4 research outputs found
Can TDDFT Describe Excited Electronic States of Naphthol Photoacids? A Closer Look with EOM-CCSD
The <sup>1</sup>L<sub>b</sub> and <sup>1</sup>L<sub>a</sub> excited
states of naphthols are characterized by using time-dependent density
functional theory (TDDFT), configuration interaction with singles
(CIS), and equation-of-motion coupled cluster singles and doubles
(EOM-CCSD) methods. TDDFT fails dramatically at predicting the energy
and ordering of the <sup>1</sup>L<sub>a</sub> and <sup>1</sup>L<sub>b</sub> excited states as observed experimentally, while EOM-CCSD
accurately predicts the excited states as characterized by natural
transition orbital analysis. The limitations of TDDFT are attributed
to the absence of correlation from doubly excited configurations as
well as the inconsistent description of excited electronic states
of naphthol photoacids revealed by excitation analysis based on the
one-electron transition density matrix
Toward Understanding the Redox Properties of Model Chromophores from the Green Fluorescent Protein Family: An Interplay between Conjugation, Resonance Stabilization, and Solvent Effects
The redox properties of model chromophores from the green
fluorescent
protein family are characterized computationally using density functional
theory with a long-range corrected functional, the equation-of-motion
coupled-cluster method, and implicit solvation models. The analysis
of electron-donating abilities of the chromophores reveals an intricate
interplay between the size of the chromophore, conjugation, resonance
stabilization, presence of heteroatoms, and solvent effects. Our best
estimates of the gas-phase vertical/adiabatic detachment energies
of the deprotonated (i.e., anionic) model red, green, and blue chromophores
are 3.27/3.15, 2.79/2.67, and 2.75/2.35 eV, respectively. Vertical/adiabatic
ionization energies of the respective protonated (i.e., neutral) species
are 7.64/7.35, 7.38/7.15, and 7.70/7.32 eV, respectively. The standard
reduction potentials (<i>E</i><sub>red</sub><sup>0</sup>) of the anionic (Chr<sup>•</sup>/Chr<sup>–</sup>) and neutral (Chr<sup>+•</sup>/Chr)
model chromophores in acetonitrile are 0.34/1.40 V (red), 0.22/1.24
V (green), and −0.12/1.02 V (blue), suggesting, counterintuitively,
that the red chromophore is more difficult to oxidize than the green
and blue ones (in both neutral and deprotonated forms). The respective
redox potentials in water follow a similar trend but are more positive
than the acetonitrile values
Phenothiazine Radical Cation Excited States as Super-oxidants for Energy-Demanding Reactions
We demonstrate that the 10-phenyl-10<i>H</i>-phenothiazine
radical cation (PTZ<sup>+•</sup>) has a manifold of excited
doublet states accessible using visible and near-infrared light that
can serve as super-photooxidants with excited-state potentials is
excess of +2.1 V vs SCE to power energy demanding oxidation reactions.
Photoexcitation of PTZ<sup>+•</sup> in CH<sub>3</sub>CN with
a 517 nm laser pulse populates a D<sub>n</sub> electronically excited
doublet state that decays first to the unrelaxed lowest electronic
excited state, D<sub>1</sub>′ (τ < 0.3 ps), followed
by relaxation to D<sub>1</sub> (τ = 10.9 ± 0.4 ps), which
finally decays to D<sub>0</sub> (τ = 32.3 ± 0.8 ps). D<sub>1</sub>′ can also be populated directly using a lower energy
900 nm laser pulse, which results in a longer D<sub>1</sub>′→D<sub>1</sub> relaxation time (τ = 19 ± 2 ps). To probe the
oxidative power of PTZ<sup>+•</sup> photoexcited doublet states,
PTZ<sup>+•</sup> was covalently linked to each of three hole
acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile
(ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs
SCE, respectively. In all three cases, photoexcitation wavelength
dependent ultrafast hole transfer occurs from D<sub>n</sub>, D<sub>1</sub>′, or D<sub>1</sub> of PTZ<sup>+•</sup> to Per,
DPA, and ACN. The ability to take advantage of the additional oxidative
power provided by the upper excited doublet states of PTZ<sup>+•</sup> will enable applications using this chromophore as a super-oxidant
for energy-demanding reactions
Extension of the Effective Fragment Potential Method to Macromolecules
The
effective fragment potential (EFP) approach, which can be described
as a nonempirical polarizable force field, affords an accurate first-principles
treatment of noncovalent interactions in extended systems. EFP can
also describe the effect of the environment on the electronic properties
(e.g., electronic excitation energies and ionization and electron-attachment
energies) of a subsystem via the QM/EFP (quantum mechanics/EFP) polarizable
embedding scheme. The original formulation of the method assumes that
the system can be separated, without breaking covalent bonds, into
closed-shell fragments, such as solvent and solute molecules. Here,
we present an extension of the EFP method to macromolecules (mEFP).
Several schemes for breaking a large molecule into small fragments
described by EFP are presented and benchmarked. We focus on the electronic
properties of molecules embedded into a protein environment and consider
ionization, electron-attachment, and excitation energies (single-point
calculations only). The model systems include chromophores of green
and red fluorescent proteins surrounded by several nearby amino acid
residues and phenolate bound to the T4 lysozyme. All mEFP schemes
show robust performance and accurately reproduce the reference full
QM calculations. For further applications of mEFP, we recommend either
the scheme in which the peptide is cut along the C<sub>α</sub>–C bond, giving rise to one fragment per amino acid, or the
scheme with two cuts per amino acid, along the C<sub>α</sub>–C and C<sub>α</sub>–N bonds. While using these
fragmentation schemes, the errors in solvatochromic shifts in electronic
energy differences (excitation, ionization, electron detachment, or
electron-attachment) do not exceed 0.1 eV. The largest error of QM/mEFP
against QM/EFP (no fragmentation of the EFP part) is 0.06 eV (in most
cases, the errors are 0.01–0.02 eV). The errors in the QM/molecular
mechanics calculations with standard point charges can be as large
as 0.3 eV