| This thesis explores the quenching of singlet and triplet excited states of organic chromophores by covalently attached stable free radicals through transient absorption spectroscopy and time resolved electron paramagnetic resonance (TREPR). The chromophores used in this thesis are perylene-3,4:9,10-bis(dicarboximide) (PDI), perylene-3,4-dicarboximide (PMI), 4-amino-1,8-naphthalenedicarboximide (ANI) and zinc tetraphenylporphyrin (ZnTPP), while the radicals used in this thesis are tert-butylphenylnitroxide radical (BPNO •), 2,2,6,6-tetramethylpiperidinoxyl (TEMPO•), and alpha,gamma-bisdiphenylene-beta-phenylallyl (BDPA•).;From transient absorption spectroscopy it was determined that the rate and yield of excited state quenching of the chromophore excited singlet and triplet states is dependent on the connectivity and type of free radical attached to the chromophore. BPNO• has the largest delocalization of spin density into its phenyl ring, and thus was found to be the most effective quencher of excited states, while TEMPO• and BDPA • have their spin densities more localized and thus generally had slower rates of quenching. Furthermore, the orientation of the radical relative to the chromophore was also found to have a profound impact on excited state quenching. Generally, fluorescence quenching was a result of exchange induced enhanced intersystem crossing (EISC) to generate the triplet state very rapidly, while the mechanisms of triplet quenching were varied for each system.;The electron spin polarization (ESP) of the radical-triplet molecules was explored by TREPR at X (9.5 GHz) and W (94 GHz) band and showed that the photogenerated three-spin systems consist of the strongly-coupled unpaired electrons confined to the triplet chromophore, which are each weakly coupled to the unpaired electron on the radical to form excited doublet (D1) and quartet (Q) states. The reverse quartet mechanism (RQM) has been used previously to describe the ESP observed for other linked radical-triplet studies. RQM was observed for several of the radical-triplet systems studied here, but other mechanisms were also identified and explained. The rates of spin polarization transfer depend on the molecular connectivity between the chromophore triplet and radical and can be rationalized in terms of the dependence on molecular structure of the through-bond electronic coupling between these species. |
