Linear and Star‐Shaped Extended Di‐ and Tristyrylbenzenes: Synthesis, Characterization and Optical Response to Acid and Metal Ions

Abstract Two linear 1,4‐distyrylbenzenes and five star‐shaped 1,3,5‐tristyrylbenzene derivatives (L2a and L2b, Y0–Y3 and YNBu) were synthesized and spectroscopically characterized. The photophysical properties, optical response to acid and metal ions were investigated. Upon backbone extension of linear distyrylbenzenes or the introduction of dibutylanilines, the electronic spectra are redshifted. Incorporation of electron‐deficient pyridyl units does not significantly affect the optical properties. Variation of the number of pyridine rings and substitution pattern tune the fluorescence response to acids and metal ions. The novel arenes discriminate Al3+, Mn2+, Fe3+, Fe2+, Cd2+, Ag+ and Hg2+.


Results and Discussion
Synthesis, X-ray crystallographica nalyses and liquid crystalline behavior L 2a is ad istyrylbenzene substituted with two pyridines. L 2b was designed as an analogueo fL 2a with al ongere ffective conjugation length. [10] Y 0 is as tar-shaped and C 3 -symmetric psystem bearing three identical arms.B ased on this knowns kel-eton (Y 0 )r eported by Maier et al., [5c] Y 1-3 and Y NBu are obtained eitherb yvariation of the number of pyridine rings or the introductiono fe lectron-rich dibutylaniline groups, respectively (Scheme 1).
We obtained single crystals for Y 0 (from DCM/methanol), L 2b and Y 3 (from DCM/n-hexane) and performed X-ray analysis. Figure 1d epicts the structure and Table S1-S3 summarize the correspondingd ata. As shown in Figure S1, all derivativesa re almostp lanar and their vinylic linkers display E-configuration. However,t he packingp atterns of L 2b , Y 0 and Y 3 deviate significantly from each other. L 2b packs in parallel layer stacks with some displacement. Y 0 displays a2 -dimensional wall-like arrangementa nd the orientation of molecular planes in adjacent parallel walls is different. In Y 3 ,apattern of parallelp lanes was observed, which extends in three directions.
Similar to Maier's star-shaped compounds, nematicl iquid crystalline phases from isotropic during cooling scans were observed for Y 0-2 as investigated by temperature-dependent polarizationo ptical micrographs ( Figure S2). [5c] No liquid crystalline behaviour was detected for oily Y NBu at room temperature.

Photophysical properties and theoretical calculations
The normalized absorption and emission spectra of the compoundsi nd ilute THF are shown in Figure 2. Table 1s ummarizes the photophysical data. Due to the meta-conjugation, absorptionspectra of star-shaped series Y 0 -Y 3 are superimposable to that of L 2a ,w ith am aximum absorption centered at around 400 nm and as houlder peak located at 326-348nm. It is mainlya ttributed to the extension of conjugation or the electron-richd ibutylaniline groups.
In THF,t he trend in the emission maxima( Figure 2b)f ollows the order Y NBu > L 2b > L 2a > Y 3 > Y 2 > Y 1 > Y 0 . L 2b exhibits ar edshiftedg reen emission with av ibronic structure relative to that of L 2 a,o wing to its longerc onjugation length. Asymmetric Y 1 and Y 2 show similar emission maxima in comparison to symmetric Y 3 while their maxima are redshifted by ca. 10 nm compared to symmetric hydrocarbon Y 0. The fluorescencem aximum of Y NBu bathochromically shiftedt o5 19 nm (Stokes shift Scheme1.Synthetic route to linear 1,4-distyrylbenzenes( L 2a -L 2b )a nd star-shaped 1,3,5-tristyrylbenzened erivatives (Y 0 -Y 3 and Y NBu ). of 4151 cm À1 ). The large Stokes shift observed for Y NBu is caused by its higherd ipole momenti ni ts excited state stabilized in more polar solvents. [12] L 2a , L 2b and Y 0-3 display quantum yields (F f )v arying from 0.64 to 0.82, while Y NBu exhibitsa green fluorescence with aq uantumy ield of 0.61. Although Y 0-3 and Y 4 have similar conjugation skeletons, dibutylaminecontaining Y 4 shows al ower quantum yield, whichm ight be attributed to the free intramolecularr otationo rt he existence of photoelectron transfer facilitated by the flexible dibutylamine groups. [13] DFT calculations (B3LYP/6-31 ++G**) [14] provide further insight into optoelectronics. The calculatedH OMO-LUMO energy gaps for L 2a /L 2b ,Y 0 -Y 3 and Y NBu are in the range from 2.56 to 3.06 eV ( Figure S4). The lower gaps of Y NBu or L 2b are a result of the extended conjugation,r esulting in distinct bathochromic shifts in the electronic absorption spectra. These results, as well as the trend of absorption maxima calculated by TDDFT methods (FigureS3) are consistent with the experiment.
Fluorochromicity Figure 3s hows the emission behaviors of SBs in solvents with different polarity.N egligible changes are observed in the emission color of L 2a and Y 1-3 with pyridine rings. L 2b and Y NBu emit redshifted with increasings olvent polarity. [12] Thee ffect of solvents on the emission features was evaluated by the Lippert-Mataga plot [15] (Supporting Information). All of the SBs displayedl inear dependence of Dn on Df together with different slopes, which was largest for L 2b and Y NBu ( Figure S5 and Ta ble S5). The slope of the fitting line for Y NBu is the highest, up to 9673 cm À1 ,c omparable to that of the X-shaped distyrylbenzenes, [16] which further indicated its larger dipole moment changes between the ground and excited states (m e -m g ), leading to the pronounced solvent sensitivity. [17]

