Synthesis Of Thienyl Substituted Arylimide As Heavy Atom-Free Triplet Photosensitizers And Study Of The Intersystem Crossing

Developing heavy atom-free triplet photosensitizers with low toxicity,cost efficiency,high molar absorptivity and longer triplet lifetime is very crucial to achieve the increasing demand of stable,economical and less toxic triplet photosensitizers in the field of energy and life science.Also,finding different channel to enhance the intersystem crossing(ISC)ability as well as mechanism of ISC is very important for basic photochemistry.Till date there are few reports on heavy atom-free triplet photosensitizers and the relationship between triplet properties and molecular structure is not clear.In order to further investigate the different strategies to promote the ISC in heavy atom-free triplet photosensitizers as well as the mechanism of ISC,we design triplet photosensitizers by substituting thienyl group on different arylimide chromophores.Thiophene is well known compound which show weak phosphorescence and have been used as electron donor.Although thienyl appended organic as well as inorganic compounds were prepared and their applications in various fields have been explored but the reports on triplet properties of thienyl substituted organic chromophores are very rare.The present work highlighted the design,synthesis and mechanism of ISC in heavy atom-free triplet photosensitizers by attaching the thienyl group on different organic chromophores.(1)Different bay-substituted perylene-3,4:9,10-bis(dicarboximide)(PBI)were prepared by the substitution of thienyl and phenyl group at the core and the substitution effect on the photophysical properties,especially the ISC efficiency was studied.MT-PBI(monothienyl substitution at bay position)shows high singlet oxygen quantum yield(?A)of 70%and longer triplet excited state lifetime(?T)up to 64.4 ?s,as compared to reference compounds(derivative with monophenyl substituent at bay position,MP-PBI,?A=23%and ?T=48.5?s).Quantum chemical calculations show that the energy gap between S1/T1 as well as the magnitude of the spin orbit couplings(SOC)between singlet and triplet state play significant role in ISC efficiency.Femtosecond transient absorption spectra show the ISC rate constant of 8.0×108s-1 for MT-PBI,and no charge transfer exists.Photo-driven electron transfer was observed with the PBI derivatives in the presence of sacrificial electron donor triethanolamine(TEOA),and reversible formation of PBI radical anion(PBl-·)was observed.Delayed fluorescence of P-type was also observed in MT-PBI with luminescence lifetime of 31.0 ?s.(2)Core-substituted naphthalenediimide(NDI)compounds were synthesized by substituting bromine group with thienyl group at 2-position(MT-NDI)and 2,6-position(DT-NDI)of NDI.The effect of thienyl group on ISC was studied and results were compared with phenyl core-substituted NDIs(MP-NDI and DP-NDI).Longer triplet lifetime(103.2 ?s for MT-NDI and 40.8 ?s for DT-NDI)was detected with nanosecond transient absorption than their reference compounds(MP-NDI and DP-NDI).Singlet oxygen quantum yield was determined as 41%(in toluene)for MT-NDI.fs-TA measurement reveals that MT-NDI and DT-NDI undergo solvent relaxation and internal conversion process within very short time range(about 10 ps)and no triplet state was observed due to slow rate of ISC.Also no radical anion of NDI or radical cation of thiophene was observed with fs-TA experiment and the ISC is not via charge separation/charge recombination.DFT/TDDFT computation shows that the matching of S1 and T2 energy levels are responsible for efficient ISC in MT-NDI.Triplet-triplet annihilation induced delayed fluorescence with luminescence lifetime up to 80 ps,was observed by using DT-NDI as triplet photosensitizer and perylene as triplet acceptor.The compounds were used to reversibly produce radical anion of NDI in the presence of triethanolamine(TEOA)by photoreduction.MT-NDI was also used as triplet photosensitizer with perylene as acceptor for triplet-triplet annihilation upconversion(TTA-UC)and high upconversion quantum yield(?UC)of 2.1%were observed.(3)Thienyl substituted arenes(perylene,anthracene and naphthaleneimide)were prepared and the substitution effect on ISC ability was studied by using steady state and time-resolved transient absorption and emission spectroscopies,as well as theoretical computations.Red-shift in absorption was observed with thienyl and phenyl substitution.DFT/TDDFT computations reveal that the change in energy level of HOMO(increased energy level)and LUMO(decreased energy level)is responsible for red-shift in absorption and emission.In case of MT-Per,high singlet oxygen quantum yield up to 60%was observed.The formation of triplet state was indicated by nanosecond transient absorption spectroscopy with triplet excited state lifetime of 282 ?s for MT-Per.Quantum chemical calculations show that the energy level of singlet and triplet states were changed as a result of thienyl substitution and S1/T2 states energy level matching is responsible for enhanced ISC of the thienyl compounds.