Indole-deacetylimide Derivatives: Dark State Excimer Formation And Efficient Energy Transfer

Excitation energy transfer plays a pivotal role in photosynthesis in the sophisticated biological systems.Moreover,it has been widely used as an important photophysical technique in many areas,for example,monitoring of protein-protein interactions,study of the conformational changes of other complex biological systems,detection of particle assembly and creation of important functional molecular devices in materials science.For practical application of energy transfer,it is very essential to confirm the process and mechanism of the energy transfer in the systems studied.It is well-known that excitation energy transfer mainly occurs via Forster mechanism and Dexter mechanism.For the former,non-radiative transfer of electronic excitations from a donor to an acceptor is mediated by electronic dipole-dipole coupling,which requests that the distance between the donor and acceptor must be relatively long(10-100 A).For Dexter mechanism,energy transfer can take place through electron-exchanging interaction when the molecular orbitals of the donor and the acceptor actually overlap,which could occur over short distances(less than 10 A).Therefore,one of the best ways to distinguish the two energy transfer mechanisms is to determine the distance between the donor and acceptor.However,it is difficult to judge the energy transfer type in some cases,for example,in the highly efficient Forster resonance energy transfer system,the donors and the acceptors are commonly integrated through covalent bonds to shorten the relative distance,which may cause appearance of Dexter energy transfer.For in-depth understanding this kind of energy transfer phenomenon,the systems are fabricated in my thesis,of which pyrene and perylene bisimide are specially chosen as the energy donor and acceptor connected by covalent bond.The thesis is composed of the following four parts:In the first part,an asymmetric perylene bisimide(PBI)derivative of pyrene(Py)and cholesterol(C)possessing energy transfer property and three reference compounds(C-Py,C-PBI,PBI-Py)were synthesized and characterized.UV-Vis absorption measurements demonstrated that the covalently combined but spatially separated Py and PBI moieties in the compound of C-PBI-Py behave as two individual chromophores,and this observation is further supported by the result from theoretical calculations.Concentration-dependent fluorescence measurements revealed the presence of H-style excimer of PBI unit in the chloroform solution of the compound at room temperature,a result of ∏－∏ stacking between the aromatic moieties as confirmed by 1H NMR spectroscopy studies.Temperature-dependent fluorescence measurements demonstrated that PBI moiety could form both H-type and J-type excimer.Further TRES measurements manifested the formation of PBI excimer via both pre-formed scheme and Birks’ scheme in the solutions of C-PBI-Py.More detailed fluorescence studies clarified that energy transfer from Py to PBI takes place inter-molecularly in the solutions of C-PBI-Py via Forster mechanism even though the emission of Py excimer does not observe directly,but the energy transfer efficiency is dependent on the solvent polarity.In the second part,a cholesterol-containing fluorescent compound,CPPBI,using Pyrene(Py)and perylene bisimide(PBI)as the main building blocks was designed and synthesized.Fluorescence studies revealed that combination of two Py moieties and one PBI unit into a single molecule using cholesterol as another structural unit resulted in a molecule showing strong tendency to form ordered and densely stacked supra-molecular assemblies both in solution and in film state.Concentration-dependent fluorescence revealed that FRET from Py to PBI takes place via intermolecular interaction even at a concentration as low as l.0×10-8 mol·L-1.Furthermore,in conjunction with the result from FRET studies,the fact of the solvent-dependence of the profile and position of the emission spectrum of CPPBI in solution state shows that the molecules of CPPBI,in particular their core structure,PBI,enjoy some mobility in the aggregated state.The results from TRES studies revealed that the energy transfer immediate,the Py excimer,may form via both the pre-formed scheme and the Birks’scheme.Further studies revealed that photochemically,the CPPBI-based film is super-stable,and sensitive to the presence of some organic liquids in vapor phase,in particular aniline.Temperature-/concentration-dependent 1H NMR spectroscopy studies revealed that inter-molecular hydrogen bonding and ∏－∏ stacking are the main driving forces for the formation of the aggregates.In the third part,a pyrenyl unit(Py)was chemically connected to a perylene bisimide moiety(PBI)via a long and flexible linker,4,7,10-trioxa-1,13-tridecanediamine(TOA),resulting in a fluorescent dyad,PBI-TOA-Py.UV-Vis absorption and fluorescence studies revealed that the two fluorescent units of PBI-TOA-Py behave independently.However,efficient Forster Resonance Energy Transfer(FRET)from the Py unit to the PBI moiety in solution state was also observed.Temperature and solvent effect studies demonstrated that the energy transfer efficiency is highly dependent upon solution temperature and solvent nature.Specifically,for the DMF solution of PBI-TOA-Py,the FRET efficiency is close to 88%at temperatures below-40 ℃,but the efficiency greatly decreased to nearly zero when the temperature exceeds-80 ℃.Moreover,addition of HAc into the DMF solution at room temperature could reduce the energy transfer efficiency to nearly zero,suggesting that the excited state energy of Py can’t be directly transferred to PBI structure even they are properly and chemically bonded.Based on the observations and time-resolved studies,it is believed that the observed efficient FRET from the Py unit to the PBI moiety occurs mainly via Py excimer formation,which could be a result of inter-molecular association of the compound.On the basis of the studies,the applications of the fluorescent dyad in solvent discrimination and trace water determination in organic solvents were verified through example studies.In the fourth part,a new perylene bisimide(PBI)derivative(PBI-2Py)containing two pyrenyl units was designed and synthesized.The main design ideas contain the four points as following,(1)the two pyrenyl units are connected by ethylenediamine to form pyrene(Py)excimer as energy donor,(2)quaternary-N moiety is specially introduced into the functional molecule between Py and PBI,which could efficiently avoid the aggregation between the aromatic groups,(3)N,N-Dimethyl-1,2-ethanediamine(tertiary amine)is introduced to the molecule,which would quench fluorescence of the PBI moiety(energy acceptor)in PBI-2Py though intra-molecular photoinduced electron transfer mechanism.Fluorescence studies revealed that the compound,PBI-2Py,shows fluorescence resonance energy transfer(FRET)phenomenon,of which the efficiency is independent on the solvents.As expected,further studies confirmed that Py excimer is the energy donor.Moreover,fluorescence of the compound is very weak in solution,but introduction of trace amount of trifluoroacetic acid could greatly enhance the emission of the PBI unit.