Chemical Tuning of Exciton versus Charge-Transfer Excited States in Conformationally Restricted Arylene Cages

Abstract

Covalent assemblies of conjugated organic chromophores provide the opportunity to engineer new excited states with novel properties. In this work, a newly developed triple-stranded cage architecture, in which meta-substituted aromatic caps serve as covalent linking groups that attach to both top and bottom of the conjugated molecule walls, is used to tune the properties of thiophene oligomer assemblies. Benzene-capped and triazine-capped 5,5′-(2,2-bithiophene)-containing arylene cages are synthesized and characterized using steady-state and time-resolved spectroscopic methods. The conformational freedom and electronic states are analyzed using time-dependent density functional theory. The benzene cap acts as a passive spacer whose electronic states do not mix with those of the chromophore walls. The excited state properties are dominated by through-space interactions between the chromophore subunits, generating a neutral Frenkel H-type exciton state. This excitonic state undergoes intersystem crossing on a 200 ps time scale while the fluorescence output is suppressed by a factor of 2 due to a decreased radiative rate. Switching to a triazine cap enables electron transfer from the chromophore-linker after the initial excitation to the exciton state, leading to the formation of a charge-transfer state within 10 ps. This state can avoid intersystem crossing and exhibits red-shifted fluorescence with enhanced quantum yield. The ability to interchange structural modules with different electronic properties while retaining the overall cage morphology provides a new approach for tuning the properties of discrete chromophore assemblies.

Novel D–A chromophores with condensed 1,2,4-triazine system simultaneously display thermally activated delayed fluorescence and crystallization-induced phosphorescence

“…Among all the acceptors used in TADF, triazines have attracted great attention because of their strong electron deficiency and ability to undergo various functionalizations. [31][32][33][34][35][36] There are two possible triazine isomers: symmetric 1,3,5-triazine (s-triazine), and asymmetric 1,2,4-triazine (as-triazine). 33 A third possible isomer, 1,2,3-triazine, is stable only when incorporated in fused ring systems and so has not received similar attention.…”

Back electron transfer rates determine the photoreactivity of donor–acceptor stilbene complexes in a macrocyclic host

“…37,38 Moreover, donor-accep-tor complexes can be excited by visible or even near-infrared light thanks to the emergence of charge transfer (CT) absorption bands. 38,39 We wondered if irradiation of these CT bands could be used to initiate [2 + 2] chemistry in appropriately designed donor-acceptor host-guest complexes. Directly accessing [2 + 2] chemistry from the CT state is generally considered improbable due to quick back electron transfer (BET) to the ground state following CT. [40][41][42][43] In fact, most studies of cross-[2 + 2] chemistry have viewed CT as a non-productive pathway.…”

Photoreactivity of donor-acceptor stilbene complexes in a macrocyclic host

“…Moreover, such complexes could be excited by visible or even near-infrared light thanks to the emergence of charge transfer (CT) absorption between the donor and acceptor substrates. 34,35 Typically, CT-initiated photochemistry requires one of the substrates to contain a leaving group because rapid cleavage prevents the initially-formed CT state from quickly relaxing back to the ground state by back electron transfer (BET). 36 Direct reaction of radical ions formed by CT has rarely been reported due to competition with BET.…”