8 research outputs found
Photochemically Switching Diamidocarbene Spin States Leads to Reversible Büchner Ring Expansions
The discovery of
thermal and photochemical control by Woodward
and Hoffmann revolutionized how we understand chemical reactivity.
Similarly, we now describe the first example of a carbene that exhibits
differing thermal and photochemical reactivity. When a singlet ground-state <i>N,N</i>′-diamidocarbene <b>1</b> was photolyzed
at 380 nm, excitation to a triplet state was observed. The triplet-state
electronic structure was characteristic of the expected biradical
σ<sup>1</sup>p<sub>π</sub><sup>1</sup> spin configuration
according to a combination of spectroscopic and computational methods.
Surprisingly, the triplet state of <b>1</b> was found to engage
a series of arenes in thermally reversible Büchner ring expansion
reactions, marking the first examples where both cyclopropanation
and ring expansion of arenes were rendered reversible. Not only are
these photochemical reactions different from the known thermal chemistry
of <b>1</b>, but the reversibility enabled us to perform the
first examples of photochemically induced arene exchange/expansion
reactions at a single carbon center
Organosulfonates aid argentophilic forces in the crystal engineering of [2+2] photodimerisations: reactivity involving 3-pyridyl groups
<div><p>Photoreactive Ag(I) organosulfonate complexes involving (<i>E</i>)-methyl-3-(pyridin-3-yl)prop-2-enoate (3-PAMe) are reported. Argentophilic interactions stack 3-PAMe to undergo a head-to-head intermolecular [2+2] photodimerisation to generate (<i>rctt</i>)-dimethyl-3,4-bis(pyridine-3-yl)cyclobutane-1,2-dicarboxylate (3,3′-MeBPCD) in the solid state regioselectively and in quantitative yield. The organosulfonate participates in C–H…O interactions with the coordinated 3-pyridyl groups to effectively aid in crystal engineering of the reactivity.</p></div
Edge-to-Edge C–H···N Hydrogen Bonds in Two-Component Co-crystals Aide a [2 + 2] Photodimerization
C–H···N hydrogen
bonds support an intermolecular
[2 + 2] photodimerization in the solid state. The photocycloaddition
occurs in a co-crystal of composition (<b>tbrb</b>)·(<b>4,4′-bpe</b>) (where <b>tbrb</b> = 1,2,4,5-tetrabromobenzene; <b>4,4′-bpe</b> = <i>trans</i>-1,2-bis(4-pyridyl)ethylene)
that consists of one-dimensional (1D) chains that self-assemble to
form corrugated sheets. Stacking of <b>4,4′-bpe</b> between
sheets conforms to the topochemical postulate of Schmidt. The alkene
reacts to form <i>rctt</i>-tetrakis(4-pyridyl)cyclobutane
(<b>4,4′-tpcb</b>) stereoselectively and in quantitative
yield. C–H···N hydrogen bonds also support the
assembly of the components in (<b>tbrb</b>)·(<b>4,4′-azo</b>) (where: <b>4,4′-azo</b> = 4,4′-azopyridine)
into planar sheets. Density-functional theory calculations reveal
identical face-to-face stacks of <b>tbrb</b> to aide in the
stabilization of the structures of the co-crystals
A solid-state [2 + 2] photodimerization involving coordination of Ag(I) ions to 2-pyridyl groups
<p>Photoreactive Ag(I) complexes of <i>p</i>-toluenesulfonate ions with the unsymmetrical alkene <i>trans</i>-1-(4-acetoxyphenyl)-2-(2-pyridyl)ethylene <b>1</b> is reported. The crystal [Ag(<i>p</i>-tol)(<b>1</b>)<sub>2</sub>]•(H<sub>2</sub>O) (<i>p</i>-tol = <i>p</i>-toluenesulfonate) undergoes a [2 + 2] photocycloaddition reaction in quantitative yield to afford the head-to-tail (ht) photoproduct <i>rctt</i>-1,3-bis(2-pyridyl)-2,4-bis(4-acetoxyphenyl)cyclobutane <b>2</b> regioselectively. The aromatic rings of the olefin participate in face-to-face π–π interactions and adopt an anti-conformation to position the carbon–carbon double bonds (C = C) in a suitable orientation to undergo photoreaction between neighboring complexes.</p
