A new triple-stranded cage design with covalent linking groups attached to both ends of the conjugated molecule walles is used to engineer rigid organic chromophore assemblies with well-defined geometries.  The properties of thiophene oligomer assemblies are tuned by using benzene-capped and triazine-capped arylene cages that were synthesized and then characterized using steady-state and time-resolved spectroscopic methods.  Density functional theory was also used to understand the conformational freedom and excited-state electronic properties of these assemblies.  The benzene-capped cage support neutral Frenkel H-type excitonic states created by through-space Coulomb coupling between the oligothiophene walls.  This state rapidly underegoes intersystem crossing within 200 ps, similar to the monomer.  But the loss of oscillator strength due to excitonic interactions reduces the fluorescence quantum yield by a factor of 2.  The triazine cap, on the other hand, enables the creation of a new charge-transfer state on a timescale of 7 ps that avoids both intersystem crossing and charge recombination, leading to enhanced fluorescence.  This approach to constrained covalent carbon-based assemblies provides a modular way tune electronic properties in a controlled way.

Chemically tuning excited states in conformationally restricted arylene cages

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