Synthesis And Photophysical Properties Of Meta-substituted Triphenylmethyl Radicals

Perchlorotriphenylmethyl radical（PTM）,tris（2,4,6-trichlorophenyl）methyl radical（TTM）and their derivatives are well-known organic radicals,which are emissive and stable at room temperature.Since these radicals possess an unpaired electron,the electron spin states of the lowest excited state and the ground state are both doublet.Thus the transition from the lowest excited state to the ground state does not suffer from the spin-forbidden.Therefore,when the radical-based material was used as a light-emitting layer,the theoretical internal quantum efficiency of the OLED can reach to 100%.Thus,stable luminescent organic radicalswith unique photophysical properties show broad application in new-generation OLEDs,bioimaging,molecular switches,spin labeling etc.However,some shortcomings of the triphenylmethyl radicals remain unsolved.Firstly,the emission spectra of the most luminescent organic radical show orange,deep red or near-infrared.Secondly,the fluorescent quantum efficiency of the most organic open-shell molecules is low,the fluorescence is quenched in the aggregated state andsensitive to solvent-polarity as well asthe brightness of OLED devices is low etc.Recently research results showed that rational molecular designing strategy is an effective way to solve the above shortcomings of free radicals.Up to now,the chemical modifications of the TTM-type radicals are mainly focused on the para-position of the benzene ring of TTM radicals.However,modification of the meta position was not reported.In this paper,to improve the photophysical properties of triphenylmethyl radicals,we carried out following three parts of work.1.Synthesis and photophysical properties of triphenylmethyl-type radicals with meta position-modified by different donor-acceptorWe selected the TTM block as a molecular framework,and introduced different substituents at the meta-position of TTM,photophysical properties of obtained three radicals,TTM-NO₂,TTM-Br,TTM-N-（CH₃）₂,were investigated systematically.We found that different substituents at the meta-position of TTM had no obvious effect on the absorption spectrum of radicals,but the luminescent properties and photostability of the radicals affected by substituents.The fluorescence quantum efficiencies（PLQY）of TTM-NO₂ and TTM-Br were 2.4%and 3.6%respectively（PLQY=3%for TTM）,while TTM-N-（CH₃）₂ did not emit light.Photostability result of the radicals showed that（TTM as a reference）the half-live times of TTM-NO₂,TTM-Br and TTM-N-（CH₃）₂ are 117 s,83 s and 10640 s respectively,which are 1.5,1.1 and 140 times higher than that of TTM.Further investigation of the radical TTM-N-（CH₃）₂ found that it can undergo a reversible protonation-deprotonation reaction,and emits orange light after protonation.2.Synthesis and photophysical properties of triphenylmethyl-type radicals with meta position-modified by benzyl substituents.We used the TTM-Br as a building block,which was synthesized in the previous chapter and introduced different benzyl groups at the meta-position of TTM through Suzuki coupling reaction,and obtained three kinds of radicals:TTM-mMe,TTM-m2Me and TTM-pMe.We found that absorption spectra of those types of radicals are similar to TTM,however,the luminescence properties and stabilities are obviously improved.The experimental results showed that the emission wavelengths of the radicals TTM-mMe,TTM-m2Me and TTM-pMe in cyclohexane solution peaked at 574 nm（625 nm）,633 nm（575 nm）and 633 nm（575 nm）,respectively.Compared with TTM,the emission wavelengths are obviously red-shifted;the PLQYs measured by integrated sphere method are 2%,2%and 2.3%,respectively;the photostabilities are 3.5,3.8 and 3.6times higher than that of TTM.Through investigation of solvation effects and theoretical calculations of free radicals,we found that the excited states of such free radicals are easily affected by the solvent polarity,so short wavelength（575 nm）emissions of TTM-mMe and TTM-m2Me are disappear in polar solvents,while long wavelength emission was increase.3.Synthesis and photophysical properties of carbazole-capped and meta position-modified triphenylmethyl-type radicalsIn this chapter,we introduced carbazole molecules into TTM-mMe,TTM-m2Me and TTM-pMe,and achieved TTM-mMe-CZ,TTM-m2Me-CZ and TTM-pMe-CZ.We investigated the photophysical properties of three radicals and found that the introduction of carbazole unit to TTM-mMe,TTM-m2Me and TTM-pMe can increase the long-wavelength absorption（600 nm）from forbidden transition.The PLQYs of the three radicals in cyclohexane solution were 60%,50.3%and 52%,respectively,and the luminescent efficiency of the radicals was significantly improved.The photostability tests showed that the photostability of the radicals TTM-mMe-CZ,TTM-m2Me-CZ and TTM-pMe-CZ were 11.6,10.2 and 9.9 times higher than that of TTM.The experimental results showed that the solvent-polarity has no obvious effect on the absorption spectra of those radicals.However,as the increase of solvent-polarity,the luminescent efficiency of this type radicals were decrease and the emission wavelength red shifted because of the excited states of free radicals are more affected by solvent polarity..