Solution‐Processable Donor–Acceptor Polymers with Modular Electronic Properties and Very Narrow Bandgaps

Abstract

Bridgehead imine-substituted cyclopentadithiophene structural units, in combination with highly electronegative acceptors that exhibit progressively delocalized π-systems, afford donor-acceptor (DA) conjugated polymers with broad absorption profiles that span technologically relevant wavelength (λ) ranges from 0.7 < λ < 3.2 μm. A joint theoretical and experimental study demonstrates that the presence of the cross-conjugated substituent at the donor bridgehead position results in the capability to fine-tune structural and electronic properties so as to achieve very narrow optical bandgaps (Eg (opt) < 0.5 eV). This strategy affords modular DA copolymers with broad- and long-wavelength light absorption in the infrared and materials with some of the narrowest bandgaps reported to date.

A high-spin ground-state donor-acceptor conjugated polymer

“…1). A salient design feature is the cross-conjugated donor, which allows intricate control of structure-property relationships, raises the highest occupied MO (HOMO), affords adjustments to molecular indices such as bond length alternation (BLA), and promotes a highly planar conjugated backbone (see the Supplementary Materials) ( 27 , 28 ). The strong, proquinoidal thiadiazoloquinoxaline (TQ) acceptor is key for lowering the lowest unoccupied MO (LUMO) and promoting strong electron correlations to form and stabilize unpaired spins in the long-chain limit.…”

Donor–acceptor polymers with tunable infrared photoresponse

“…8 This design motif also permits careful control of structural and electronic features necessary to overcome conjugation saturation behavior and achieve solution-processable DA polymers with very narrow optical bandgaps (E opt g < 0.5 eV). 9 It seemed reasonable that similar considerations should apply to copolymers comprised of bridgehead olefin (CvCPh) substituted cyclopentadithiophene (CPDT) structural units, with the advantage of increasing the LUMO energies of the resultant polymers to facilitate photoinduced electron transfer (PET) to conventional fullerene acceptors.…”

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity

“…The cross-conjugated donors with a highly planar molecular backbone have a high degree of electronic coherence through extended π-conjugation, thereby leading to a small bandgap ( London et al., 2017 , 2019 ; Wang et al., 2019 ; Foster et al., 2014 ). For example, the cyclopentadithiophene (CPDT) donor, when conjugated with thiadiazoloquinoxaline (TQ) acceptor unit, provides a vanishingly small singlet-triplet energy gap upon increasing oligomer length, resulting into triplet to be the ground state at a longer chain length ( London et al., 2019 ).…”