PYSCF is a Python-based general-purpose electronic structure platform that both supports first-principles simulations of molecules and solids, as well as accelerates the development of new methodology and complex computational workflows. The present paper explains the design and philosophy behind PYSCF that enables it to meet these twin objectives. With several case studies, we show how users can easily implement their own methods using PYSCF as a development environment. We then summarize the capabilities of PYSCF for molecular and solid-state simulations. Finally, we describe the growing ecosystem of projects that use PYSCF across the domains of quantum chemistry, materials science, machine learning and quantum information science.

Site-selective functionalization of complex molecules is a grand challenge in chemistry. Protecting groups or catalysts must be used to selectively modify one site among many that are similarly reactive. General strategies are rare such the local chemical environment around the target site is tuned for selective transformation. Here we show a four amino acid sequence (Phe-Cys-Pro-Phe), which we call the “π-clamp”, tunes the reactivity of its cysteine thiol for the site-selective conjugation with perfluoroaromatic reagents. We used the π-clamp to selectively modify one cysteine site in proteins containing multiple endogenous cysteine residues (e.g. antibodies and cysteine-based enzymes), which was impossible with prior cysteine modification methods. The modified π-clamp antibodies retained binding affinity to their targets, enabling the synthesis of site-specific antibody-drug conjugates (ADCs) for selective killing of HER2-positive breast cancer cells. The π-clamp is an unexpected approach for site-selective chemistry and provides opportunities to modify biomolecules for research and therapeutics.

Donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2 were synthesized and their emissive properties were studied. They exhibited a blue-green fluorescence with emission lifetimes on the order of a microsecond in cyclohexane at room temperature. The long lifetime emission is quenched by O 2 and is attributed to thermally activated delayed florescence (TADF). Unimolecular TADF is made possible by the separation and weak coupling due to homo-conjugation of the HOMO and LUMO on different arms of the three-dimensional donoracceptor triptycene. Organic light emitting devices (OLEDs) were fabricated using TPA-QNX(CN)2 and TPA-PRZ(CN)2 as emitters which displayed electroluminescence with efficiencies as high as 9.4% EQE.Since the first report by Tang and Van Slyke in 1987 1 , multi-layered organic light emitting diodes (OLEDs) have attracted interest for utilization in high efficiency illumination and flexible displays.2 OLEDs using fluorescent materials 3 have low internal quantum efficiencies (IQEs) of ≈ 25%, 4 due in part to the inherent limitation of electrical excitation, which generates singlets and triplets in a 1:3 ratio. 5High quantum yield OLEDs with Ir or Pt phosphorescent materials have been intensely investigated for the last several decades 5,6 and now achieve 100% IQE. 5aAlthough phosphorescent materials have defined the present state of OLED technology, there are significant issues including cost, stability of blue emitters, and strong triplet-triplet annihilation at high current density.7 As a result of recent efficiency increases, thermally activated delayed fluorescence (TADF) has become a viable alternative for harvesting both singlet and triplet state in OLEDs. 8,9 TADF is based on reversible intersystem crossing from thermally equilibrated triplet and singlet excited states, and competitive luminescence from the singlet states. If non-radiative pathways are negligible then TADF can achieve 100% electroluminescence IQE. 9f An advantage of TADF materials is that they can be purely organic materials and do not Typical TADF designs employ an electron donor and electron acceptor, which are connected directly but have a twisted geometry ( Figure 1a) to minimize the HOMO-LUMO overlap. 9,13An alternative approach is a through-space interaction wherein electronic systems are in communication by homo-conjugation 14 but are sufficiently separated to create a small singlet-triplet ∆E ST (Figure 1b). The design we report herein places the donor and acceptor on the different fins of a triptycene scaffold. These structures display homoconjugation and many triptycene derivatives display intrinsically high thermal stability, which is critical to OLED manufacturing and opperation. We designed the donor-acceptor triptycences, TPA-QNX(CN)2 and TPA-PRZ(CN)2, as novel TADF materials ( Figure 1c). The triphenylamine functions as the donor and dicyanoquinoxaline or dicyanopyrazine as the acceptor. Our designs were guided by time-dependent density functional theory (TD-DFT) calculations, which prov...

We introduce a simple quantum-mechanical model for thermally activated delayed fluorescence (TADF).The Hamiltonian is represented in the basis of four spin-mixed diabatic states representing pure charge transfer (CT) and local excitations (LE). The model predicts that it is possible to realize lowest-lying adiabatic singlet (S1) and triplet (T1) states with a small singlet-triplet gap, differing CT/LE contributions, and appreciable LE component in the S1 state. These characteristics can explain the coexistence of fast T1→S1 reverse intersystem crossing and S1→S0 radiative decay in some chromophores. Through the sampling of the parameter space and statistical analysis of the data, we show which parameter combinations contribute the most to the TADF efficiency. We also show that conformational fluctuations of a single model donor-acceptor system sample a significant region of the parameter space and can enhance the TADF rate by almost three orders of magnitude. This study provides new guidelines for optimization of TADF emitters by means of electronic structure and conformation engineering.

Organic molecules with charge-transfer (CT) excited states are widely used in industry and are especially attractive as candidates for fabrication of energy efficient OLEDs, as they can harvest energy from nonradiative triplets by means of thermally activated delayed fluorescence (TADF). It is therefore useful to have computational protocols for accurate estimation of their electronic spectra in order to screen candidate molecules for OLED applications. However, it is difficult to predict the photophysical properties of TADF molecules with LR-TDDFT, as semilocal LR-TDDFT is incapable of accurately modeling CT states. Herein, we study absorption energies, emission energies, zero-zero transition energies, and singlet-triplet gaps of TADF molecules using a restricted open-shell Kohn-Sham (ROKS) approach instead and discover that ROKS calculations with semilocal hybrid functionals are in good agreement with experiments-unlike TDDFT, which significantly underestimates energy gaps. We also propose a cheap computational protocol for studying excited states with large CT character that is found to give good agreement with experimental results without having to perform any excited-state geometry optimizations.