Opticalresponse to protons
Pyridines or dibutylanilines are basic.All SBs (except Y 0 )d isplay an acidochromic change of their absorption and emission spectra ( Figure 4). SBs with pyridine units experience ar edshift, accompanied with al oss of fluorescencei ntensity. Upon protonation the donor-acceptor character of these SBs increases and therefore internal charge transfer is favored. The additiono f TFAt oY NBu leads to strongly blue emissive species, in which the dibutylamino groups are protonated.T he HOMO is stabilized by protonation, resulting in an increased HOMO-LUMO gap. The color change is detected by eye. As expected, Y 3 is more sensitive to protonation compared to Y 1 and Y 2 ,a st here are three pyridine rings as interaction sites.
As shown in Figure

Opticalresponse to metali ons
Dilute solutions of all the SBs in DCM were exposed to an excess of salts of 13 cations (Al 3 + ,Z n 2 + ,C u 2 + ,C u + ,M n 2 + , Fe 2 + ,F e 3 + ,C o 2 + ,N i 2 + ,C d 2 + ,A g + ,P b 2 + and Hg 2 + ,a dded as perchlorates, and CuI) (Figure 6a). Only Hg 2 + quenches lumi-nescenceofY 0 .Exceptf or Zn 2 + ,C u 2 + ,N i 2 + ,t he addition of the remaining ten metal ions leads to either quenching or ar edshift in emission of SBs with pyridine units (L 2a , L 2b and Y 1-3 ). In the case of Y NBu ,A l 3 + ,M n 2 + ,F e 3 + ,F e 2 + ,C d 2 + ,A g + ,a nd Hg 2 + induceablueshift in emission, as expected for ac oordination at the aniline nitrogen.A l 3 + ,M n 2 + ,C d 2 + and Co 2 + are quenchers for Y 1 and Y 2 ,b ut they less impactp hotoluminescence of Y 3 .A l 3 + ,M n 2 + ,F e 3 + ,F e 2 + ,C d 2 + ,A g + and Hg 2 + can be easily distinguished from each other through the responses of the SBs by the naked eye. It is challenging to discriminate Cd 2 + from Zn 2 + . [18] SBs with pyridine or dibutylaniline groups display fluorescencer esponses in the presence of Cd 2 + but no response with Zn 2 + . s m; n r; g ðÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P SBs7 SBs1 r n À r m ðÞ 2 þ g n À g m ðÞ 2 3*7 r ð1Þ Statistical evaluation of differences in emission colors after exposure to metal ions was performed. Figure6bs hows the autocorrelation plot of their response. The brightness independentc olor coordinates rg of the RAWd ata of the photographsw ere determined and treated with MANOVAs tatistics (Eq. (1)). [7c, 19] Zn 2 + ,C u 2 + and Ni 2 + are hard to discern due to their weakc oordination and thus have similar color responses. All of the other investigated metal ions are distinguished.

Conclusions
We have synthesized linear 1,4-distyryla nd star-shaped 1,3,5tristyrylbenzene derivatives (L 2a /L 2b , Y 0 -Y 3 and Y NBu ). These are strongly fluorescent in dilutes olutions. Y NBu works as polarity sensor due to its response to different solvents. Upon protona-   tion, all of the pyridine-containing compounds, L 2a /L 2b and Y 0 -Y 3 ,s how ap ronouncedr edshift, and the fluorophore with dibutylaniline groups Y NBu displays ab lueshifti ne mission. Ten metal ions such as Al 3 + ,M n 2 + ,F e 3 + ,F e 2 + ,C d 2 + ,A g + and Hg 2 + were well discriminated.

Experimental Section
General procedure 1( GP1) Synthesis of intermediates 4-6 by Heck reaction. The reaction was performed in ah eat-gun-dried 50 mL Schlenk tube under an itrogen atmosphere. The brominated intermediate (1.0 equiv) and the vinyl compound (1.14 equiv) were dissolved in dry DMF.P d(OAc) 2 (5 mol %), tris(o-tolyl)phosphine (0.1 equiv) and dry triethylamine (0.8 mL) were added and the mixture was stirred at 110 8Cf or 48 h. After the reaction mixture was cooled to ambient temperature, it was poured into water to give as uspension which was extracted with DCM. The combined organic layers were washed with brine, dried over MgSO 4 and the solvents were removed under reduced pressure. The residues were purified by column chromatography.