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Internal Oriented Electric Fields as a Strategy for Selectively Modifying Photochemical Reactivity
摘要： Time dependent density functional theory calculations have been performed on acetophenone derivatives to explore the possibility of using charged functional groups as internal electric fields, the orientation of which can be altered to change photochemical behavior at will. Results demonstrate that non-conjugated charged groups can significantly alter, by up to -1.44 eV, the stabilities of excited states. Specifically, a non-conjugated negatively charged group in the para-position will destabilize the nπ* and stabilize the ππ* transitions, while a positively charged group will do the opposite. These electrostatic effects can be tuned by moving these substituents into the meta and ortho positions. Through use of acids and bases, these charged groups can switched on or off with pH, allowing for selective alteration of the energy levels and photochemical reactivity. Solvent effects are shown to attenuate the electric field effect with increasing dielectric permittivity; however electrostatic effects are shown remain significant even in quite polar solvents. Using charged functional groups to deliver the position-dependent electrostatic (de)stabilization effects is therefore a potential route to improving the efficiency of desirable photochemical processes.
关键词： electric fields,acetophenone derivatives,TD-DFT,excited states,photochemical reactivity
更新于2025-09-10 09:29:36
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Structural, optical and nonlinear optical properties and TD-DFT analysis of heteroleptic bis-cyclometalated iridium(III) complex containing 2-phenylpyridine and picolinate ligands
摘要： In this work, we studied the structural, optical and nonlinear optical properties and UV–visible absorption spectrum of the heteroleptic bis-cyclometalated iridium(III) complex (tfmppy)2Ir(pic) recently synthesized with tfmppy = 5-trifluoromethyl-2-phenylpyridine and pic = picolinate. The calculations were performed by means of density functional theory (DFT) and time-dependent density functional (TD-DFT) methods using four functionals B3LYP, PBE0, CAM-B3LYP and M06-2X. Calculated geometric parameters agree with the experimental data. CAM-B3LYP and M06-2X lower the HOMO energy level and give a large energy gaps compared with B3LYP and PBE0. The four functionals show that LUMO is delocalized over ??? ppy and dIr orbitals. The studied complex gets a remarkably large first-order NLO response. B3LYP would provide good estimates of the energy gap and shows the strongest values of the first hyperpolarizabilities βHRS; M06-2X and CAM-B3LYP functionals overestimate the gaps and lower βHRS values. PBE0 and B3LYP spectra agree better with the experimental spectrum in the visible region, while CAM-B3LYP and M06-2X are more accurate in UV-C region. Natural transition orbital analysis shows that the weak band observed at 468 nm corresponds to MLCT/LLCT charge transfer transitions and the intense band observed at 270 nm is mainly assigned to intra-ligand state.
关键词： Excited states,Wiberg indices,Absorption spectrum,Hyperpolarizability,Complexes,Iridium,NTO,TD-DFT,Optical, nonlinear optical,Picolinate
更新于2025-09-10 09:29:36
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Investigating core-excited states of nitrosyl chloride (ClNO) and their break-up dynamics following Auger decay
摘要： The fragmentation of ClNO upon resonant core-electron excitation to the LUMO and LUMO+1 orbitals at the N and O K-edges is investigated. The produced fragment ions were detected in coincidence with a position sensitive ion time-of-flight detector which enables deduction of the angular distribution of the ions. This facilitates a comparison between the two resonances and the two K-edges with respect to fragmentation time, transition dipole moment orientation, fragment yield of single-ion and ion-pair channels, and fragmentation mechanisms. We observe significant correlations between the core-excited site and the location of the bonds that are broken, as well as the dissociation time. Moreover, we observe preferential cleavage of specific bonds upon excitation to the LUMO and LUMO+1 states which can be attributed to their orbital character.
关键词： ClNO,core-excited states,transition dipole moment,Auger decay,fragmentation dynamics
更新于2025-09-10 09:29:36
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Selective negative-ion formation from core-valence doubly excited states of the water molecule
摘要： This study focuses on the role of negative-ion formation in the decay of core-valence doubly excited water molecules. Combining negative- and positive-ion coincidence measurements with calculated energies of core-valence doubly excited states, we find that O? and H? production is enhanced selectively. In particular, we suggest that O? production is correlated to double occupancy of the antibonding 4a1 virtual orbital, while H? appears at electronic states with double occupancy in 2b2. We also show that H? and O? can be created as a result of electron recapture close to the O 1s ionization potential.
