Range-separated
hybrid (RSH) functionals have been shown to overcome
the tendency of traditional density functional theory to underestimate
the fundamental orbital gap. More recently, the screened RSH (SRSH)
approach has been developed as a means to extend these functionals
to address the effect of the electrostatic environment on the fundamental
gap. Here, we report a scheme that combines the SRSH formulation with
the polarized continuum model (PCM) within a consistent framework
for addressing long-range screened electrostatic interactions, which
is further improved by optimal tuning (OT). The quantitative predictive
power of the new OT-SRSH-PCM scheme is demonstrated by addressing
fundamental gaps in thin films of organic semiconducting materials.
This is especially impressive as the approach is based on single molecule
calculations. We also discuss the advantages of this approach over
alternative schemes combining PCM with RSH. In particular, we show
that it avoids the well-documented tendency of standard OT to collapse
the range separation parameter when performed within a dielectric
continuum.
A screened-range
separated hybrid (SRSH) functional in combination
with a polarized continuum model (PCM) was recently implemented within
a consistent dielectric polarization treatment. The SRSH-PCM demonstrated
excellent agreement of the calculated fundamental orbital gaps with
measured energies in the condensed phase. Here we develop a linear
response time-dependent DFT (TDDFT) approach to obtain solvated charge
transfer state energies. We show that the calculated excited state
energies of solvated electron-donor–acceptor complexes are
in excellent agreement with measured benchmark values. Specifically
we consider donor–acceptor complexes of functionalized anthracenes
with tetracyanoethylene in methylene chloride. Our proposed SRSH-PCM
calculated energies earn a mean absolute deviation (MAD) from the
benchmark values as low as 0.04 eV with optimal tuning in PCM, whereas
values based on simpler RSH-PCM, without proper treatment of dielectric
screening, are associated with a 0.27 eV MAD.
Long range-corrected
(LRC) or range-separated hybrid (RSH) functionals
where the long-range (LR) limit of electronic interactions is set
to the exact exchange have been shown to correct the tendency of traditional
density functional theory (DFT) to underestimate the frontier orbital
gap. Consequently, the use of such functionals in calculating electronic
excited states using linear response based time-dependent DFT (TDDFT)
has been successful in correcting the tendency for underestimating
the energies of charge transfer states by DFT-based calculations.
More recently formulations of functionals that attenuate the LR limit
to address condensed-phase effects to polarize the electronic density
have been reported. In particular screened RSH (SRSH) combined with
polarizable continuum model (PCM) was benchmarked successfully in
reproducing the fundamental gap and charge transfer state energies
of molecular systems in the condensed phase. Here we use SRSH-PCM
to address triplet excited states, and show its success in obtaining
correspondence of the low-lying triplet states to the singlet–triplet
gap in a similar way that the fundamental orbital gap corresponds
to electron removal and addition energies. Importantly, the accuracy
of the SRSH-PCM in calculating triplet excitations stands on the polarization
consistent framework in addressing the scalar dielectric constant
and without affecting the optimal tuning by triplet
energies. The prospect of even further improving the SRSH-PCM accuracy
in calculating triplet states can be achieved by optimal tuning on
the basis of the spin multiplicity gap.
Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The watersolvated capsule effect on the coumarin's electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blueshifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state.
The special pair, a bacteriochlorophyll a (BChl) dimer found at the core of bacterial reaction centers, is known to play a key role in the functionality of photosystems as a precursor to the photosynthesis process. In this paper, we analyze the inherent affinity of the special pair to rectify the intrapair photo-induced charge transfer (CT). In particular, we show that the molecular environment affects the nuclear geometry, resulting in symmetry breaking between the two possible intrapair CT processes. To this end, we study the relationships of the intrapair CT and the molecular geometry with respect to the effective dielectric constant provided by the molecular environment. We identify the special pair structural feature that breaks the symmetry between the two molecules, leading to CT rectification. Excited state energies, oscillator strengths, and electronic coupling values are obtained via time-dependent density functional theory, employing a recently developed framework based on a screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM). We analyze the rectification capability of the special pair by calculating the CT rates using a first-principles-based Fermi's golden rule approach.
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