Synthesis And Photophysical Properties Of 2,6-substituted Naphthalenediimide Triplet Photosensitizers

Naphthalene-1,8:4,5-bis(dicarboximide)or commonly called naphthalenediimide(NDI)is a well-known chromophore and has been used in many fields such as semiconducting devices,molecular sensors,supramolecular assemblies and photosynthetic mimics.However,its triplet state is less studied,due to its absorption in the short wavelength region which makes it unsuitable for many applications for instance photocatalysis,photodynamic therapy(PDT)and triplet-triplet annihilation(TTA)upconversion.The aim of this thesis was to prepare NDI based triplet photosensitizer which can absorb in longer wavelength region and show intersystem crossing(ISC)via different mechanisms.By introducing two 2,6-diiodo-bodipy(diiodo-BDP)units on 2,6-position of NDI via a flexible triazole linker,a novel broadband absorbing triplet photosensitizer(diiodoBDP-NDI)was synthesized by Click reaction in good yield(78%).This molecule is capable of absorbing in a broad range of spectrum(500?620 nm).The compound was studied for detailed photophysical properties and femtosecond transient absorption(fs TA)spectroscopic studies revealed that ISC of diiodo-BDP(kISC=3.3×109s-1)is out competed by FRET(kFRET=2.5 x 1011 s-1 and NDI localized triplet state(38 ?s;??=39%)was observed.The diiodoBDP-NDI was used as broadband excitable photosensitizer and a high upconversion quantum yield(?UC=2.8%)was obtained using 9,10-bis(phenylethynyl)-anthracene(BPEA)as triplet acceptor.To obtain a further bathochromic shift in the absorbance of NDI,a red light absorbing triplet photosensitizer(T-S-NDI;?abs=674 nm,?=25000 M-1cm-1)was synthesized by replacing oxygen atom of carbonyl group on NDI with a sulfur atom using Lawesson's reagent in good yield(42%).This compound exhibits efficient singlet oxygen generation(??=58%)with triplet state lifetime of 14 ?s.The fs TA studies showed that ISC in T-S-NDI is fast(kISC=1.2 × 1010 s-1).Based on TD-DFT computations,we proposed that the efficient ISC of T-SNDI is due to the S1? T3 transition,which is in 1(?-?*)?3(n-?*)feature,that is allowed by the El-Sayed rules.We further studied a new approach of radical enhanced intersystem crossing(REISC)by connection of a stable radical with the NDI to red shift the absorption of NDI up to 600 nm.Two novel compounds(NDI-Ac-Ph-Vz and NDI-Ph-Vz)were synthesized in good yield(-46%)by covalently connecting NDI to a stable verdazyl radical via different linkers,i.e.phenyl or ethynylphenyl.Our molecular designing goal was to attain REISC via electron spinspin exchange interaction between electronically excited chromophore and the radical unit.The fs TA studies revealed that an electron transfer(<150 fs)takes place in both dyads followed by charge recombination induced ISC(ca.1.40-1.85 ps).The triplet state on the chromophore interacts with the doublet state of verdazyl radical and biexponential decay was observed by nanosecond transient absorption(ns TA)spectroscopy.The fast-decaying component(50 ns,population ratio:80%)is tentatively attributed to the spin allowed D1?D0 internal conversion,and the slow-decaying component(2 ?s,population ratio:20%)is assigned to the Q1->D0 ISC.These dyads are used in the photoreduction in the presence of(triethanolamine)TEOA to form radical anion of NDI.A new electron donor-acceptor approach was used to design spin orbit charge transfer(SOCT-ISC)based molecular triads.Two carbazole units were introduced on 2,6-position of NDI via different substitution position of carbazole.We observed that electronic coupling matrix element(VDA)is smaller in triads with C-connected carbazole moieties(VDA=0.12 eV;?=4000 M-1 cm-1);while in compounds where there is ?-conjugation between electron donor and accepter with N-connection of the carbazole moiety,the electronic coupling is stronger(VDA=0.35 eV),thus the CT absorption band is more significant(?=16500 M-1 cm-1).NDICz-Ph and NDI-Ph-Cz show ISC(??=26%)and thermally activated delayed fluorescence(TADF)with a lifetime of?2 ns(99.2%)/?20 ?s(0.8%).These TADF based triplet photosensitizers were used for TTA upconversion and high upconversion quantum yield(?UC=8.2%)was observed at very low concentration of photosensitizer and acceptor,which is rare for TADF based upconversion system.