Optical probes of the quantum-entangled triplet-triplet state in a heteroacene dimer

Abstract

The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron π-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron theory of the electronic structure of the triplet-triplet, and of complete wavelength-dependent excited state absorptions (ESAs) from all three states in a heteroacene dimer of interest in the field of intramolecular singlet fission. The theory allows direct comparisons of ESAs with existing experiments as well as experimental predictions, and gives physical understandings of transient absorptions within a pictorial exciton basis that can be carried over to other experimental systems.

Free Triplets Versus Bound Triplet–Triplet Biexciton in Intramolecular Singlet Fission Materials: Structure–Property Correlations

“…2(a ). Similar (but stronger) CT absorptions are seen also in BPn and PTn, experimentally 6,13 and within our many-body computations 34,35 . There is a subtle difference between the CT contributions to p − 2 and m − 2.…”

“…The absorptions in the 550−600 nm region, common to both p − 2 and m − 2, are due to intramonomer molecular excitations. The absorption at ∼ 700 nm in p − 2 is to T 2 , which is of CT character and occurs also in BP1 and PT1 35 . Transient absorptions from T 1 are then predicted to be different in p − 2 and m − 2.…”

“…Our calculations are done using a localized exciton basis that allows pictorial representations of eigenstates. [33][34][35] . The Hamiltonian (Eq.…”

“…We are however confident that the ranges and the overall trend for the quantities computed within our many-body approach are accurate and more importantly, that the predicted structure-property trends (2-2 versus 6-6 , and para versus meta links) are correct. Computational results on the photophysics of BPn and PTn have been discussed previously 34,35 . Present work focuses on p − 2 and m − 2, but comparisons are made with the earlier results on BPn and PTn to point out similarities and differences, as appropriate.…”

Emissive spin-0 triplet-pairs are a direct product of triplet–triplet annihilation in pentacene single crystals and anthradithiophene films

“…Spectral signatures of this state are widely debated in part because the main method used to study it, transient absorption spectroscopy, provides very similar signatures for 1 (TT), (T..T) and free triplets. Calculations [12][13][14] suggest that this is particularly true in the visible spectral region where most experiments are performed. In addition, paramagnetic resonance techniques [15][16][17] are blind to spin-zero states such as 1 (TT).…”