Sensitized Singlet Fission in Rigidly Linked Axial and Peripheral Pentacene-Subphthalocyanine Conjugates

Abstract

The goal of harnessing the theoretical potential of singlet fission (SF), a process in which one singlet excited state is split into two triplet excited states, has become a central challenge in solar energy research. Covalently linked dimers provide crucial models for understanding the role of chromophore arrangement and coupling in SF. Sensitizers can be integrated into these systems to expand the absorption bandwidth through which SF can be accessed. Here, we define the role of the sensitizer-chromophore geometry in a sensitized SF model system. To this end, two conjugates have been synthesized consisting of a pentacene dimer (SF motif) connected via a rigid alkynyl bridge to a subphthalocyanine (the sensitizer motif) in either an axial or a peripheral arrangement. Steady-state and time-resolved photophysical measurements are used to confirm that both conjugates operate as per design, displaying near unity energy transfer efficiencies and high triplet quantum yields from SF. Decisively, energy transfer between the subphthalocyanine and pentacene dimer occurs ca. 26 times faster in the peripheral conjugate, even though the two chromophores are ca. 3 Å farther apart than in the axial conjugate. Following a theoretical evaluation of the dipolar coupling, V dip 2, and the orientation factor, κ2, of both the axial (V dip 2 = 140 cm–2; κ2 = 0.08) and the peripheral (V dip 2 = 724 cm–2; κ2 = 1.46) arrangements, we establish that this rate acceleration is due to a more favorable (nearly co-planar) relative orientation of the transition dipole moments of the subphthalocyanine and pentacenes in the peripheral constellation.

Tetracene Dimers: A Platform for Intramolecular Down- and Up-conversion

“…As expected, the contribution of the CT state strongly depends on the nature of the spacer and the solvent polarity. In line with our recent work on pentacene dimers 32 and the work from Wasielewski and co-workers on terylenediimide dimers, 29−31 we postulate that the third species is a superposition of (S 1 S 0 ) rel , the CT state, and 1 (T 1 T 1 ) to give a mixed state, which we refer to as (S 1 S 0 ) (T1T1)…”

“…When turning to the coherent scenario, a superposition of (S 1 S 0 ), 1 (T 1 T 1 ), and the CT state is formed upon photoexcitation. ,− Strong mixing between all three states is realized if the energetic differences are small. The composition of the coherent superposition changes as a function of time, and different products evolve as a result of dephasing.…”

One‐Pot Synthesis and Excited‐State Dynamics of Null Exciton‐Coupled Diketopyrrolopyrroles Oligo‐Grids

“…Previous investigations on pentacene dimers [ 54 ] and terylenediimide dimers [ 50,55,56 ] reveal that one of the several SF mechanisms is based on coherence. It is mediated by a superposition of the singlet excited state (S 1 S 0 ), the correlated triplet pair 1 (T 1 T 1 ), and CT. For gridized oligo‐DPPs, [2]Grid and [3]Grid in benzonitrile, a similar intermediate state is observed.…”

Sequence-Defined Conjugated Oligomers in Donor–Acceptor Dyads

“…Boron subphthalocyanines (BsubPc) are a class of ambivalent donor/acceptors that have garnered interest in recent years due to the ease and scalability of their synthesis, in addition to numerous methods for covalent modifications to precisely tune their bandgap. − Previous research has also established that energy and charge transfer can occur between non-sequence-defined short-chain oligothiophenes (bi/quarter-) or polythiophene and BsubPc. , As such, BsubPc is an ideal platform for assessing the effect of the oligothiophene length and sequence on the transfer of photoexcitations between the two components. Furthermore, axial substitution of BsubPc has limited impacts on the molecular orbital energy levels of the acceptor in addition to offering an avenue for both regio- and chemoselective substitution. − Both of these factors are important to consider in deconvoluting the effect of the covalent substitution itself from the effect of the oligothiophene length or sequence. The efficiency of alkoxylation reactions between alcohols and axially brominated BsubPc (Br-BsubPc) has also been established previously .…”

“…This compound exhibits photophysical properties characteristic of axially substituted BsubPcs with no indication of intramolecular energy, electron, or hole transfer within the dyad (Figure ). − …”