12 research outputs found
Picosecond Control of Photogenerated Radical Pair Lifetimes Using a Stable Third Radical
Photoinduced electron transfer reactions
in organic donorāacceptor
systems leading to long-lived radical ion pairs (RPs) have attracted
broad interest for their potential applications in fields as diverse
as solar energy conversion and spintronics. We present the photophysics
and spin dynamics of an electron donor ā electron acceptor
ā stable radical system consisting of a <i>meta</i>-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide
(ANI) electron-accepting chromophore as well as an Ī±,Ī³-bisdiphenylene-Ī²-phenylallyl
(BDPA) stable radical. Selective photoexcitation of ANI produces the
BDPAāmPD<sup>+ā¢</sup>āANI<sup>āā¢</sup> triradical in which the mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup> RP spins are strongly exchange coupled. The presence of BDPA is
found to greatly increase the RP intersystem crossing rate from the
initially photogenerated BDPAā<sup>1</sup>(mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup>) to BDPAā<sup>3</sup>(mPD<sup>+ā¢</sup>āANI<sup>āā¢</sup>),
resulting in accelerated RP recombination via the triplet channel
to produce BDPAāmPDā<sup>3*</sup>ANI as compared to
a reference molecule lacking the BDPA radical. The RP recombination
rates observed are much faster than those previously reported for
weakly coupled triradical systems. Time-resolved EPR spectroscopy
shows that this process is also associated with strong spin polarization
of the stable radical. Overall, these results show that RP intersystem
crossing rates can be strongly influenced by stable radicals nearby
strongly coupled RP systems, making it possible to use a third spin
to control RP lifetimes down to a picosecond time scale
Effect of Perylene Photosensitizer Attachment to [Pd(triphosphine)L]<sup>2+</sup> on CO<sub>2</sub> Electrocatalysis
Two new covalently
linked chromophoreāCO<sub>2</sub> reduction catalyst systems
were prepared using a perylene chromophore and a bisĀ[(dicyclohexylphosphino)Āethyl]ĀphenylphosphinopalladiumĀ(II)
catalyst. The primary goal of this study is to probe the influence
of photosensitizer attachment on the electrocatalytic performance.
The position either para or meta to the phosphorus on the phenyl group
of the palladium complex was linked via a 2,5-xylyl group to the 3
position of perylene. The electrocatalytic CO<sub>2</sub> reduction
activity of the palladium complex is maintained in the meta-linked
system, but is lost in the para-linked system, possibly because of
unfavorable interactions of the perylene chromophore with the glassy
carbon electrode used. Following selective photoexcitation of the
perylene, an enhanced perylene excited-state decay rate was observed
in the palladium complexes compared to perylene attached to the free
ligands. This decrease is accompanied by formation of the perylene
cation radical, showing that electron transfer from perylene to the
palladium catalyst occurs. Electron transfer and charge recombination
were both found to be faster in the para-linked system than in the
meta-linked one, which is attributed to stronger electronic coupling
in the former. These results illustrate the need to carefully tune
the electronic coupling between a photosensitizer chromophore and
the catalyst to promote photodriven electron transfer yet inhibit
adverse electronic effects of the chromophore on electrocatalysis
Carrier Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> Films
The
dynamics of photoexcited lead-free perovskite films, CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub>, were studied using broadband
transient absorption and time-resolved fluorescence spectroscopy.
Similar to its lead analogue CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, we show that free carrier (electrons and holes) recombination is
also the dominant relaxation pathway in CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> films. The slow hot carrier relaxation time is 0.5
ps. Long carrier diffusion lengths for electrons (279 Ā± 88 nm)
and holes (193 Ā± 46 nm) were obtained from fluorescence quenching
measurements. We also show that SnF<sub>2</sub> doping in the CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> film effectively increases the
fluorescence lifetime up to 10 times and gives diffusion lengths exceeding
500 nm. These results suggest that the photophysics of CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3</sub> perovskite are as favorable as those
of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>, demonstrating that
it is a promising nontoxic lead-free replacement for lead iodide perovskite-based
solar cells
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>+ā¢</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>+ā¢</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>ā² (Ļ < 0.3 ps), followed
by relaxation to D<sub>1</sub> (Ļ = 10.9 Ā± 0.4 ps), which
finally decays to D<sub>0</sub> (Ļ = 32.3 Ā± 0.8 ps). D<sub>1</sub>ā² can also be populated directly using a lower energy
900 nm laser pulse, which results in a longer D<sub>1</sub>ā²āD<sub>1</sub> relaxation time (Ļ = 19 Ā± 2 ps). To probe the
oxidative power of PTZ<sup>+ā¢</sup> photoexcited doublet states,
PTZ<sup>+ā¢</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>ā², or D<sub>1</sub> of PTZ<sup>+ā¢</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>+ā¢</sup> will enable applications using this chromophore as a super-oxidant
for energy-demanding reactions
Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers
The
use of multiple chromophores as photosensitizers for catalysts
involved in energy-demanding redox reactions is often complicated
by electronic interactions between the chromophores. These interchromophore
interactions can lead to processes, such as excimer formation and
symmetry-breaking charge separation (SB-CS), that compete with efficient
electron transfer to or from the catalyst. Here, two dimers of perylene
bound either directly or through a xylyl spacer to a xanthene backbone
were synthesized to probe the effects of interchromophore electronic
coupling on excimer formation and SB-CS using ultrafast transient
absorption spectroscopy. Two time constants for excimer formation
in the 1ā25 ps range were observed in each dimer due to the
presence of rotational isomers having different degrees of interchromophore
coupling. In highly polar acetonitrile, SB-CS competes with excimer
formation in the more weakly coupled isomers followed by charge recombination
with Ļ<sub>CR</sub> = 72ā85 ps to yield the excimer.
