Tunable Electronic Coupling and Driving Force in Structurally Well-Defined Tetracene Dimers for Molecular Singlet Fission: A Computational Exploration Using Density Functional Theory

Abstract

Singlet fission (SF), a process by which two excited states are formed in a chromophoric system following the absorption of a single photon, has the potential to increase the theoretical efficiency of solar energy conversion devices beyond the single-junction Shockley-Quiesser limit. Although SF is observed with high yield in the solid state of certain molecules, linearly linked dimers based on these same constituents exhibit small yields in part due to small interchromophore electronic coupling. Previous work from our group demonstrated enhancement of SF yield in polycrystalline tetracene (Tc) via excitation of intermolecular motions, which increased direct overlap of monomer π-systems. In this current work, a series of norbornyl-bridged bistetracene (BT) dimers are investigated using DFT and the ability to control SF thermodynamics along with important interchromophore electronic coupling parameters via bridging geometry is shown. Although the electronic coupling of a series of C2v-symmetric dimers (BT1-BT3) that differ in norbornyl bridge length is larger than in previously studied Tc dimers, a key nonhorizontal electron-transfer (ET) matrix element used in determining the SF rate is zero due to symmetry. In these systems, SF may be expected but electronic excitation will require coupling to vibrational modes that break symmetry. Singly bridged dimer isomers BT1-trans and BT1-cis, which break the C2v symmetry of BT1 by exploiting attachment of the norbornyl bridge at the 1,2 instead of the 2,3 Tc positions, are expected to be significantly more favorable for SF due to an exoergic driving force, increased electronic coupling, a lower charge-transfer-state energy (particularly in the case of BT1-cis), and nonhorizontal ET matrix elements that are nonzero.

Solution-Phase Singlet Fission in a Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer

“…47 This analysis predicts a ratio of 2.3 for observed S 3a ← S 0 and S 3b ← S 0 absorption features, in good agreement with the 1.9 ratio observed in the electronic absorption spectrum and the ratio of 2.6 previously predicted using TD-DFT. 47 As a final point in these discussions of BT1 electronic absorption, we comment on the significant number of smaller peaks with an onset of ∼405 nm that complicate the lowerenergy vibronic progression involving the S 1b state. These can be assigned as involving dipole-allowed transitions to S 2a and S 2b .…”

“…30 An average of energies for the 0−0 features in both the absorption and emission spectra suggest that the S 1 lies at 2.66 eV, in reasonable agreement with our previous TD-DFT prediction of The Journal of Physical Chemistry A Article 2.76 eV for the geometry-optimized state (TD-DFT with gradients; ω-B97XD range-corrected functional and 6-31G* basis set). 47 BT1 in Toluene. Comparative photoluminescence and absorption spectra for the dimer BT1 in room-temperature toluene are shown in Figure 3 (right).…”

Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran

“…19 Covalently bound tetracene dimers with an "edge-to-edge" or "face-to-face" geometry undergo SF in nonpolar solvents with similarly low efficiencies, 20−22 although those with stronger coupling via an ethynyl linker have shown much higher triplet yields. 23,24 This behavior is similar to that of functionalized pentacene dimers, which in solution have exhibited efficient SF characterized through the interchromophore distance dependence 25 and the interplay between SF and TT annihilation in dimers constructed with para, meta, and ortho linking geometries 26 and in a T-shaped dimer. 27 In the present study, we investigate dimer 4 (Figure 1), in which the attachment occurs as a para−para′ link between the phenyl groups of neighboring chromophores of 1.…”

Tuneable Singlet Exciton Fission and Triplet–Triplet Annihilation in an Orthogonal Pentacene Dimer

“…Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%.…”