Regiocontrol of the [2 + 2] Photodimerization in the Solid State Using Isosteric Resorcinols: Head-to-Tail Cyclobutane Formation via Unexpected Embraced Assemblies
Regiocontrolled head-to-head and
head-to-tail [2 + 2] photodimerizations
of (<i>E</i>)-methyl-3-(pyridin-3-yl)prop-2-enoate
are achieved in the solid state using 4,6-di-X-res (where X = Cl,
Br, I; res = resorcinol) as small-molecule templates. The components
in each co-crystal form discrete, three-component supramolecular assemblies
sustained by two O–H···N hydrogen bonds. Whereas
the head-to-head photoproduct (<i>rctt</i>)-dimethyl-3,4-bis(pyridin-3-yl)cyclobutane-1,2-dicarboxylate
forms using 4,6-di-X-res (X = Cl or Br), the head-to-tail regioisomer
(<i>rctt</i>)-dimethyl-2,4-bis(pyridin-3-yl)cyclobutane-1,3-dicarboxylate
forms using 4,6-di-I-res. The head-to-tail product is generated via
unexpected embraced dimeric assemblies. The stereochemistry of the
head-to-tail product is confirmed using single-crystal X-ray diffraction
Synthesis, Spectroscopic Properties, and Photoconductivity of Black Absorbers Consisting of Pt(Bipyridine)(Dithiolate) Charge Transfer Complexes in the Presence and Absence of Nitrofluorenone Acceptors
The
diimine–dithiolato ambipolar complexes Pt(dbbpy)(tdt) and Pt(dmecb)(bdt)
(dbbpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine;
tdt<sup>2–</sup> = 3,4-toluenedithiolate; dmecb = 4,4′-dimethoxyester-2,2′-bipyridine;
bdt<sup>2–</sup> = benzene-1,2-dithiolate) are prepared herein.
Pt(dmecb)(bdt) exhibits photoconductivity that remains constant (photocurrent
density of 1.6 mA/cm<sup>2</sup> from a 20 nm thin film) <i>across
the entire visible region of the solar spectrum</i> in a Schottky
diode device structure. Pt(dbbpy)(tdt) acts as donor when combined
with the strong nitrofluorenone acceptors 2,7-dinitro-9-fluorenone
(DNF), 2,4,7-trinitro-9-fluorenone (TRNF), or 2,4,5,7-tetranitro-9-fluorenone
(TENF). Supramolecular charge transfer stacks form and exhibit various
donor–acceptor stacking patterns. The crystalline solids are
“black absorbers” that exhibit continuous absorptions
spanning the entire visible region and significant ultraviolet and
near-infrared wavelengths, the latter including long wavelengths that
the donor or acceptor molecules alone do not absorb. Absorption spectra
reveal the persistence of donor–acceptor interactions in solution,
as characterized by low-energy donor/acceptor charge transfer (DACT)
bands. Crystal structures show closely packed stacks with distances
that underscore intermolecular DACT. <sup>1</sup>H NMR provides further
evidence of DACT, as manifested by upfield shifts of aromatic protons
in the binary adducts versus their free components, whereas 2D nuclear
Overhauser effect spectroscopy (NOESY) spectra suggest coupling between
dithiolate donor protons with nitrofluorenone acceptor protons, in
correlation with the solid-state stacking. The NMR spectra also show
significant peak broadening, indicating some paramagnetism verified
by magnetic susceptibility data. Solid-state absorption spectra reveal
further red shifts and increased relative intensities of DACT bands
for the solid adducts vs solution, suggesting cooperativity of the
DACT phenomenon in the solid state, as further substantiated by ν<sub>C–O</sub> and ν<sub>N–O</sub> IR bands and solid-state
tight-binding computational analysis