3), which were synthesized by Suzuki-Miyaura crosscoupling reaction of the corresponding bromopyrenes with 4methoxyphenyl boronic acid, and characterized by single-crystal Xray diffraction, 1 H/ 13 C NMR spectra, mass spectrometry as well as elemental analysis.These results supported our conclusions on the bromination mechanism, namely that it was possible to regioselectively generate the mono-to tetrabromopyrenes from the 2-tert-butylpyrene (1) via a stepwise electrophilic substitution using a FeBr 3 -catalyzed rearrangement.Otherwise, the spectroscopic data, DFT calculations and electron chemistry results of the arylfunctionalized pyrenes indicate that the HOMOs and LUMOs level can be tuned by both the number of substitution and substutitedposition cooperativity; the numbers of substituent moieties contribute to higher oxidation potential and the reduction potential was attributed to position-substituted.The series of new molecular materials combines excellent optical features with reasonable thermal stabilities, making such molecules potential candidates in optoelectronic applications such as OLED-like devices.Further investigations on their usefulness in organic electroluminescent devices are in progress in our laboratory.

Experimental Section
Materials: Unless otherwise stated, all other reagents used were purchased from commercial sources and used without further purification.The preparations of 2-tert-butylpyrene (1) 35 was reported previously.
All melting points (Yanagimoto MP-S 1 ) are uncorrected. 1H/ 13 C NMR spectra (300 MHz) were recorded on a Nippon Denshi JEOL FT-300 NMR spectrometer.IR spectra were measured for samples as KBr pellets in a Nippon Denshi JIR-AQ2OM spectrophotometer.Mass spectra were obtained with a Nippon Denshi JMS-HX110A Ultrahigh Performance Mass Spectrometer at 75 eV using a directinlet system.Elemental analyses were performed by Yanaco MT-5.UV/ Vis spectra were obtained with a Perkin-Elmer Lambda 19 UV/Vis/NIR spectrometer in various organic solvents.Fluorescence spectroscopic studies were performed in various organic solvents in a semimicro fluorescence cell (Hellma ® , 104F-QS, 10 × 4 mm, 1400 µL) with a Varian Cary Eclipse spectrophotometer.Fluorescence quantum yields were measured using absolute methods.Thermogravimetric analysis (TGA) was undertaken using a SEIKO EXSTAR 6000 TG/ DTA 6200 unit under nitrogen atmosphere at a heating rate of 10 °C min −1 .Differential scanning calorimeter (DSC) was performed using a Perkin-Elmer Diamond DSC Pyris instrument under nitrogen atmosphere at a heating rate of 10 °C min −1 .Photoluminescence spectra were obtained using a FluroMax-2 (Jobin-Yvon-Spex) luminescence spectrometer.Electrochemical properties of HOMO and LUMO energy levels were determined by Electrochemical Analyzer.The quantum chemistry calculation was performed on the Gaussian 03W (B3LYP/6-31G* basis set) software package. 36

Run 9: Lewis acid-catalysed bromination of 2e/2f
A mixture of 2e/2f (100 mg, 0.17 mmol) and iron powder (50 mg, 0.9 mmol) were added to CH 2 Cl 2 (15 mL) at 0 °C with stirring for 15min.A solution of Br 2 (0.025 mL, 0.49 mmol) in CH 2 Cl 2 (5 mL) was slowly added drop-wise with vigorous stirring.After this addition, the reaction mixture was continuously stirred for 8 h at 28 °C.The mixture was quenched with Na 2 S 2 O 3 (10%) and extracted with CH 2 Cl 2 (2 × 20 mL).The combined organic extracts were washed with water and brine and evaporated.The residue (85 mg) was dried and identified by 1 H NMR spectroscopy.The yield was evaluated by 1 H NMR spectral analysis (25 % for 2e, 75 % for 2f).

Synthesis of 4-methoxylphenyl substituted pyrene derivatives 3
The pyrene derivatives 3 were synthesized from resultant bromopyrenes with 4-methoxylphenylboronic acid by Suzuki-Miyaura cross coupling reaction in good yield.Although the mixture of bromopyrenes, 2d/2f and 2e/2f could not be separated, the final products 3d, 3e and 3f were isolated by column chromatography without complication.

Figure 7 .
Figure 7. (a) Normalized UV-vis absorption and (b) emission spectra of 3 and 5 in thin films.

Figure 8 .
Figure 8. Computed molecular orbital plots (B3LYP/6-31G*) of compounds of 3 and 5; the upper plots represent the HOMOs, and the lower plots represent the LUMOs.

Table 1
Bromination of 2-tert-butylpyrene 1 under the various experimental conditions.a a The isolated yields are shown in square bracket.bYieldswere determined by1H NMR analysis and shown in parentheses.

Table 2 .
The photophysical and electrochemical properties of compounds 3, 4 and 5.

Table S2
Summary of crystal data of pyrene derivatives 3 a] DFT/B3LYP/6-31G * using Gaussian,[b]HOMO and LUMO energy levels were calculated according to equations: -(4.8+E ox onset ) and LUMO = HOMO + E g , [c] E ox 1/2 is the half-wave potential of the oxidative waves,[d]E ox onset is the onset potential of the first oxidative wave, with potentials versus Fc/Fc + couple.[e]E g : estimated from UV-vis absorption spectra in solution.nd.No determination. [