Varying the Interpentacene Electronic Coupling to Tune Singlet Fission

Abstract

We have designed and used four different spacers, denoted A–D, to connect two pentacenes and to probe the impact of intramolecular forces on the modulation of pentacene–pentacene interactions and, in turn, on the key steps in singlet fission (SF), that is, the 1(S1S0)-to-1(T1T1) as well as 1(T1T1)-to-5(T1T1) transitions by means of transient absorption and electron paramagnetic resonance measurements. In terms of the 1(S1S0)-to-1(T1T1) transition, a superexchange mechanism, that is, coupling to a higher-lying CT state to generate a virtual intermediate, enables rapid SF in A–D. Sizeable electronic coupling in A and B opens, on one hand, an additional pathway, that is, the population of a real intermediate, and changes, on the other hand, the mechanism to that of hopping. In turn, A and B feature much higher 1(T1T1) quantum yields than C and D, with a maximum value of 162% for A. In terms of the 1(T1T1)-to-5(T1T1) transition, the sizable electronic coupling in A and B is counterproductive, and C and D give rise to higher 5(T1T1)-to-(T1 + T1) quantum yields than A and B, with a maximum value of 85% for D.

Singlet Fission in Enantiomerically Pure Pentacene Dimers

“…[48] In the case of (S,S)-and (R,R)cis-Pt, drastic differences are observed in the spectra, including the loss of fine-structure and an additional peak/shoulder at around 695 nm (Figure 1b-c). [33,39,[48][49][50] Previous work using a platinum-linked pentacene monomer (Pt-Mono) and cis-platinum pentacene dimer without enantiomeric ligands (cis-Pt) produced similar observations ( Figure 2). [41] In accordance with our previous work, concentrationdependent experiments in solvents of different polarity, namely toluene (Tol), tetrahydrofuran (THF), and benzonitrile (BN), rule out aggregation, exciton splitting, and charge transfer as possible origins for the feature at 695 nm ( Figure S8), as Pt-Mono also possessed this feature.…”

“…Recently, a similar mixing, that is, the superimposition of 1 (T 1 T 1 ), (S 1 S 0 ), and (CT), has been postulated in a series of flexible linked pentacene dimers, as well as in a covalent slip-stacked terrylene-3,4 : 11,12 bis (dicarboximide) dimer. [49,53,54] Ground-state bleaching, in the form of minima at 642 and 695 nm, is also detected during the transition from (S 1 S 0 ) to 1 (T 1 T 1 ). Of great relevance is the fact that the transition from (S 1 S 0 ) to 1 (T 1 T 1 ) is linked to an overall increase of the groundstate bleaching and, in turn, to triplet excited state quantum yields well above 100 %, namely via SF.…”

“…In sound agreement with previous studies, a solvent rearrangement occurs in response to the more polar character of (S 1 S 0 ) compared to (S 0 S 0 ) and implies mixing between (S 1 S 0 ) and higher-lying (CT). [48,49] The initial (S 1 S 0 ) state has a lifetime of about 60 ps and then transforms rapidly into a new transient. Features of the newly developing species include sharp maxima at 486, 518, 860, and 966 nm, as well as a broad and featureless maximum at 1073 nm.…”

Modulating the Electronic and Solid‐State Structure of Organic Semiconductors by Site‐Specific Substitution: The Case of Tetrafluoropentacenes

“…A promising approach to optimize SF efficiencies for applications is chemical modification, which in turn often affects molecular arrangement . Due to the coupled triplet‐pair state, formation of which is a crucial step in the SF process, SF is highly sensitive to changes in molecular packing and small modifications in the orbital overlap can be used to tune SF …”

“…[59] Ap romising approach to optimize SF efficiencies for applicationsi s chemicalmodification,which in turn often affects molecular arrangement. [60,61] Due to the coupled triplet-pair state, formation of which is ac rucial step in the SF process, SF is highly sensitivet oc hanges in molecular packing [60][61][62][63][64][65][66] and small modificationsint he orbital overlap can be used to tune SF. [62] Given that the molecular arrangementi nt hin films often differs from the single-crystal phase, we measured the structure of 80 nm thick films using X-ray reflectivity (Figure 11).…”