We improve the methodology to construct a complete active space-configuration interaction (CAS-CI) expansion for density-matrix renormalization group (DMRG) wave functions using a matrix-product state representation, inspired by the sampling-reconstructed CAS [SR-CAS; Boguslawski, K.; J. Chem. Phys. 2011, 134, 224101] algorithm. In our scheme, the genetic algorithm, in which the “crossover” and “mutation” processes can be optimized based on quantum information theory, is employed when reconstructing a CAS-CI-type wave function in the Hilbert space. Analysis of results for ground and excited state wave functions of conjugated molecules, transition metal compounds, and a lanthanide complex illustrate that our scheme is very efficient for searching the most important CI expansions in large active spaces.

Metal coordination bonds are widely found in natural adhesives and load-bearing and protective materials, in which they are thought to be responsible for the high mechanical strength and toughness. However, it remains unknown how metal–ligand complexes could give rise to such superb mechanical properties. Here, we developed a single-chain nanoparticle based force spectroscopy to directly quantify the mechanical properties of individual catechol–Fe3+ complexes, the key elements accounting for the high toughness and extensibility of byssal threads of marine mussels. We found that catechol–Fe3+ complexes possess a unique combination of mechanical features, including high mechanical stability, fast reformation kinetics, and stoichiometry-dependent mechanics. Therefore, they can serve as sacrificial bonds to efficiently dissipate energy in the materials, quickly recover the mechanical properties when load is released, and respond to pH and Fe3+ concentrations. Especially, we revealed that the bis-catechol–Fe3+ complex is mechanically ∼90% stronger than the tris-catechol–Fe3+ complex. Quantum calculation study suggested that the distinction between mechanical strength and thermodynamic stability of catechol–Fe3+ complexes is due to their different mechanical rupture pathways. Our study provides the nanoscale mechanistic understanding of the coordination bond-mediated mechanical properties of biogenetic materials, and could guide future rational design and regulation of the mechanical properties of synthetic materials.Keywords: atomic force microscopy; dopa; load-bearing materials; mussel foot protein; surface adhesion;

•New fragmentation-based approach for evaluating the intra-chain excitonic coupling.•Frenkel exciton model can nicely reproduce full TDDFT calculations in weakly coupled systems.•Description for strongly coupled system require the inclusion of higher excited states in fragments.For computing the intra-chain excitonic couplings in polymeric systems, here we propose a new fragmentation approach. A comparison for the energetic and spatial properties of the low-lying excited states in PPV between our scheme and full quantum chemical calculations, reveals that our scheme can nicely reproduce full quantum chemical results in weakly coupled systems. Further wavefunction analysis indicate that improved description for strongly coupled system can be achieved by the inclusion of the higher excited states within each fragments. Our proposed scheme is helpful for building the bridge linking the phenomenological descriptions of excitons and microscopic modeling for realistic polymers.Download high-res image (118KB)Download full-size image

Carbon–carbon double bonds (C═C) are ubiquitous in natural and synthetic polymers. In bulk studies, due to limited ways to control applied force, they are thought to be mechanically inert and not to contribute to the extensibility of polymers. Here, we report a single molecule force spectroscopy study on a polymer containing C═C bonds using atomic force microscope. Surprisingly, we found that it is possible to directly observe the cis-to-trans isomerization of C═C bonds at the time scale of ∼1 ms at room temperature by applying a tensile force ∼1.7 nN. The reaction proceeds through a diradical intermediate state, as confirmed by both a free radical quenching experiment and quantum chemical modeling. The force-free activation length to convert the cis C═C bonds to the transition state is ∼0.5 Å, indicating that the reaction rate is accelerated by ∼109 times at the transition force. On the basis of the density functional theory optimized structure, we propose that because the pulling direction is not parallel to C═C double bonds in the polymer, stretching the polymer not only provides tension to lower the transition barrier but also provides torsion to facilitate the rotation of cis C═C bonds. This explains the apparently low transition force for such thermally “forbidden” reactions and offers an additional explanation of the “lever-arm effect” of polymer backbones on the activation force for many mechanophores. This work demonstrates the importance of precisely controlling the force direction at the nanoscale to the force-activated reactions and may have many implications on the design of stress-responsive materials.Keywords: atomic force microscope; carbon−carbon double bonds; cis-to-trans isomerization; force-induced rotation; mechanochemistry;

