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Next‐generation quantum theory of atoms in molecules for the ground and excited state of the ring‐opening of cyclohexadiene
摘要： The factors underlying the experimentally observed branching ratio (70:30) of the (1,3-cyclohexadiene) CHD ! HT (1,3,5-hexatriene) photochemical ring-opening reaction are investigated. The ring-opening reaction path is optimized by a high-level multi-reference DFT method and the density along the path is analyzed by the quantum theory of atoms in molecules (QTAIM) and stress tensor methods. The performed density analysis suggests that, in both S1 and S0 electronic states, there exists an attractive interaction between the ends of the fissile σ-bond of CHD that steers the ring-opening reaction predominantly in the direction of restoration of the ring. It is suggested that opening of the ring and formation of the reaction product (HT) can only be achieved when there is a sufficient persistent nuclear momentum in the direction of stretching of the fissile bond. As this orientation of the nuclear momentum vector can be expected relatively rare during the dynamics, this explains the observed low quantum yield of the ring-opening reaction.
关键词： QTAIM,ring-opening,cyclohexadiene,conical intersection,stress tensor
更新于2025-09-23 15:23:52
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Exploring approximate geometries of minimum energy conical intersections by TDDFT calculations
摘要： An approach is proposed to obtain approximate geometries for minimum energy conical intersections between the ground and first excited singlet electronic states (S0/S1-MECIs) using the time-dependent density functional theory (TDDFT). This approach uses the energy shift method to avoid discontinuities on TDDFT potential energy surfaces around conical intersections. It is shown numerically that the approximate S0/S1-MECIs of benzene and naphthalene obtained by this approach qualitatively reproduce the geometries and energies of the S0/S1-MECIs obtained by multireference theories. Moreover, the performance of the present approach when combined with an automated MECI searching method is examined through applications to benzene and naphthalene.
关键词： energy shift method,time dependent density functional theory,gradient projection,single component artificial force induced reaction,conical intersection,global reaction route mapping
更新于2025-09-23 15:23:52
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Photorelaxation Pathways of 4-(N,N-Dimethylamino)-4a?2-nitrostilbene Upon S1 Excitation Revealed by Conical Intersection and Intersystem Crossing Networks
摘要： Multi-state n-electron valence state second order perturbation theory (MS-NEVPT2) was utilized to reveal the photorelaxation pathways of 4-(N,N-dimethylamino)-4'-nitrostilbene (DANS) upon S1 excitation. Within the interwoven networks of five S1/S0 and three T2/T1 conical intersections (CIs), and three S1/T1 intersystem crossings (ISCs), those competing nonadiabatic decay pathways play different roles in trans-to-cis and cis-to-trans processes, respectively. After being excited to the Franck–Condon (FC) region of the S1 state, trans-S1-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, trans-S1-FC → ISC-S1/T1-twist → trans- or cis-S0. The singlet relaxation pathway mediated by CI-S1/S0-twist-c is hindered by the prominent energy barrier on S1 surface and by the reason that CI-S1/S0-twist-t are both not energetically accessible upon S1 excitation. On the other hand, the cis-S1-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S1/S0-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between αC=C twisting (cis-S1-FC → CI-S1/S0-twist-c → trans- or cis-S0) and DHP formation relaxation pathways (cis-S1-FC → CI-S1/S0-DHP → DHP-S0) on the S1 surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, cis-S1-FC → ISC-S1/T1-cis → DHP-T1 → DHP-S0, however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for cis-S1-FC mediated by ISC-S1/T2-cis are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S1 → T2 → T1 or S1 → T2 → S1 processes. The present study reveals photoisomerization dynamic pathways via conical intersection and intersystem crossing networks and provides nice physical insight into experimental investigation of DANS.
关键词： MS-NEVPT2,intersystem crossing,photorelaxation,conical intersection
更新于2025-09-23 15:21:01
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Locating Cytosine Conical Intersections by Laser Experiments and <i>Ab Initio</i> Calculations
摘要： The decay mechanism of S1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250?1200 cm?1 above the 000 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5?C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (1ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm?1.
