Pressure Regulates The Optical Properties Of Stimuli-responsive AIE Molecules And Explores The Mechanism Through Ultrafast Dynamics

Stimuli-responsive aggregation-induced emission(AIE)materials are intelligent functional materials,which not only change the fluorescence color/intensity under the stimulation of external conditions,but also could overcome the defect of aggregationcaused Quenching(ACQ).Therefore,they have been widely used in the field of intelligence,optoelectronic data storage,biological imaging and other fields.Since high pressure can induce changes in the excited state properties of molecules,stimuliresponsive AIE molecules can effectively enhance fluorescence intensity or regulate fluorescence color under pressure.In this thesis,we designed and selected four stimuli-responsive AIE molecules with different structures.By combining in-situ high-pressure anvil technology,a variety of spectral detection methods and theoretical calculation methods,we explored the influence of high pressure on the excited state proton transfer,charge transfer and torsional charge transfer of molecules,further revealed the relevant photophysical/chemical mechanisms.The detailed research contents as follows:(1)We investigated the effect on the fluorescence intensity of(E)-4-((naphthalene-1-ylmethylene)amino)-N,N-diphenylamine(CNN)which like propeller structure under high pressure.The fluorescence intensity of CNN molecules in tetrahydrofuran/water mixed solvent increased by 1.72 times at atmospheric pressure,and its fluorescence intensity increased significantly by 96.14 times at 0.82 GPa.By analyzing the results of the high-pressure femtosecond transient absorption spectra as well as the kinetic fitting,it was found that the ICT process of the excited state is accelerated under the pressure of the CNN molecules,and the lifetime of the TICT process in the excited state is reduced from 5029 ps at atmospheric pressure to3382 ps.The torsion of intramolecular groups was suppressed,resulting in an increase of fluorescence intensity.(2)The emission peak regulation of AIE molecule E)-1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol(NNH)with ESIPT properties was achieved by high pressure.The steady-state fluorescence spectra showed that NNH molecule produced a new emission peak at 465 nm under pressure.Under atmospheric pressure,the single peak disappeared at 565 nm,and the fluorescence intensity increased by 10 times.According to theoretical calculation,the emission peak of NNH molecule at atmospheric pressure was generated by ESIPT process.Comparing the fluorescence peak of the NNH molecule with its fluorescence peak without a hydroxyl derivative,it was found that high pressure opened the channel of TPA luminescence.Combined with femtosecond transient absorption spectroscopy and high-pressure raman spectroscopy,it was revealed that the opening of TPA channel was caused by the inhibition of intramolecular rotation.The increasing in fluorescence intensity was due to pressure inhibition of the excited state TICT process and the intramolecular vibration for the NNH molecules.(3)By using high-pressure techniques,the fluorescence color and fluorescence intensity of AIE molecule(E)5-((4-(diethylamino)-2-hydroxybenzylidene)amino)isophthalic acid dimethyl ester(SBOH)with ESIPT and torsional amino ethyl structure were regulated.From the steady-state fluorescence spectroscopy,it was found that the pressure caused the emission peak at 604 nm of the SBOH molecule to disappear at atmospheric pressure,and its fluorescence increased by 9.5 times.The CIE chromaticity diagram showed that as the pressure increased,the fluorescence color of SBOH molecule changed from yellow-green to blue-green.The density functional theory study revealed that the two fluorescence peaks at atmospheric pressure of SBOH molecule belonged to E* and TICT state emission,respectively.The kinetic process obtained by high-pressure femtosecond transient absorption spectroscopy showed that the pressure made the ESIPT process disappeared,and the TICT process slowed down from 79.54 ps at 1.0 atm to 296.90 ps at 2.0 GPa.It was due to the fact that at high pressure more particles were stored in the E* state and were difficult to transfer to the TICT state,and the rotation of the-COO-Me portion and the ethylamine group of the SBOH molecule was limited.(4)The double fluorescence emission is appeared induced by pressure in the 6,6’,6’’,6’’,6’’-(1,9-dihydropyrene-1,3,6,8-tetrayl)tetramethyl tetra(2-naphthoate)(TBT)molecules with high conjugated symmetry.The TBT molecules exhibited single fluorescence peak emission at atmospheric pressure,and a new fluorescence peak appeared at 638 nm as the pressure increased to 0.25 GPa.Combining the geometry of the ground state and excited state of the TBT molecule obtained by the timedependent density functional theory method,we attributed the new fluorescence peak to the fluorescence emitted by the TICT state.The fitting results of femtosecond transient absorption spectra showed that the ICT process of TBT molecules accelerated with the increase of pressure.Our study showed that pressure could inhibit the torsion of the four arms for the TBT molecules,inducing the luminescence of the TICT state.