Anticipating Acene-Based Chromophore Spectra with Molecular Orbital Arguments

Abstract

Recent synthetic studies on the organic molecules tetracene and pentacene have found certain dimers and oligomers to exhibit an intense absorption in the visible region of the spectrum which is not present in the monomer or many previously-studied dimers. In this article we combine experimental synthesis with electronic structure theory and spectral computation to show that this absorption arises from an otherwise dark charge-transfer excitation 'borrowing intensity' from an intense UV excitation.Further, by characterizing the role of relevant monomer molecular orbitals, we arrive at a design principle that allows us to predict the presence or absence of an additional absorption based on the bonding geometry of the dimer. We find this rule correctly explains the spectra of a wide range of acene derivatives and solves an unexplained structure-spectrum phenomenon first observed seventy years ago. These results pave the way for the design of highly absorbent chromophores with applications ranging from photovoltaics to liquid crystals.

Switching between Coherent and Incoherent Singlet Fission via Solvent-Induced Symmetry Breaking

“…intensity borrowing. 77 This description closely reproduces the experimental trend of Figure 1c for the molar extinction coefficient, which is minimized in intermediate-polarity solvents. The calculated molar extinction coefficient is shown in SI Section S1.3 for a range of solvents.…”

On Exciton–Vibration and Exciton–Photon Interactions in Organic Semiconductors

“…For intermediate-polarity solvents, we find that CT ↓ and LE B are energetically very close, leading to a strong mixing and transfer of oscillator strength from LE to CT, i.e. intensity borrowing [318]. This description closely reproduces the experimental trend of Figure 9.1c for the molar extinc- If the equivalently destabilised state CT ↑ were to form, we would in principle expect it to relax into CT ↓ .…”

On the electronic structure of alternant conjugated organic radicals for light-emitting diode applications

“…We stress that there are many other theoretical techniques which may be of use in explaining the functionality of radical OLEDs, and which have previously been successfully applied in optoelectronic systems, such as group theory 27 which can inform orbital mixing 16,19,28 and spin-orbit coupling, 28,29 as well as intensity borrowing theory. 2,16,30 Further application of these theories to radicals is left as future work.…”

“…6 Since the discovery in 2018 of a highly-efficient radical OLED, there has been great interest in radical optoelectronics, both synthetic 2, 5 and computational, 5,[7][8][9][10] as well as review articles from an organic synthetic perspective 3 and an applied physics viewpoint. 11 However, there has been comparatively little research into the theoretical electronic structure of organic radicals, and the success of methods such as Hückel theory and Pariser-Parr-Pople (PPP) theory [12][13][14][15] for closed-shell optoelectronic species [16][17][18][19] suggests that this could be a promising avenue of research for radicals too. We therefore seek to go beyond the qualitiative (but usually correct) depiction of radical electronic structure as shown in Fig.…”