The results of this study of perylene molecular dimers can inform
the design of chromophoreācatalyst systems for solar fuel production
that utilize multiple perylene chromophores
Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy
Facile
exciton transport within ordered assemblies of Ļ-stacked
chromophores is essential for developing molecular photonic and electronic
materials. Excimer states having variable charge transfer (CT) character
are frequently implicated as promoting or inhibiting exciton mobility
in such systems. However, determining the degree of CT character in
excimers as a function of their structure has proven challenging.
Herein, we report on a series of cyclophanes in which the interplanar
distance between two phenyl-extended viologen (<b>ExV</b><sup><b>2+</b></sup>) chromophores is varied systematically using
a pair of <i>o</i>-, <i>m</i>-, or <i>p</i>-xylylene (<i><b>o</b></i>-, <i><b>m</b></i>-, or <i><b>p</b></i><b>-Xy</b>) covalent
linkers to produce <i><b>o</b></i><b>-ExBox</b><sup><b>4+</b></sup> (3.5 Ć
), <i><b>m</b></i><b>-ExBox</b><sup><b>4+</b></sup> (5.6 Ć
), and <i><b>p</b></i><b>-ExBox</b><sup><b>4+</b></sup> (7.0 Ć
), respectively. The cyclophane structures are characterized
using NMR spectroscopy in solution and single-crystal X-ray diffraction
in the solid state. Femtosecond transient mid-IR and stimulated Raman
spectroscopies show that the CT contribution to the excimer states
formed in <i><b>o</b></i><b>-ExBox</b><sup><b>4+</b></sup> and <i><b>m</b></i><b>-ExBox</b><sup><b>4+</b></sup> depends on the distance between the chromophores
within the cyclophanes, while in the weak interaction limit, as represented
by <i><b>p</b></i><b>-ExBox</b><sup><b>4+</b></sup> (7.0 Ć
), the lowest excited singlet state of <b>ExV</b><sup><b>2+</b></sup> exclusively photo-oxidizes the <i><b>p</b></i><b>-Xy</b> spacer to give the <i><b>p</b></i><b>-Xy</b><sup><b>+ā¢</b></sup>-<b>ExV</b><sup><b>+ā¢</b></sup> ion pair.
Moreover, the vibrational spectra of the excimer state show that it
assumes a geometry that is intermediate between that of the locally
excited and CT states, approximately reflecting the degree of CT character
Quantitative Determination of the Differential Raman Scattering Cross Sections of Glucose by Femtosecond Stimulated Raman Scattering
Femtosecond stimulated
Raman spectroscopy (FSRS) is a vibrational
spectroscopy technique that has been used in a wide variety of applications:
from transient vibrational signature tracking to amplifying weak normal
Raman scattering signals. Presented here is an application of FSRS
to quantify the differential Raman scattering cross sections (DRSCs)
of glucose. In using FSRS to determine the DRSCs of multiple glucose
vibrational modes, we demonstrate the applicability of both stimulated
Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS.
