First isolation and characterization of biphenalenylidenes, which have long been unidentified reactive intermediates on the decomposition pathway of phenalenyl radical, were accomplished. ...Photoinduced electrocyclic ring-opening reaction of anti-dihydroperopyrene resulted in a successful conversion to E-biphenalenylidene, which enabled a detailed investigation of the electronic structure of E-biphenalenylidene by means of spectroscopic techniques. A stereoisomer, Z-biphenalenylidene, was also observed by suppressing a facile E–Z isomerization to E-biphenalenylidene in a rigid matrix. Furthermore, Z-biphenalenylidene demonstrated a thermal ring-closure in conrotatory process, which is not conforming to the Woodward–Hoffmann rule. These unusual reactivities of biphenalenylidene are ascribed to the ground states destabilized by its singlet biradical character, which was fully supported by theoretical calculations. The presence of E-biphenalenylidene on the decomposition pathway of phenalenyl was confirmed experimentally, leading to the full understanding of the decomposition mechanism of phenalenyl.
Vibronic coupling plays a crucial role in singlet fission whereby a singlet exciton splits into two triplet excitons. In order to reveal the physicochemical origin of the vibronic coupling associated ...with singlet fission as well as to clarify its relationship with chemical structure, we evaluate relevant vibronic couplings from the viewpoint of their spatial contributions described by vibronic coupling density. From the analysis using a model tetracene dimer, a typical singlet fission system, the frequency dependence of vibronic couplings in each electronic state is found to be significantly different from that of another depending on the nature of the electronic structure (intra/intermolecular excitation) and the related vibrational motion. These findings contribute not only to the fundamental understanding of the singlet fission mechanism from the viewpoint of vibronic couplings but also to opening a new path to designing highly efficient singlet fission materials through phonon–bath engineering.
The singlet fission (SF) dynamics of realistic/artificial pentacene dimer models are investigated using the quantum master equation method in order to obtain new insight into the SF dynamics and its ...rational design guidelines. We comprehensively clarify the effects of the energy offsets of diabatic Frenkel exciton (FE) and charge transfer (CT) exciton states to the double-triplet (TT) exciton state, excitonic couplings, and state-dependent vibronic couplings on the exciton population dynamics using relative relaxation factors (RRFs) between the adiabatic exciton states. As shown in previous studies, efficient sequential/superexchange CT-mediated SF is observed in the energy level matching region (E(TT) – E(FE) < 0). On the other hand, it is predicted that almost the perfect energy level matching (E(TT) – E(FE) ∼ 0) causes the significant reduction of TT yields though exhibits remarkably fast SF rates, when the corresponding adiabatic double-triplet (TT′) and Frenkel exciton (FE′) states are near-degenerate to each other with common diabatic configurations. The excitonic coupling is also found to have a possibility of causing significant change of SF dynamics when it has a large amplitude comparable to those of the other electronic coupling elements. Furthermore, the large vibronic coupling of CT state shows striking enhancement of SF rates with keeping high TT yields in the CT-mediated superexchange region, while the large vibronic couplings of FE and TT states do not show such striking enhancement. These features are understood by analyzing their RRFs, which are proportional to the product of the square of common diabatic exciton configuration coefficients in the concerned two adiabatic exciton states, multiplied by the spectral density (vibronic coupling).
Using the time-dependent tuned long-range corrected density functional theory method, the feasibility for singlet fission in oligorylenes has been investigated within the scope of the diradical ...character based guideline and of the energy level matching conditions for the isolated monomers. It is found that the relatively small-size oligorylenes, that is, terrylene and quaterrylene, which present intermediate diradical character without significant tetraradical character, are possible candidates for energetically efficient singlet fission. In relation to this result, we also raise the possibility that the unsettled ultrafast dynamics previously observed on quaterrylene is evidence for singlet fission.