The exciton dissociation in a model donor/acceptor heterojunction with electron–phonon couplings is simulated by a full quantum dynamical method, in which ultrafast long-range charge separation is observed. Such a novel scenario does not undergo short-range interfacial (pinned) charge transfer states, but can be mainly ascribed to the quantum resonance between local Frenkel excited states and a broad array of long-range charge transfer (LRCT) states assisted by the moderate off-diagonal vibronic couplings. The entropy-increasing effect associated with the very dense density of states for LRCT states is also found to be beneficial for lowering the free energy barrier for charge generation in organic solar cells.

Two-dimensional (2D) π-conjugated microporous polymers recently attracted tremendous interest due to their unique structural and electronic properties compared with 1D polymers and also graphene. In this paper, we present a comprehensive electronic structure investigation of several representative 2D conjugated polymers by virtue of first-principles density functional theory (DFT) calculations. A comparison of how spatial distribution of frontier molecular orbitals and charge carriers evolves in 1D and 2D conjugated oligomers as a function of system size is given. We also report the relationships between HOMO–LUMO gaps/ionization potential (IP)/electron affinity (EA)/structural reorganization energy upon charge doping and the oligomer size. These findings are insightful for understanding the electronic structure difference between 1D and 2D π-conjugated polymers and informative for designing new functional materials.

The electronic excited states of amorphous polymeric semiconductor MEH-PPV are investigated by first principles quantum chemical calculations based on trajectories from classical molecular dynamics simulations. We inferred an average conjugation length of ∼5–7 monomers for lowest vertical excitations of amorphous MEH-PPV at room temperature and verified that the normal definition of a chromophore in a polymer based on purely geometric “conjugation breaks” is not always valid in amorphous polymers and a rigorous definition can be only on the basis of the evaluation of the polymer excited state wave function. The charge transfer character is observed to be nearly invariant for all excited states in low energy window while the exciton delocalization extent is found to increase with energy. The interchain excitonic couplings for amorphous MEH-PPV are shown to be usually smaller than 10 meV suggesting that the transport mechanism across chain can be described by incoherent hopping. All these observations about the energetic and spatial distribution of the excitons in polymer as well as their couplings provide important qualitative insights and useful quantitative information for constructing a realistic model for exciton migration dynamics in amorphous polymer materials.

Efficient organic solar cells require a high yield of exciton dissociation. Herein we investigate the possibility of having more than one charge-transfer (CT) state below the first optically bright Frenkel exciton state (FE) for common molecular donor (D)/acceptor (A) pairs and the role of the second-lowest CT state (CT2) in the exciton dissociation process. This situation, previously explored only for fullerene acceptors, is shown to be rather common for other D/A pairs. By considering a phenomenological model of a large aggregate, we reveal that the position of CT2 can remarkably modulate the exciton dissociation rate by up to more than two orders of magnitude. Thus, controlling the alignment of CT2 is suggested as a promising rule for designing new D/A heterojunctions.

In this paper, we apply the recently developed ab initio renormalized excitonic method (REM) to the excitation energy calculations of various molecular aggregates, through the extension of REM to the time-dependent density functional theory (TDDFT). Tested molecular aggregate systems include one-dimensional hydrogen-bonded water chains, ring crystals with π–π stacking or van der Waals interactions, two dimensional benzene aggregates and the general aqueous systems with polar and nonpolar solutes. The basis set factor as well as the effect of the exchange-correlation functionals are also investigated. The results indicate that the REM-TDDFT method with suitable basis set and exchange-correlation functionals can give good descriptions of excitation energies and excitation area for lowest electronic excitations in the molecular aggregate systems with economic computational costs. It is shown that the deviations of REM-TDDFT excitation energies from those by standard TDDFT are much less than 0.1 eV and the computational time can be reduced by one order.