Unified model for singlet fission within a non-conjugated covalent pentacene dimer

Abstract

When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Using Structurally Well-Defined Norbornyl-Bridged Acene Dimers to Map a Mechanistic Landscape for Correlated Triplet Formation in Singlet Fission

“…formation of 1 TT based on results from a growing number of pentacene-based systems in the literature. [5][6][7][10][11][12]14,15,[17][18][19] It is noted that in the communication of our synthetic methodology, we showed preliminary spectral evidence for 1 TT at 10 ps following photoexcitation. 18 However, that work did not establish time constants or yields.…”

Pentacene Dimers as a Critical Tool for the Investigation of Intramolecular Singlet Fission

“…Furthermore, the authors demonstrate that iSF can be directed to the direct mechanism, by incorporating dimer 44 into av iscous polymer matrix (polystyrene, polyvinyl acetate, polyethylene oxide)t hat impedes intramolecular rotation along the 6-,6'-bond. The combinedr esults for 44 and 45 are in good agreement with the other 6-,6'-linked pentacene dimers 27-30, 34-35 and 41,w hich also suggest the typical involvement of chargetransfer states in the iSF mechanism.I nc ontrastt o6 ,6'-linked pentacene dimers, [20,78,83,87,88] however,2 ,2'-linked pentacene dimers such as 4-6 do not show solvent dependency of the SF kinetics,t riplet decay,o rt riplet quantum yield, which supports ad irect mechanism in these cases (i.e.,Step 2, Figure 2).…”

“…In particular,X -ray crystallography clearly documentst he spatial differences in, for example, the cross-conjugated dimer 35 [83] and non-conjugated analogue 41. [20] Most telling is the distance between the 6-and6 ' positions, which increases from 8.8 to 9.2 in 35 and 41,r espectively.A lso evident is the increased spacingb etween the pentacene moieties, albeit the distances (3.3 and 7.2 )a re, to some extent,a ffected by the solid-state geometry and do not necessarily reflects olutionstate valueso f35 and 41.…”

“…To this end, two pentacene dimers have been designed in which through-bond coupling is changed,w hile molecular geometry (and therefore through-space coupling) is held constant.A ss pacers, meta-diethynylbenzene and 1,3-diethynyladamantanea ccomplish this goal (Figure 4), giving rise to the pairo fd imers 29 (Scheme 5) and 41 (Scheme 7). [20,78] The assembly of 41 is shown in Scheme 7, based on the desymmetrization of 6,13-pentacenequinone via nucleophilic addition of LiÀCCSiiBu 3 at low temperature to provide unsymmetricalk etone 42. [79] Nucleophilic addition of lithiated 1,3-diethynyladamantane (43)to42,following workup and reductive aromatization, gave the non-conjugated pentacene dimer 41.…”

“…[15][16][17] Following formation of the multiexciton state 1 (T 1 T 1 ), population of aq uintet state, 5 (T 1 T 1 ), is possible, formed via spin evolution from the 1 (T 1 T 1 )s tate (Step 4,Figure2). [18][19][20] Subsequently,s pin decoherence of either the 1 (T 1 T 1 )o r 5 (T 1 T 1 )s tates allows for the generation of the two non-interacting, free triplet (T 1 + T 1 )s tates (Step 5, Figure 2). The free triplet states are then available for "harvesting", [21] completing the process of solar energy conversion.…”