An
organic dye with three electron donating groups is studied that
has broad full visible spectrum panchromatic absorption in solution.
The donor groups introduce six significant optical transitions spaced
throughout the visible spectrum to give a dye with near uniform molar
absorptivity across the visible spectrum. This unique material is
characterized by optical, electrochemical, and computational methods.
Within dye-sensitized solar cell (DSC) devices, this material shows
a strong photocurrent output of ∼20 mA/cm2 with
no cosensitization. DSC devices are characterized through impedance
spectroscopy, current–voltage curve analysis, and incident
photon-to-current conversion efficiency measurements. Importantly,
minimal DSC device performance loss is observed after 500 h of continuous
irradiation.
Few anions exhibit electronically excited states, and, if they do, the one or two possible excitations typically transpire beyond the visible spectrum into the near-infrared. These few, red-shifted electronic absorption features make anions tantalizing candidates as carriers of the diffuse interstellar bands (DIBs), a series of mostly unknown, astronomically ubiquitous absorption features documented for over a century. The recent interstellar detection of benzonitrile implies that cyano-functionalized polycyclic aromatic hydrocarbon (PAH) anions may be present in space. The presently reported quantum chemical work explores the electronic properties of deprotonated benzene, naphthalene, and anthracene anions functionalized with a single cyano group. Both the absorption and emission properties of the electronically excited states are explored. The findings show that the larger anions absorption and emission energies possess both valence and dipole bound excitations in the 450–900 nm range with oscillator strengths for both types of >1×10−4. The valence and dipole bound excited state transitions will produce slightly altered substructure from one another making them appear to originate with different molecules. The known interstellar presence of related molecules, the two differing natures of the excited states for each, and the wavelength range of peaks for these cyano-functionalized PAH anions are coincident with DIB properties. Finally, the methods utilized appear to be able to predict the presence of dipole-bound excited states to within a 1.0 meV window relative to the electron binding energy.
Dicyano-functionalized benzene and naphthalene anion derivatives exhibit a relatively rich population of electronically excited states in stark contrast to many assumptions regarding the photophysics of anions in general. The present work has quantum chemically analyzed the potential electronically excited states of closed-shell anions created by replacing hydrogen atoms with valence-bound lone pairs in benzene and naphthalene difunctionalized with combinations of -CN and -C2H. Dicyanobenzene anion derivatives can exhibit dipole-bound excited states as long as the cyano groups are not in para position to one another. This also extends to cyanoethynylbenzene anions as well as deprotonated dicyano- and cyanoethynylnaphthalene anion derivatives. Diethynyl functionalization is less consistent. While large dipole moments are created in some cases for deprotonation on the -C2H group itself, the presence of electronically excited states beyond those that are dipole-bound is less consistent. Beyond these general trends, 2-dicyanonaphthalene-34 gives strong indication for exhibiting a quadrupole-bound excited state, and the 1-cyanoethynylnaphthalene-29 and -36 anion derivatives are shown to possess as many as two valence-bound excited states and one dipole-bound excited state. These photophysical properties may have an influence on regions where polycyclic aromatic hydrocarbons are known to exist such as in various astrochemical environments or even in combustion flames.
Nonminimum carbonic acid clusters provide excitation
energies and
oscillator strengths in line with observed ice-phase UV absorptions
better than traditional optimized minima. This equation-of-motion
coupled cluster quantum chemical analysis on carbonic acid monomers
and dimers shows that shifts to the dihedral angle for the internal
heavy atoms in the monomer produce UV electronic excitations close
to 200 nm with oscillator strengths that would produce observable
features. This τ(OCOO) dihedral is actually a relatively floppy
motion unlike what is often expected for sp2 carbons and
can be distorted by 30° away from equilibrium for an energy cost
of only 11 kcal/mol. As this dihedral decreases beyond 30°, the
excitation energies decrease further. The oscillator strengths do,
as well, but only to a point. Hence, the lower-energy distortions
of τ(OCOO) are sufficient to produce structures that exhibit
excitation energies and oscillator strengths that would red-shift
observed spectra of carbonic acid ices away from the highest UV absorption
feature at 139 nm. Such data imply that colder temperatures (20 K)
in the experimental treatment of carbonic acid ices are freezing these
structures out after annealing, whereas the warmer temperature experiments
(80 K) are not.
A readily accessible (less than four synthetic steps) dye molecule with potential properties well-beyond the current state-of-the-art for use in dye-sensitized solar cells (DSCs) is realized from extensive quantum chemical...
Functionalizing deprotonated polycyclic aromatic hydrocarbon (PAH) anion derivatives gives rise to electronically excited states in the resulting anions. While functionalization with −OH and −C2H, done presently, does not result in the richness of electronically excited states as it does with −CN done previously, the presence of dipole-bound excited states and even some valence excited states are predicted in this quantum chemical analysis. Most notably, the more electron withdrawing −C2H group leads to valence excited states once the number of rings in the molecule reaches three. Dipole-bound excited states arise when the dipole moment of the corresponding neutral radical is large enough (likely around 2.0 D), and this is most pronounced when the hydrogen atom is removed from the functional group itself regardless of whether functionalized by a hydroxyl or enthynyl group. Deprotonatation of the hydroxyl group in the PAH creates a ketone with a delocalized highest occupied molecular orbital (HOMO) unlike deprotonation of a hydrogen on the ring where a localized lone pair on one of the carbon atoms serves as the HOMO. As a result, hydroxyl functionlization and subsequent deprotonation of PAHs creates molecules that begin to exhibit structures akin to nucleic acids. However, the electron withdrawing −C2H has more excited states than the electron donating −OH functionalized PAH. This implies that the −C2H electron withdrawing group can absorb a larger energy range of photons, which signifies an increasing likelihood of being stabilized in the harsh conditions of the interstellar medium.
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