multiexciton interactions in singlet fission and triplet fusion upconversion dendrimers

Singlet fission (SF) and triplet-triplet annihilation upconversion (TTA-UC) are two multiexciton processes intimately related to the dynamic interaction between one high-lying energy singlet and two low-lying energy triplet excitons. Here, we introduce a series of dendritic macromolecules that serve as a platform to study the effect of interchromophore interactions on the dynamics of multiexciton generation and decay as a function of dendrimer generation. The dendrimers (generations 1-4) consist of trimethylolpropane (TMP) core and 2,2-bis(methylol)propionic acid (bis-MPA) dendrons that provide exponential growth of the branches, leading to a corona decorated with pentacenes for SF or anthracenes for TTA-UC. The findings reveal a trend where a few highly ordered sites emerge as the dendrimer generation grows, dominating the multiexciton dynamics, as deduced from optical spectra, and transient absorption spectroscopy. While the dendritic structures enhance TTA-UC at low annihilator concentrations in the largest dendrimers, the paired chromophore interactions induce a broadened and red-shifted excimer emission. In SF dendrimers of higher generations, the triplet dynamics become increasingly dominated by pairwise sites exhibiting strong coupling (Type II), which can be readily distinguished from sites with weaker coupling (Type I) by their spectral dynamics and decay kinetics.

Methods
An dendrimer:
Normalized absorbance spectra in chloroform at 0.03 mg/mL.
UCPL data were performed with PdTPTBP (50 uM) with corresponding annihilator concentration as shown.

Pc dendrimer:
UV-vis and fluorescence spectra were measured using the diluted pentacene dendrimer solution.
Transient absorption data were collected by Helios and EOS excited by 600 nm pulse  Here, fs-TA data are after chirp correction and ns-TA are after time zero correction for figures in the manuscript.

Sensitization measurements were performed in toluene solution of anthracene (~10 mM) with low concentrations of dendrimers excited by 360 nm pulse. The data here are after time zero correction.