Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers

Abstract

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absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpublished experimental transient absorptions in the near and mid infrared we conclude that the end product of photoexcitation in these particular bipentacenes is the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...

Understanding the Bound Triplet-Pair State in Singlet Fission

“…A similar result was found in theoretical work by Mazumdar and coworkers who assigned a photoinduced transition near 1400 nm in pentacene dimers to from 1 (T 1 T 1 ) / S 2 . 61,62 Such transitions between one and two exciton manifolds have been reported previously in work by the Knoester group. 63 Additional information can be gleamed from TAS by probing even further into the infrared; Grieco et al have identified transitions between 1 (T 1 T 1 ) and S 1 that lie near 0.3 eV.…”

“…An innovative approach to reduce overlapped signals was reported in two independent studies on crystals of rubrene, 65 and hexacene. 57 In both cases, highly polarized singlet absorption was used to provide a reduction in signal overlap and provided a key 15,55,61 insight: the coherent pathway for m (T 1 T 1 ) promotion. The presence of a nearly instantaneous signal at time zero is most clearly observed because of the suppression of singlet signals based on the selective probing along a specific facet.…”

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

“…(iii) Finally, to what extent does the 1 (TT) 1 ESA resemble ESA from the free triplet T 1 as the number of phenyl linkers is increased? Experiments indicate decreasing coupling [13,35] between the two triplets with increasing n. Our previous calculations for BPn could not be performed for n > 1 because of the large size of the TIPS-pentacene monomer [17].…”

“…Absorptions from FE excitations shown in Fig. 2(a) are polarized along the short axes of the monomers [17,29] while CT excitations are predominantly polarized along the long molecular axis of the dimer.…”

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…Contiguous dimers have shown that the singlet and triplet pair energy manifolds are mixed, with allowed TT-S n optical transitions. 32,62 Furthermore, radiationless decay of triplet pairs has been shown to slow down as chromophore proximity decreases. From this, we suggest that the triplet pair decays through coupling to singlet vibronic modes, permitting a rapid and spin-allowed route back to the ground state.…”