Using the two analogous FSRS techniques, SRG and SRL, we determine
that the DRSCs of glucose excited at 514.5 nm range from a low of
5.0 Ā± 1.1 Ć 10<sup>ā30</sup> to a high of 8.9 Ā±
0.9 Ć 10<sup>ā30</sup> cm<sup>2</sup> molecule<sup>ā1</sup> sr<sup>ā1</sup>. This work establishes both the compatibility
of SRL for measuring DRSCs and values for the DRSC of multiple vibrational
modes of glucose
Solvent-Templated Folding of Perylene Bisimide Macrocycles into Coiled Double-String Ropes with Solvent-Sensitive Optical Signatures
A series
of semirigid perylene bisimide (PBI) macrocycles with
varied ring size containing two to nine PBI chromophores were synthesized
in a one-pot reaction and their photophysical properties characterized
by fluorescence, steady-state, and transient absorption spectroscopy
as well as femtosecond stimulated Raman spectroscopy. These macrocycles
show solvent-dependent conformational equilibria and excited-state
properties. In dichloromethane, the macrocycles prevail in wide-stretched
conformations and upon photoexcitation exhibit symmetry-breaking charge
separation followed by charge recombination to triplet states, which
photosensitize singlet oxygen formation. In contrast, in aromatic
solvents folding of the macrocycles with a distinct oddāeven
effect regarding the number of PBI chromophore units was observed
in steady-state and time-resolved absorption and fluorescence spectroscopy
as well as femtosecond stimulated Raman spectroscopy. These distinctive
optical properties are attributable to the folding of the even-membered
macrocycles into exciton-vibrational coupled dimer pairs in aromatic
solvents. Studies in a variety of aromatic solvents indicate that
these solvents embed between PBI dimer pairs and accordingly template
the folding of even-membered PBI macrocycles into ropelike folded
conformations that give rise to solvent-specific exciton-vibrational
couplings in UVāvis absorption spectra. As a consequence of
the embedding of solvent molecules in the coiled double-string rope
architecture, highly solvent specific intensity ratios are observed
for the two lowest-energy exciton-vibrational bands, enabling assignment
of the respective solvent simply based on the absorption spectra measured
for the tetramer macrocycle
Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores
Facilitating photoinduced electron transfer (PET) while
minimizing
rapid charge-recombination processes to produce a long-lived charge-separated
(CS) state represents a primary challenge associated with achieving
efficient solar fuel production. Natural photosynthetic systems employ
intermolecular interactions to arrange the electron-transfer relay
in reaction centers and promote a directional flow of electrons. This
work explores a similar tactic through the synthesis and ground- and
excited-state characterization of two Cu(I)bis(phenanthroline) chromophores
with homoleptic and heteroleptic coordination geometries and which
are functionalized with negatively charged sulfonate groups. The addition
of sulfonate groups enables solubility in pure water, and it also
induces assembly with the dicationic electron acceptor methyl viologen
(MV2+) via bimolecular, dynamic electrostatic interactions.
The effect of the sulfonate groups on the ground- and excited-state
properties was evaluated by comparison with the unsulfonated analogues
in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated
complexes are similar to what we expect from previous literature,
with the exception of the sulfonated heteroleptic complex whose metal-to-ligand
charge-transfer (MLCT) lifetime in water has two components that are
fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced
MV+ā¢ in both solvent environments following MLCT
excitation, but control measurements in 1:1 acetonitrile/water with
the unsulfonated analogues showed no PET to MV2+, indicating
that electrostatically driven supramolecular assemblies of the sulfonated
complexes with MV2+ facilitate the observed PET. Additionally,
the strength of the intermolecular interactions driving the formation
of these assemblies changes drastically with the solvent environment.
In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes
with quantum yields (Ī¦ET) of 2ā3% but increased
to a remarkable 98% for the sulfonated heteroleptic complex with a
3 Ī¼s CS-state lifetime in water
Spin-Selective Photoreduction of a Stable Radical within a Covalent DonorāAcceptorāRadical Triad
Controlling
spināspin interactions in multispin molecular
assemblies is important for developing new approaches to quantum information
processing. In this work, a covalent electron donorāacceptorāradical
triad is used to probe spin-selective reduction of the stable radical
to its diamagnetic anion. The molecule consists of a perylene electron
donor chromophore (D) bound to a pyromellitimide acceptor (A), which
is, in turn, linked to a stable Ī±,Ī³-bisdiphenylene-Ī²-phenylallyl
radical (R<sup>ā¢</sup>) to produce D-A-R<sup>ā¢</sup>. Selective photoexcitation of D within D-A-R<sup>ā¢</sup> results
in ultrafast electron transfer to form the D<sup>+ā¢</sup>-A<sup>āā¢</sup>-R<sup>ā¢</sup> triradical, where D<sup>+ā¢</sup>-A<sup>āā¢</sup> is a singlet spin-correlated
radical pair (SCRP), in which both SCRP spins are uncorrelated relative
to the R<sup>ā¢</sup> spin. Subsequent ultrafast electron transfer
within the triradical forms D<sup>+ā¢</sup>-A-R<sup>ā</sup>, but its yield is controlled by spin statistics of the uncorrelated
A<sup>āā¢</sup>-R<sup>ā¢</sup> radical pair, where
the initial charge separation yields a 3:1 statistical mixture of
D<sup>+ā¢</sup>-<sup>3</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>) and D<sup>+ā¢</sup>-<sup>1</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>), and subsequent reduction of R<sup>ā¢</sup> only occurs in D<sup>+ā¢</sup>-<sup>1</sup>(A<sup>āā¢</sup>-R<sup>ā¢</sup>). These findings inform the design of multispin
systems to transfer spin coherence between molecules targeting quantum
information processing using the agency of SCRPs