| Heavy atom-free triplet photosensitizers(PSs)with intense visible absorption are always desired in photochemistry due to economic synthetic route,negligible dark toxicity and longlived triplet excited state.However,designing efficient triplet PSs without inclusion of any heavy atom is still considered a challenging task in photochemistry.Though different strategies are introduced to develop heavy atom-free PSs but the research is still in its infancy and structural relation to the triplet excited state properties is unclear.Owning our interest in developing the heavy atom-free PSs and to further elucidate the intersystem crossing(ISC)mechanism,we designed some PSs by linking donor moieties with different electron donating strength at various position of perylene-3,4,9,10-bis(dicarboximide)(PBI)acceptor.PBI is a well-known chromophore,which has been extensively studied for singlet state properties but rarely investigated for triplet state properties.Intense visible absorption and availability of various decoration sites,make it quite interesting for triplet state studying.Thus,this dissertation aims to prepare efficient PBI-derived heavy atom-free PSs based on charge transfer mediated ISC and explore their ISC mechanism.A series of bay substituted compact PBI-anthracene donor-acceptor dyads/triad with different mutual chromophore orientation was investigated for their photophysical properties,especially the charge recombination-induced ISC.Employing the different spectroscopic approaches,it was observed that the dihedral angle and mutual orientation of donor and acceptor units control the photophysical/photochemical properties,especially the spin orbit charge transfer intersystem crossing(SOCT-ISC).The devised PBI-anthracene dyads,PBI-9AN(the dyad in which anthracene was connected from 9th position to the PBI core)and PBI2-AN(the dyad with substitution at the 2nd position of anthracene)exhibited drastically different photophysical properties.The dyad with orthogonal geometry(PBI-9-AN)showed the high singlet oxygen generation ability(ΦΔ=86%)compared to that of non-orthogonal dyad(PBI-2-AN;ΦΔ=57%),proving that the orthogonal geometry is more beneficial for triplet population via SOCT-ISC.Moreover,it was found that the energy gap between the charge separated state(CSS)and triplet state also control the SOCT-ISC efficiency.All the PBIanthracene dyads/triad showed long-lived(τT=100～139 μs)PBI localized triplet state,compared to conventional heavy atom based PBI triplet PSs in which triplet state is much short lived(0.37-22.3 μs).The time-resolved electron paramagnetic resonance(TR-EPR)spectroscopy showed the different spin polarization pattern of triplet state for dyad PBI-2-AN(e,a,e,a,e,a)and triad PBI-Bis-2-AN(e,e,e,a,a,a),with nearly similar ZFS parameters(|D|=43.4 mT,|E|=4.2 mT for PBI-2-AN and |D|=39.0 mT,|E|=4.1 mT for PBI-Bis-2-AN),revealing that the triplet state is localized on the PBI chromophore and polarization pattern is not only dependent upon the geometry but also on the structure of compound.PBI-anthracene derivatives were subjected to photoreduction and reversible PBI-· and PBI2-formation was observed and it was shown that the selective excitation into near infrared absorption bands of PBI-· doesn’t produce the PBI2-.Generally,orthogonal geometry is considered beneficial for SOCT-ISC.To elucidate this prospect and structural correlation to the SOCT-ISC efficiency,a series of imide substituted PBI-carbazole donor-acceptor dyads with different length of spacer between donor and acceptor units,were investigated for the triplet excited state properties.Steady-state absorption/emission studies showed the strong fluorescence quenching and presence of negligible ground state electronic interaction among donor and acceptor units in these imide-substituted PBI dyads.Theoretical computations revealed that the dihedral angle between donor(carbazole)and acceptor(PBI)in these novel PBI-carbazole dyads is not orthogonal,yet the ISC is efficient.The dyad having short length of spacer,PBI-Cz-3 exhibited efficient ISC(ΦΔ=72%)as compared to dyad with a long spacer(PBI-Cz-2;ΦΔ=45%)and phenyl substituted PBI derivative(PBI-N-Ph),which exhibited negligible triplet yield.Compared with conventional heavy atom-free PBI derived PSs based on S1/Tn energy level matching approach(τT=30～60μs),PBI-Cz dyads showed significantly longer triplet lifetime(τT=110～189μs).TR-EPR showed the similar spin polarization pattern(e,a,e,a,e,a)for all the PBI-Cz dyads,confirming the SOCT-ISC mechanism of triplet population,which is different from the reported imide substituted phenothiazine-PBI derivatives exhibiting radical pair intersystem crossing.Through TR-EPR study of PBI-Cz-2,which exhibited the Cz triplet state,we demonstrated that intramolecular triplet-triplet energy transfer(TTET)process become slow at low temperature.SOCT-ISC showing boron-dipyrromethene(B DP)labelled PBI donor-acceptor dyads/triad were prepared by substituting same donor(BDP)at the bay and imide position of the PBI.The mutual orientation and electronic interaction between the BDP and PBI units was varied and its effect on ISC efficiency was explored.For BDP-PBI-3,the PBI derivative with imide substitution,high singlet oxygen generation(ΦΔ=85%)was observed as compared to the bay substituted derivative,BDP-PBI-1(ΦΔ=30%).Ultrafast transient absorption studies showed the slow charge separation(CS:1.55 ns)in BDP-PBI-3 compared to that bay substituted derivative,BDP-PBI-1,in which fast CS(3.6 ps)was observed.The transient absorption studies revealed the first BDP triplet excited state population via charge recombination and then PBI triplet state generate through slow intramolecular TTET.The observation of upper triplet excited state population in SOCT-ISC and slow intramolecular TTET(14.5 μs)is rare.We have provided the first experimental evidence of the upper triplet excited state population in donor-acceptor dyads,which will not only help to clarify SOCT-ISC mechanism but will paves the way for future designing of triplet PSs based on this approach. |
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