C–N Bond Rotation Controls Photoinduced Electron Transfer in an Aminostyrene–Stilbene Donor–Acceptor System

We investigate energy transfer and electron transfer in a dimethylsilylene-spaced aminostyrene–stilbene donor–acceptor dimer using time-dependent density functional theory calculations. Our results confirm that the vertical S3, S2, and S1 excited states are, respectively, a local excitation on the aminostyrene, local excitation on the stilbene, and the charge-transferred (CT) excited state with electron transfer from aminostyrene to stilbene. In addition, an energy minimum with the C–N bond of the amino group twisted at about 90° is also identified on the S1 potential energy surface. This S1 state exhibits a twisted intramolecular charge transfer (TICT) character. A potential energy scan along the C–N bond torsional angle reveals a conical intersection between the S2 stilbene local excitation and the S1 CT/TICT state at a torsional angle of ∼60°. We thus propose that the conical intersection dominates the electron transfer dynamics in the donor–acceptor dimer and copolymers alike, and the energy barrier along the C–N bond rotation controls the efficiency of such a process. Moreover, we show that despite the zero oscillator strength of the S1 excited states in the CT and TICT minima, an emissive S1 state with a V-shaped conformational structure can be located. The energy of this V-shape CT structure is thermally accessible; therefore, it is expected to be responsible for the CT emission band of the dimer observed in polar solvents. Our data provide a clear explanation of the complex solvent-dependent dual emission and photoinduced electron transfer properties observed experimentally in the dimer and copolymer systems. More importantly, the identifications of the conical intersection and energy barrier along the C–N bond rotation provide a novel synthetic route for controlling emissive properties and electron transfer dynamics in similar systems, which might be useful in the design of novel organic optoelectronic materials

Multisite Constrained Model of <i>trans</i>-4‑(<i>N,N</i>-Dimethylamino)-4′-nitrostilbene for Structural Elucidation of Radiative and Nonradiative Excited States

A constrained model compound of trans-4-(N,N-dimethylamino)-4′-nitrostilbene (DNS), namely, compound DNS-B3 that is limited to torsions about the phenyl-nitro C–N bond and the central CC bond, was prepared to investigate the structural nature of the radiative and nonradiative states of electronically excited DNS. The great similarities in solvent-dependent electronic spectra, fluorescence decay times, and quantum yields for fluorescence (Φf) and trans → cis photoisomerization (Φtc) between DNS and DNS-B3 indicate that the fluorescence is from a planar charge-transfer state and torsion of the nitro group is sufficient to account for the nonradiative decay of DNS. This conclusion is supported by TDDFT calculations on DNS-B3 in dichloromethane. The structure at the conical intersection for internal conversion is associated with not only a twisting but also a pyramidalization of the nitro group. The mechanism of the NO2 torsion is discussed in terms of the effects of solvent polarity, the substituents, and the volume demand. The differences and analogies of the NO2- vs amino-twisted intramolecular charge-transfer (TICT) state of trans-aminostilbenes are also discussed

The effect of static disorder on the center line slope in 2D electronic spectroscopy

Two-dimensional electronic spectroscopy (2DES) is a powerful tool for investigating the dynamics of complex systems. However, analyzing the resulting spectra can be challenging, and thus may require the use of theoretical modeling techniques. The center line slope (CLS) method is one of such approaches, which aims to extract the time correlation function (TCF) from 2DES with minimal error. Since static disorder is widely observed in complex systems, it may be interesting to ask whether the CLS approach still work in the presence of the static disorder. In this paper, the effect of the static disorder on the TCF obtained through the CLS method is investigated. It is found that the steady-state value of the CLS increases monotonically with respect to the static disorder, which suggests that the amplitude of the static disorder can be determined using the CLS in the long-time limit. Additionally, as the static disorder rises, the decay rate of the CLS first decreases to a certain value and remains at this value until the static disorder is sufficiently large. Afterward, the CLS begins to fluctuate significantly and thus results in obtaining the decay rate through the CLS method unreliable. Based on these discoveries, we propose a method to fix the error and obtain the TCF. Our findings may pave the way for obtaining reliable system-bath information by analyzing 2DES in the practical situations

Clustered Geometries Exploiting Quantum Coherence Effects for Efficient Energy Transfer in Light Harvesting

Elucidating quantum coherence effects and geometrical factors for efficient energy transfer in photosynthesis has the potential to uncover nonclassical design principles for advanced organic materials. We study energy transfer in a linear light-harvesting model to reveal that dimerized geometries with strong electronic coherences within donor and acceptor pairs exhibit significantly improved efficiency, which is in marked contrast to predictions of the classical Förster theory. We reveal that energy tuning due to coherent delocalization of photoexcitations is mainly responsible for the efficiency optimization. This coherence-assisted energy-tuning mechanism also explains the energetics and chlorophyll arrangements in the widely studied Fenna–Matthews–Olson complex. We argue that a clustered network with rapid energy relaxation among donors and resonant energy transfer from donor to acceptor states provides a basic formula for constructing efficient light-harvesting systems, and the general principles revealed here can be generalized to larger systems and benefit future innovation of efficient molecular light-harvesting materials