关键词： CASPT2//CASSCF computations,jet-cooled spectroscopy,internal conversion,substitution effect,R2PI spectra,decay mechanism,cytosine,conical intersection
更新于2025-09-23 15:19:57
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Dissociative Nature of C(sp2)-N(sp3) bonds of Carbazole Based Materials via Conical Intersection: Simple Method to Predict the Exciton Stability of Host Materials for blue OLEDs: A Computational Study
摘要： In this work, the origin of singlet and triplet exciton-induced degradation of host materials with C(sp2)?N(sp3) bonds around nitrogen (carbazoles, acridines etc), connecting donor and acceptor units, were unravelled using DFT and CASSCF methods. The results reveal that molecules (employed in OLEDs) with basic unit containing C(sp2)?N(sp3) bonds (nitrogen connected to carbon in triangular fashion) have natural tendency to fragment at C-N bond through S1/S0 conical intersection (CI). The calculation of barrier heights, to reach dissociation point, indicates that degradation via triplet states is kinetically less feasible (?G* T1-TS >25 kcal mol-1) compared to first singlet excited state (?G* S1-TS ?7-30 kcal mol-1). However, long lifetime of triplets (as compared to singlets) aids in the reverse intersystem crossing from triplet to singlet state for subsequent degradation. From the results and inference, ?G* S1-TS and ?ES1-T1 are proposed to be the controlling factors for exciton-induced degradation of host materials with C(sp2)?N(sp3) bonds. Further, multiple functionalization of carbazole moieties reveal that polycyclic aromatic systems employed as acceptor unit of host materials are best suited for PhOLEDs as they will increase their lifetime due to larger ?G* S1-TS and ?ES1-T1. For TADF-based devices, materials with fused ring systems (with N(sp3) at the centre) in the donor unit is the most recommended one based on the findings of this work, as it avoids the dissociative channel altogether. Negative linear correlation between ?G* S1-TS and HOMO-LUMO gap is observed, which provides indirect way to predict the kinetic stability of these materials in excitonic states. These initial results are promising for future development of QSAR-type approach for smart designing of host materials for long-life blue OLEDs.
关键词： C(sp2)?N(sp3) bonds,OLED,DFT,degradation,host materials,conical intersection,CASSCF
更新于2025-09-23 15:19:57
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How the methyl group position influences the ultrafast deactivation in aromatic radicals
摘要： Excited xylyl (methyl–benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry. Depending on the substitution we find different deactivation channels after excitation into the D3(2A00) state (310 nm, 4 eV). While the ortho and para isomer exhibit deactivation rates similar to the benzyl radical, meta-xylyl sticks out and depletes twice as fast into the vibrationally hot ground state. We found that a ring deformation mode rather than the methyl pseudorotation enables access to a conical intersection, which is responsible for the faster deactivation. Transitions in the photoelectron spectrum can be assigned to several Rydberg series with mostly d angular momentum components. Absorption of two 4 eV photons triggers hydrogen loss reactions on a femtosecond timescale.
关键词： photoelectron spectroscopy,mass spectrometry,conical intersection,Rydberg states,femtosecond spectroscopy,ultrafast deactivation,xylyl radicals
更新于2025-09-19 17:15:36
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Unveiling coupled electronic and vibrational motions of chromophores in condensed phases
摘要： The quest for capturing molecular movies of functional systems has motivated scientists and engineers for decades. A fundamental understanding of electronic and nuclear motions, two principal components of the molecular Schr?dinger equation, has the potential to enable the de novo rational design for targeted functionalities of molecular machines. We discuss the development and application of a relatively new structural dynamics technique, femtosecond stimulated Raman spectroscopy with broadly tunable laser pulses from the UV to near-IR region, in tracking the coupled electronic and vibrational motions of organic chromophores in solution and protein environments. Such light-sensitive moieties hold broad interest and significance in gaining fundamental knowledge about the intramolecular and intermolecular Hamiltonian and developing effective strategies to control macroscopic properties. Inspired by recent experimental and theoretical advances, we focus on the in situ characterization and spectroscopy-guided tuning of photoacidity, excited state proton transfer pathways, emission color, and internal conversion via a conical intersection.
关键词： emission color,excited state proton transfer,electronic and vibrational motions,chromophores,photoacidity,femtosecond stimulated Raman spectroscopy,conical intersection,internal conversion
更新于2025-09-11 14:15:04
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Collective Jahn-Teller Interactions through Light-Matter Coupling in a Cavity
摘要： The ultrafast nonradiative relaxation of a molecular ensemble coupled to a cavity mode is considered theoretically and by real-time quantum dynamics. For equal coupling strength of single molecules to the cavity mode, the nonradiative relaxation rate from the upper to the lower polariton states is found to strongly depend on the number of coupled molecules. The coupling of both bright and dark polaritonic states among each other constitutes a special case of (pseudo-)Jahn-Teller interactions involving collective displacements the internal coordinates of the molecules in the ensemble, and the strength of the first order vibronic coupling depends exclusively on the gradient of the energy gaps between molecular electronic states. For N > 2 molecules, the N ? 1 dark light-matter states between the two optically active polaritons feature true collective conical intersection crossings, whose location depends on the internal atomic coordinates of each molecule in the ensemble, and which contribute to the ultrafast nonradiative decay from the upper polariton.
关键词： conical intersection crossings,nonradiative relaxation,cavity mode,Jahn-Teller interactions,polaritonic states,molecular ensemble
更新于2025-09-04 15:30:14