4 research outputs found
Ruthenium-Phenothiazine Electron Transfer Dyad with a Photoswitchable Dithienylethene Bridge: Flash-Quench Studies with Methylviologen
A molecular ensemble composed of a phenothiazine (PTZ)
electron
donor, a photoisomerizable dithienylÂethene (DTE) bridge, and
a RuÂ(bpy)<sub>3</sub><sup>2+</sup> (bpy = 2,2âČ-bipyridine)
electron acceptor was synthesized and investigated by optical spectroscopic
and electrochemical means. Our initial intention was to perform flash-quench
transient absorption studies in which the RuÂ(bpy)<sub>3</sub><sup>2+</sup> unit is excited selectively (âflashâ) and
its <sup>3</sup>MLCT excited state is quenched oxidatively (âquenchâ)
by excess methylviologen prior to intramolecular electron transfer
from phenothiazine to RuÂ(III) across the dithienylethene bridge. However,
after selective RuÂ(bpy)<sub>3</sub><sup>2+1</sup>MLCT excitation of
the dyad with the DTE bridge in its open form, <sup>1</sup>MLCT â <sup>3</sup>MLCT intersystem crossing on the metal complex is followed
by tripletâtriplet energy transfer to a <sup>3</sup>ÏâÏ*
state localized on the DTE unit. This energy transfer process is faster
than bimolecular oxidative quenching with methylviologen at the ruthenium
site (RuÂ(III) is not observed); only the triplet-excited DTE then
undergoes rapid (10 ns, instrumentally limited) bimolecular electron
transfer with methylviologen. Subsequently, there is intramolecular
electron transfer with PTZ. The time constant for formation of the
phenothiazine radical cation via intramolecular electron transfer
occurring over two <i>p</i>-xylene units is 41 ns. When
the DTE bridge is photoisomerized to the closed form, PTZ<sup>+</sup> cannot be observed any more. Irrespective of the wavelength at which
the closed isomer is irradiated, most of the excitation energy appears
to be funneled rapidly into a DTE-localized singlet excited state
from which photoisomerization to the open form occurs within picoseconds
Photoswitchable Organic Mixed Valence in Dithienylcyclopentene Systems with Tertiary Amine Redox Centers
The electronic structures of the radical cations of two dithienylperfluorocyclopentene molecules with appended tertiary amine units were investigated by electrochemical and optical spectroscopic methods. The through-bond NâN distances in the photocyclized (closed) forms of the two systems are 9.3 and 17.6 Ă
, respectively, depending on whether the nitrogen atoms are attached directly to the two thienyl units or whether xylyl spacers are in between. In the case of the radical cation with the longer NâN distance, photocyclization of the dithienylperfluorocyclopentene core induces a changeover from class I to class II mixed valence behavior. In the case of the shorter system, the experimental data is consistent with assignment of the photocyclized form to a class III mixed valence species
A TriarylamineâTriarylborane Dyad with a Photochromic Dithienylethene Bridge
A molecular triad composed of a triarylamine donor, a
triarylborane
acceptor, and a photoisomerizable dithienylethene bridge has been
synthesized and explored by cyclic voltammetry, UVâvis, and
luminescence spectroscopy. The effects of irradiation with UV light
and fluoride addition on the electrochemical and optical spectroscopic
properties of the donorâbridgeâacceptor molecule were
investigated. Photoisomerization of the dithienylethene bridge affects
the triarylboron reduction potential, but not the triarylamine oxidation
potential. UVâvis experiments reveal that the association constant
for fluoride binding at the triarylborane site is independent of the
isomerization state of the bridge. Irradiation of a THF solution of
our donorâbridgeâacceptor molecule with UV light, followed
by F<sup>â</sup> addition, leads to a different color of the
sample than UV irradiation alone or F<sup>â</sup> addition
alone
Charge Delocalization in a Homologous Series of α,αâČ-Bis(dianisylamino)-Substituted Thiophene Monocations
A homologous series of three molecules containing thiophene,
bithiophene,
and terthiophene bridges between two redox-active tertiary amino groups
was synthesized and explored. Charge delocalization in the one-electron-oxidized
forms of these molecules was investigated by a combination of cyclic
voltammetry, near-infrared optical absorption spectroscopy, and EPR
spectroscopy. All three cation radicals can be described as organic
mixed-valence species, and for all of them the experimental data are
consistent with strong delocalization of the unpaired electron. Depending
on what model is used for analysis of the optical absorption data,
estimates for the electronic coupling matrix element (<i>H</i><sub><i>AB</i></sub>) range from âŒ5000 to âŒ7000
cm<sup>â1</sup> for the shortest member of the homologous series.
According to optical absorption and EPR spectroscopy, even the terthiophene
radical appears to belong either to RobinâDay class III or
to a category of radicals commonly denominated as borderline class
II/class III systems. The finding of such a large extent of charge
delocalization over up to three adjacent thiophene units is remarkable