Coordination of Azobisisobutyronitrile with Cobalt Complexes in Cobalt-Mediated Radical Polymerization Disclosed by Linear Correlation between the Equilibrium Constant and Half-Wave Potential

In cobalt-mediated radical polymerization (CMRP) of methyl acrylate with a series of Co­(salen) complexes, a linear correlation between the equilibrium constants (KCMRP) and half-wave potential (E1/2) was observed. However, this correlation was inconsistent with the simulated trend obtained by density functional theory (DFT) calculations until the axial coordination of azobisisobutyronitrile (AIBN) with organocobalt­(III) was added to the model. Control studies using V601, an initiator similar to AIBN with no cyano-group, and tert-butyl cyanide further confirmed that AIBN was the substituent axially coordinating to the organocobalt­(III) species. According to the DFT calculation, the function of the coordinated nitrile was mainly to stabilize organocobalt­(III), possibly through the formation of an octahedral configuration, rather than to weaken the cobalt–carbon bond through the trans-effect

Quantum Coherence Enabled Determination of the Energy Landscape in Light-Harvesting Complex II

The near-unity efficiency of energy transfer in photosynthesis makes photosynthetic light-harvesting complexes a promising avenue for developing new renewable energy technologies. Knowledge of the energy landscape of these complexes is essential in understanding their function, but its experimental determination has proven elusive. Here, the observation of quantum coherence using two-dimensional electronic spectroscopy is employed to directly measure the 14 lowest electronic energy levels in light-harvesting complex II (LHCII), the most abundant antenna complex in plants containing approximately 50% of the world’s chlorophyll. We observe that the electronically excited states are relatively evenly distributed, highlighting an important design principle of photosynthetic complexes that explains the observed ultrafast intracomplex energy transfer in LHCII

Preparation of Ketimines from Aryldiazonium Salts, Arenes, and Nitriles via Intermolecular Arylation of <i>N</i>‑Arylnitrilium Ions

A transition-metal-free approach for the preparation of <i>N</i>-arylketimines has been developed from the direct reaction of aryldiazonium salts, arenes, and nitriles in a one-pot fashion with the consecutive formation of N–C and C–C bonds. This approach proceeds via an <i>in situ</i> generation of <i>N</i>-arylnitrilium intermediate, which then undergoes intermolecular arylation. This three-component strategy offers a step- and atom-efficient way to <i>N</i>-arylketimines from easily accessible reagents under mild reaction conditions. The characterization of stereochemistry of ketimine was achieved by X-ray crystallographic structure and theoretical calculation. Operational simplicity, shorter reaction time, excellent functional group compatibility, and scalability are the key features of this report

Preparation of Ketimines from Aryldiazonium Salts, Arenes, and Nitriles via Intermolecular Arylation of <i>N</i>‑Arylnitrilium Ions

A transition-metal-free approach for the preparation of <i>N</i>-arylketimines has been developed from the direct reaction of aryldiazonium salts, arenes, and nitriles in a one-pot fashion with the consecutive formation of N–C and C–C bonds. This approach proceeds via an <i>in situ</i> generation of <i>N</i>-arylnitrilium intermediate, which then undergoes intermolecular arylation. This three-component strategy offers a step- and atom-efficient way to <i>N</i>-arylketimines from easily accessible reagents under mild reaction conditions. The characterization of stereochemistry of ketimine was achieved by X-ray crystallographic structure and theoretical calculation. Operational simplicity, shorter reaction time, excellent functional group compatibility, and scalability are the key features of this report

Synthesis of Metallacyclobutenes of Late Transition Metals via Nucleophilic Addition of Allenyl or Propargyl Complexes

The regioselective addition of NEt3, PPh3, or pyridine to the central carbon of a cationic η3-allenyl/propargyl complex of platinum, {Pt(PPh3)2(η3-C3H3)}(BF4) (1), leads to the formation of the new cationic = NEt3 (2a), PPh3 (2b), C5H5N (2c)) via formation of a C−N or C−P bond, respectively. Complex 2c can transform into {cis-Pt(PPh3)2(Py)(η1-CHCCH2)}(BF4) (3). The reverse reaction has not been observed. It is suggested that nucleophilic addition of 1 likely involves external attack at the central carbon of the η3-allenyl/propargyl ligand. Protonation of 2b yields {Pt(PPh3)2[η3-CH2C(PPh3)CH2]}(BF4)2 (7). Addition of PPh3 to a labile η1-allenyliridium complex, (OC-6-42)-Ir(Cl)(PPh3)2(OTf)(CO)(η1-CHCCH2) (4), results in the The single-crystal X-ray structure of 5 has been determined