Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wavefunctions of the optical singlets, the lowest triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish between the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpublished experimental transient absorptions in the near and mid infrared we conclude that the end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet and not free triplets. We predict additional transient absorptions at even longer wavelengths, beyond 1500 nm, to the equivalent of the classic 2 1 A − g in linear polyenes. PACS numbers:The consequences of strong electron correlations in π-conjugated systems have been investigated most intensively at the two extremes of system sizes, (a) small molecules such as linear polyenes, and (b) extended systems, such as π-conjugated polymers and single-walled carbon nanotubes. The occurrence of the lowest twophoton 2 , where the phenomena of interest are exciton formation [6,7] and the consequence thereof on nonlinear optical spectroscopy [8]. Understanding the photophysics of discrete but large molecular systems of intermediate size poses new challenges [9][10][11][12]. We consider members of one such family of large π-conjugated molecules here, dimers of bis(triisopropylsilylethynyl) (TIPS) pentacene, covalently linked by 0, 1 and 2 phenyl groups (see Fig. 1) [13]. Following the original investigators we will refer to these molecules as bipentacenes BPn, with n = 0, 1 and 2, respectively. The photophysics of these and other bipentacenes [13][14][15][16][17] and related covalently linked dimeric molecular systems [18][19][20][21] are of strong current interest as candidates for intramolecular singlet fission (iSF), as we briefly discuss below.SF is the process [22] in which an optical spin-singlet exciton S 1 dissociates into two spin triplet excitons T 1 . If each photogenerated triplet dissociates with 100% efficiency at the donor-acceptor interface of an organic solar cell, the photoconductivity is doubled. Enhanced external quantum efficiencies using SF have been reported for pentacene/C 60 solar cells [23,24]. The bulk of the theoretical [25][26][27][28][29][30][31][32][33][34] and experimental [35][36][37][38][39][40][41][42][43][44] literature until now had focused on intermolecular SF (xSF), in which the two triplets are generated on neighboring weakly coupled monomer molecules in a thin film or crystal. Recent investigations of dimer molecules [13][14][15][16][17][18][19][20][21], in which the monomers are linked by covalent bonds, is driven by the belief that the stronger coupling between the monomer components will give higher SF efficienc...

Recent experiments in several singlet-fission materials have found that the triplet-triplet biexciton either is the primary product of photoexcitation or has a much longer lifetime than believed until now. It thus becomes essential to determine the difference in the spectroscopic signatures of the bound triplet-triplet and free triplets to distinguish between them optically. We report calculations of excited state absorptions (ESAs) from the singlet and triplet excitons and from the triplet-triplet biexciton for a pentacene crystal with the herringbone structure and for nanocrystals of bis(triisopropylsilylethynyl) (TIPS)-pentacene. The triplet-triplet biexciton absorbs in both the visible and the near-infrared (NIR), while the monomer free triplet absorbs only in the visible. The intensity of the NIR absorption depends on the extent of intermolecular coupling, in agreement with observations in TIPS-pentacene nanocrystals. We predict additional weak ESA from the triplet-triplet but not from the triplet, at still lower energy.

Recent advances in singlet-fission research make it imperative that structure-property correlations that determine optical signatures of the triplet-triplet spin biexciton as well as its binding energy be understood precisely. We report many-body calculations of excited state absorptions from the triplet exciton and the triplet-triplet biexciton from two transversally linked dimers of pentacene derivatives. Comparison of experiment against theory leads to new interpretations of experiments performed earlier. We show that in the para-linked isomer the triplet-triplet does not dissociate to free triplets through the duration of the measurements. In contrast, even as calculated and experimental transient absorptions agree in the meta-isomer, the experimental observations here are more difficult to interpret, indicating the strong role structural variations can play in determining the rate and yield of free triplets. We also report many-body calculations of the spin gap, the energy difference between the spin quintet versus spin singlet triplet-triplet, as well as the binding energy of the spin singlet triplet-triplet, defined as the energy difference between two free triplets and the bound biexciton. The spin gap and the binding energy of the spin singlet triplet-triplet are different quantities in all but coupled two-level systems. The experimental behavior in the transversally linked dimers as well as previously studied longitudinally linked dimers agree with the trends that would be predicted from the computed biexciton binding energies. arXiv:1904.08539v2 [cond-mat.str-el]

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.