Cofacial Versus Coplanar Arrangement in Centrosymmetric Packing Dimers of Dipolar Small Molecules: Structural Effects on the Crystallization Behaviors and Optoelectronic Characteristics

Abstract

Two D-π-A-A molecules (MIDTP and TIDTP) composed of an electron-rich ditolylamino group (D) and an electron-deficient 5-dicyanovinylenylpyrimidine (A-A) fragment bridged together with indeno[1,2-b]thiophene (IDT) were synthesized. These molecules provide an opportunity to examine in-depth the impact of side-chain variations (methyl vs p-tolyl) on the crystallization behaviors, solid-state morphology, physical properties, and optoelectronic characteristics relevant for practical applications. X-ray analyses on single-crystal structures indicate that methyl-substituted MIDTP forms "coplanar antiparallel dimers" via C-H···S interactions and organizes into an ordered slip-staircase arrays. In contrast, p-tolyl-bearing TIDTP shows "cofacial centrosymmetric dimers" via π-π interactions and packs into a less-ordered layered structures. The X-ray diffraction analyses upon thermal treatment are consistent with a superior crystallinity of MIDTP, as compared to that of TIDTP. This difference indicates a greater propensity to organization by introduction of the smaller methyl group versus the bulkier p-tolyl group. The increased propensity for order by MIDTP facilitates the crystallization of MIDTP in both solution-processed and vacuum-deposited thin films. MIDTP forms solution-processed single-crystal arrays that deliver OFET hole mobility of 6.56 × 10(-4) cm(2) V(-1) s(-1), whereas TIDTP only forms amorhpous films that gave lower hole mobility of 1.34 × 10(-5) cm(2) V(-1) s(-1). MIDTP and TIDTP were utilized to serve as donors together with C70 as acceptor in the fabrication of small-molecule organic solar cells (SMOSCs) with planar heterojunction (PHJ) or planar-mixed heterojunction (PMHJ) device architectures. OPV devices based on higher crystalline MIDTP delivered power conversion efficiencies (PCEs) of 2.5% and 4.3% for PHJ and PMHJ device, respectively, which are higher than those of TIDTP-based cells. The improved PCEs of MIDTP-based devices are attributed to better hole-transport character.

Molecular generation targeting desired electronic properties via deep generative models

“…A low HOMO-LUMO gap, and by extension a low optical gap, ensures the generated oligomers are promising materials for organic photovoltaics, and a low dipole moment in the ground state is desired as it has been shown that this can help with directing self-assembly towards supramolecular arrangements that promote macroscopic properties such as charge-carrier mobility. 33 Our approach would be equally applicable to a focus on a different region of property space and we would note that for optoelectronic device applications such as OPVs and OLEDs, it is typical for there to be multiple material properties and device characteristics contributing to high performance, and these would need to be considered and appropriately weighted on a case-by-case basis. We discuss the challenges for further extension of the method in the discussion.…”

Electron‐Withdrawing Substituted Quinazoline Push‐Pull Chromophores: Synthesis, Electrochemical, Photophysical and Second‐Order Nonlinear Optical Properties

“…Over the last 25 years, there has been a considerable interest in the design of organic push‐pull chromophores having electron‐donating group (D) linked to an electron‐withdrawing one (A) via a π‐conjugated bridge (D–π‐A structure) In these compounds, intramolecular charge transfer (ICT) occurs and generation of new low‐energy molecular orbital induces unique optical and electrochemical properties . These chromophores have found key applications as fluorescent sensors, in optoelectronic, particularly for photovoltaics, organic field‐effect transistors (OFETs) and in organic light‐emitting diodes (OLEDs) . The generation of a molecular dipole in push‐pull chromophores induces also nonlinear optical (NLO) properties such as second order NLO phenomena and two‐photon absorption (TPA).…”

Molecular Generation Targeting Desired Electronic Properties via Deep Generative Models

“…A low HOMO-LUMO gap ensures the generated oligomers are promising materials for organic photovoltaics, and a low dipole moment in the ground state is desired as it has been shown that this can help with directing selfassembly towards supramolecular arrangements that promote macroscopic properties such as charge-carrier mobility. 30 Our approach would be equally applicable to a focus on a different region of property space.…”