关键词： core-valence doubly excited states,electron recapture,negative-ion formation,water molecule,O 1s ionization potential
更新于2025-09-09 09:28:46
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Influence of stacking on the ground and excited states of 2-aminopyridine
摘要： In the present work, the ground and excited electronic state properties of seven isomers of 2-aminopyridine (2AMP) stacked dimer have been investigated at the estimated CCSD(T)/CBS limit and RI-CC2/def2-TZVP levels of theory, respectively. The interaction energy values for the seven isomers fall in the range of -7.7 to -4.9 kcal/mol. The most stable dimer is stabilized by two weak hydrogen bonds as well as by stacking interaction between the two moieties. The performance of several density functional theoretical methods has been tested for the ground state as well as the excited states. For the ground state, the functionals B3LYP-D3 and B97D were found to perform the best, giving a mean absolute deviation of 0.3 kcal/mol. For the excited states, the use of the functionals M06-2X and CAM-B3LYP led to results in good agreement with those of the RI-CC2 method, giving a mean absolute deviation of 2.3 kcal/mol. In addition, the vertical excitation energy values for a single proton transfer in the hydrogen bonded (planar) 2-aminopyridine dimer and double proton transfer in the stacked dimer of (2AMP)2 are also reported.
关键词： stacked dimer,2-aminopyridine,ground state geometry,electronic excited states
更新于2025-09-09 09:28:46
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Excited-State Switching between Ligand-Centered and Charge Transfer Modulated by Metal–Carbon Bonds in Cyclopentadienyl Iridium Complexes
摘要： Three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands were synthesized and structurally characterized, [Cp*Ir(tpy)L]n+ (tpy = 2-tolylpyridinato; n = 0 or 1), [Cp*Ir(piq)L]n+ (piq = 1-phenylisoquinolinato; n = 0 or 1), and [Cp*Ir(bpy)L]m+ (bpy = 2,2′-bipyridine; m = 1 or 2), featuring a range of monodentate carbon-donor ligands within each series [L = 2,6-dimethylphenylisocyanide; 3,5-dimethylimidazol-2-ylidene (NHC); methyl)]. The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H]+ were examined to establish electronic structure?photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge-transfer (MLCT) character. However, the molecules featuring anionic and strongly σ-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of CT character. Density functional theory calculations suggest that the lowest energy triplet states in these complexes are composed of a mixture of MLCT and ligand-to-ligand CT originating from both the Cp* and methyl or hydride ancillary ligands. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C^N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac-Ir(C^N)3 or Ir(C^N)2(acac) that have lowest-energy triplet excited states characterized as primarily LC character with a more moderate MLCT admixture.
关键词： cyclopentadienyl iridium complexes,charge transfer,photophysical properties,Iridium(III) complexes,ligand-centered excited states
更新于2025-09-09 09:28:46
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[Topics in Current Chemistry Collections] Multidimensional Time-Resolved Spectroscopy || Multidimensional Vibrational Coherence Spectroscopy
摘要： Multidimensional vibrational coherence spectroscopy has been part of laser spec-troscopy since the 1990s and its role in several areas of science has continuously been increasing. In this contribution, after introducing the principals of vibrational coherence spectroscopy (VCS), we review the three most widespread experimen-tal methods for multidimensional VCS (multi-VCS), namely femtosecond stimu-lated Raman spectroscopy, pump-impulsive vibrational spectroscopy, and pump-degenerate four wave-mixing. Focus is given to the generation and typical analysis of the respective signals in the time and spectral domains. Critical aspects of all multidimensional techniques are the challenges in the data interpretation due to the existence of several possible contributions to the observed signals or to opti-cal interferences and how to overcome the corresponding difficulties by exploiting experimental parameters including higher-order nonlinear effects. We overview how multidimensional vibrational coherence spectroscopy can assist a chemist in under-standing how molecular structural changes and eventually photochemical reactions take place. In order to illustrate the application of the techniques described in this chapter, two molecular systems are discussed in more detail in regard to the vibra-tional dynamics in the electronic excited states: (1) carotenoids as a non-reactive system and (2) stilbene derivatives as a reactive system.
关键词： Excited states,Photoisomerization,Multidimensional spectroscopy,Coherence spectroscopy,Vibronic coupling,Ultrafast laser spectroscopy,Raman,Vibrational spectroscopy
更新于2025-09-04 15:30:14