Carbon atoms have the potential to produce a variety of fascinating all-carbon structures with amazing electronic and mechanical properties. Over the last few decades, several efforts have been made ...using experimental and computational techniques to functionalize graphene, carbon nanotubes and fullerenes for potential use in modern hi-tech electronic, medicinal, optical and nonlinear optical (NLO) applications. Since photons replaced electrons as a carrier of information, the field of NLO material design has drawn immense interest in contemporary materials science. There have been several reports of bridging the gap between the exciting fields of carbon nanomaterials and NLO materials by functionalizing carbon nanomaterials for potential NLO applications. In contrast to previous reports of the design of third-order NLO materials using conventional closed-shell materials, a novel strategy using open-shell diradical molecular systems has recently been proposed. Quantum chemically, diradical character is explained in terms of the instability of the chemical bonds in open-shell molecular systems. Interestingly, several carbon nanomaterials, which naturally possess open-shell singlet configurations, have recently gained momentum in the design of these classes of open-shell NLO materials with excellent NLO properties, stability and diversity. The present review establishes a systematic sequence of different studies (spanning over two decades of intense research efforts), starting from the simplest theoretical two-site diradical model, continuing to its experimental and theoretical realization in actual chemical systems, and finally applying the abovementioned model/rule to novel carbon nanomaterials to tune their NLO properties, particularly their second hyperpolarizability (γ). In the present report, we highlight several recent efforts to functionalize carbon nanomaterials by exploiting their open-shell diradical character to achieve efficient third-order NLO properties. Several issues and opportunities are discussed, including the inherited disadvantages of both experimental (the high reactivity and short life of diradical compounds) and quantum (need for multi-reference methodology) techniques when dealing with carbon nanomaterials. A comparative analysis of several quantum chemical investigations, along with contemporary experimental results, will be performed to emphasize the core issues and opportunities related to carbon nanomaterials with singlet open-shell diradical character. Thus, the present review will highlight carbon nanomaterials with diradical/biradical character for their prospective applications in the NLO field.
A signature of singlet open-shell character on the optically allowed singlet excitation energy and singlet–triplet energy gap is theoretically illuminated for open-shell singlet molecules. On the ...basis of a two-site diradical model with two electrons in two orbitals, the linear dependence of these excitation energies on the transfer integral, which is equivalent to the energy gap between the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals in the Hückel theory, is found to be broken down in the high diradical character region due to an increase in electron correlation in the open-shell singlet ground state. A series of polyacenes shows the similar behavior of the optically allowed singlet excitation energies obtained by time-dependent spin–flip density functional theory calculations and experiments, which bears testimony to the singlet open-shell character in long polyacenes.
A copper(I)-catalyzed anomeric aminoalkynylation reaction of unprotected aldoses was realized. Use of an electron-deficient phosphine ligand, boric acid to stabilize the iminium intermediate, and a ...protic additive (IPA) to presumably enhance reversible carbohydrate–boron complexation were all essential for efficient conversion. The reaction proceeded well even with a natural disaccharide substrate, suggesting that the developed catalytic reaction could be useful for the synthesis of glycoconjugates with minimum use of protecting groups.
Contrary to the enormous number of previous studies on carbon nanotubes (CNTs), herein, we realized the origin of the intrinsic open-shell diradical character and second hyperpolarizability γ using a ...broken symmetry approach. This study was inspired by our recent findings (S. Muhammad, et al., Nanoscale, 2016, 8, 17998 and Nakano, et al., J. Phys. Chem. C, 2016, 120, 1193). We performed structural modifications through a unique asymmetric donor-nanotube framework, which led to a novel paradigm of modified CNTs with tunable open-shell diradical character and remarkably superior NLO response properties. Interestingly, asymmetry and diradical character were found to be the crucial factors to modulate the second hyperpolarizability γ. We initially performed a comparative analysis of the diradical characters and γ amplitudes of boron nitride nanotubes (BNNTs) and CNTs possessing significant ionic characters and covalent characters, respectively. The basic findings for these simple configurations were further extended to the donor-acceptor CNT paradigm, which finally led to excellent asymmetric donor-CNT configurations with remarkably larger γ amplitudes. Furthermore, among the CNTs, finite length zigzag CNT(6,0)3 were modified with different donor-acceptor configurations. Interestingly, for the first time, unique donor-nanotube configurations 1,4-(NH2)2CNT-(6,0)3 and 1,4-(NH2)2CNT-(6,0)5 were found; they showed significantly robust γ amplitudes as large as 2519 × 103 and 4090 × 103 a.u. at the LC-UBLYP(μ = 0.33)/6-31G* level of theory. Additionally, several molecular level insights have been obtained for these novel donor-nanotube configurations using their odd electron densities, molecular electrostatic maps, densities of states and γ density analyses to highlight the realization of these novel materials for highly efficient optical and NLO applications.
Using the density functional theory method, the crystalline packing effect on the singlet fission (SF) rate of oligorylenes, some of which are found to exhibit SF in crystal forms, is revealed by ...evaluating the effective electronic coupling (|V eff|), the square of which is proportional to the SF rate. The |V eff| values for terrylene and quaterrylene dimer models are investigated for a variety of slip-stacked forms. It is found that these values show similar dependences on the intermolecular packing as a function of lateral and longitudinal displacements of monomer frameworks, and that they are maximized in several configurations of one monomer slipped from another along the longitudinal axis. The present estimation method of the SF rate is also found to qualitatively explain the experimental SF rate difference between terrylene derivatives with different packing forms. Furthermore, by analyzing the effect of electronic couplings on the adiabatic electronic states related to SF, we predict several favorable molecular packings leading to a fast SF with a high triplet yield.
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