Design,Synthesis And Stimuli-Response Fluorescence Switching Of Twisting Donor-Acceptor Molecules

The photophysical properties(fluorescent color,and brightness)are altered by heating,pressure and so on,due to the change of packing modes.Presently,stimuli-response materials has a wide range of applications in information storage,sensors and anti-counterfeiting.In general,donor(D)–acceptor(A)luminophors generate an intramolecular charge transfer(CT)excited state in polar environments due to the twisting of the D-A bond.Interestingly,the CT state,which is highly sensitive to changes in the external environment,can induce tremendous shifts in the PL spectra.The intramolecular CT-type luminophor presented a larger colour difference and higher sensitivity.However,such a CT state generally causes forbidden electronic transitions,leading to PL quenching.Thus,preparing near-IR luminophors with high PL quantum yields(PLQYs)is quite difficult.To solve this problem,Ma and another group reported D–A luminophores showing very bright near-IR fluorescence arising from the presence of a hybridized local and charge transfer(HLCT)state,which is a special excited state that possesses the characteristics of a locally excited(LE)state and CT character.This combination is a “win-win” tactic: the former contributes to high PLQYs,while the latter guarantees high sensitivity to external stimuli.Herein,we designed and synthesized the HLCT-type π-conjugated materials based on aromatic amines,to investigate the relationship between external environmental factors(force,acid and water),molecular aggregateion,and fluorescence changing.A novel donor–π–acceptor structure stimuli-responsive fluorescent material of DBPAN was designed and synthesized,with cyano-group and pyridine as the acceptors(A)and triphenylamine as the donor(D).The PLYQs of its crystalline powder and doped films reach as high as 77.2% and 82.5% due to a hybridized local and charge-transfer(HLCT)state that simultaneously possesses the locally-excited(LE)state and charge transfer(CT)state.The LE state ensures relatively high fluorescence efficiency while the CT state provides multi-stimuli responsive fluorescent behaviors because it is easily tuned by the surrounding environment.Firstly,DBPAN exhibits “on-off-on” fluorescent properties in the mixture of water/tetrahydrofuran(THF)with the increasing water content.For the “on-off” part,a good linear relationship between fluorescent intensity and water fraction is achieved,which is ascribed to the synergistic effect of proton in water and intramolecular charge-transfer(ICT)effect depending on solvent polarity.The “off–on” part demonstrates the aggregation-induced enhanced emission(AIEE)character of o N-TPA.Secondly,DBPAN can be used as a protonic acid sensor to detect trifluoroacetic acid(TFA)in solvent and HCl vapour in solid due to the binding of the proton to the pyridine group.Finally,DBPAN presents a remarkable and reversible mechanochromic fluorescence switching between 552 nm and 642 nm(90 nm red-shift)during pressurizing–depressurizing process.Two cruciform-shaped organic luminophores,PDPA and PCbz,were designed and synthesized.Their photophysical properties was surveyed in detail.As a result,PDPA exhibited a higher PLQY than PCbz,owing to the formation of the HLCT excited state.Thereafter,crystal structures showed that the configuration of PCbz is twistier than that of PDPA.Furthermore,high-pressure measurements indicated that PDPA displayed a higher sensitivity(9.4 nm/GPa)than PCbz(6.4 nm/GPa)under the same pressure range.Combined with in situ UV-vis absorption and Raman spectroscopy experiments,the results indicated that the easy planarization of DPA in PDPA is the main cause.In contrast,the large dihedral between the central benzene ring and Cbz in the PCbz crystal effectively restrains the coplanarization of the whole PCbz molecule.All the above results illustrated that molecular design played a key role in piezochromic materials,which could directly influence the piezochromic contrast and sensitivity.Both the planarization and HLCT excited-state characteristics made PDPA a high-performance fluorescence switching.A novel AIE-active building block as the electron-withdrawing unit was successfully developed to achieve high-brightness NIR fluorescence.The introduction of rigid planar BTA into the AIE-active TPAN unit significantly extended the effective molecular conjugation,which could contribute to the increase in orbital overlap(LE-dominance)and red-shift of the PL spectra.As a result,the luminophore BPMT with such a building block,was effective at suppressing the nonradiative energy loss and emitted deep-red fluorescence at 695 nm with PLQYs as high as 30.3%.Endowed with distinct photophysical characteristics,relatively high PLQYs and AIE behaviour,the BPMT crystals were successfully used to realize strong-penetrability piezochromism with a redshift of 120 nm and high sensitivity of 20.8 nm/GPa.In addition,the BPMT-doped hemispheres were self-assembled on a DBR via a hydrophobic effect,which gave rise to the 705.9 nm NIR lasing with high-quality factor Q of 1970.