The Design And Development Of Fluorescent And Phosphorescent Materials Based On Intramolecular Charge-transfer States

Fluorescence has been applied in the area of biological sciences dramatically during the past few years.Fluorescence spectroscopy and time-resolved fluorescence already become important research tools in the area of biochemistry and biophysics.Fluorescence has been widely exploited for medical diagnostics,DNA sequencing,genetic analysis,biology sensing,etc.due to the advantages such as non-invasiveness,high sensitivity and real-time capability compared to the traditional radioactive tracers.As it is known to all,the background of fluorescence imaging is usually noisy because of the organism autofluorescence.Hence,long life-time phosphorescence might be able to decrease background noise and get better signals when used in organisms to displace short life-time fluorescence.Since phosphorescence is from the radiative decay of triplet back to ground state,and the transitions from the ground state to the triplet state are forbidden,enhancing the efficiency of intersystem crossing(ISC)is the key step to increase the yield of triplet,i.e.phosphorescence.Traditional phosphorescence systems are based on heavy-atom chemistry(bromine,iodineetc)or organometallics(uranium,palladium,etc).However,these systems are usually toxic to organisms and also not stable,hence we mainly work on developing purely organic room temperature phosphorescence(RTP)and find that RTP can also be mediated by intramolecular charge transfer(ICT)states.We also study the corresponding spectra and application in organisms.The dissertation includes three parts:1.Since quinoline derivatives have frequently been used in fluorescence imaging and sensing applications,we design and synthesize a series of green fluorescent quinolines containing both amino and trifluoromethyl groups.From comparative studies with quinolines lacking either one of the two groups,it is shown that quinolines with both groups can be synthesized under much milder conditions and exhibit strong intramolecular charge transfer fluorescence with larger Stokes shifts.More interestingly,some of the derivatives can be used for Golgi apparatus imaging with two-photon fluorescence microscopy.2.Design and synthesize a series of alkyl-substituted tetra-coordinate organoboronium bisdiketone complexes and dramatic luminescence thermochromism(25--196?)is noticed in organic solvents.In glass-forming alcohols,these complexes exhibit reversible aqua to orange-red to greenish yellowluminescence emission colour change upon cooling.It is proposed that these boron complexes have strong tendency to form luminescent molecular aggregates and these aggregates can efficiently quench fluorescence and enhance phosphorescence,phosphorescence becomes the dominant radiative decay at low temperatures.The mechanism of presented luminescence thermochromism is coined"aggregation induced intersystem crossing(AIX)" which is thought to be mediated by narrowed singlet-triplet energy gap in the excited states due to singlet exciton splitting among the aggregates.Well-defined nanostructures obtained from less polar solvents were investigated by SEM and TEM.Luminescence thermochromism is also present within these macroscopic molecular assemblies.AIX could be a general strategy to harness enhanced phosphorescence from organic aggregates.3.Purely organic materials with room-temperaturephosphorescence(RTP)are currently under intense investigation because of their potential applications in sensing,imaging,and displaying.Inspired by certain organometallic systems where RTP can be mediated by triplet ligand-to-metal(LMCT)or metal-to-ligand charge transfer(MLCT)states,we find that donor-to-acceptor CT states from the same organic molecule can also mediate localized RTP.In the model system of N-substituted naphthalimides(NNIs),the relatively large energy gap between the NNl-localized singlet and triplet states of the aromatic ring can be bridged by intramolecular CT states to decrease the energy difference and phosphorescence arises.These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules,which can be used for bioimaging and RTP microscopy.