24 research outputs found
Wavelength-optimized two-photon polymerization using initiators based on multipolar aminostyryl-1,3,5-triazines
Two-photon induced polymerization (2PP) based 3D printing is a powerful microfabrication tool. Specialized two-photon initiators (2PIs) are critical components of the employed photosensitive polymerizable formulations. This work investigates the cooperative enhancement of two-photon absorption cross sections (Ï2PA) in a series of 1,3,5-triazine-derivatives bearing 1-3 aminostyryl-donor arms, creating dipolar, quadrupolar and octupolar push-pull systems. The multipolar 2PIs were successfully prepared and characterized, Ï2PA were determined using z-scan at 800ânm as well as spectrally resolved two-photon excited fluorescence measurements, and the results were compared to high-level ab initio computations. Modern tunable femtosecond lasers allow 2PP-processing at optimum wavelengths tailored to the absorption behavior of the 2PI. 2PP structuring tests revealed that while performance at 800ânm is similar, at their respective Ï2PA-maxima the octupolar triazine-derivative outperforms a well-established ketone-based quadrupolar reference 2PI, with significantly lower fabrication threshold at exceedingly high writing speeds up to 200âmm/s and a broader window for ideal processing parameters
SoluteâSolvent Interactions and Excited-State Symmetry Breaking: Beyond the DipoleâDipole and the Hydrogen-Bond Interactions
Symmetry breaking
of the excited state of a linear quadrupolar
acceptorâdonorâacceptor molecule was investigated using
time-resolved infrared spectroscopy in 55 solvents allowing the influence
of several soluteâsolvent interactions to be examined separately.
No symmetry breaking was found in nonpolar solvents irrespective of
their refractive index, indicating that differences in dispersion
interactions between the two arms of the molecule do not suffice to
induce an asymmetric distribution of the excitation. However, symmetry
breaking was observed in nondipolar but quadrupolar solvents like
benzene to an extent that can be as large as that found in medium
dipolar solvents like THF. Whereas larger symmetry breaking occurs
in the most dipolar solvents, the strongest are observed in protic
solvents due to hydrogen bonding. Strong evidence of the formation
of halogen bonds in the excited state is also presented, confirming
the idea of symmetry-breaking-induced asymmetrical photochemistry
More than a Solvent: DonorâAcceptor Complexes of Ionic Liquids and Electron Acceptors
The
applicability of room-temperature ionic liquids (RTILs) as
inert solvents is generally based on their electrochemical window.
We herein show that this concept has its limitations if RTILs are
exposed to an oxidizing environment in the presence of light. Acetonitrile
solutions of RTILs with 1-methyl-3-ethylimidazolium as cation and
five different anions, including thiocyanate (SCN<sup>â</sup>) and dicyanamide (DCA<sup>â</sup>), were investigated. Upon
addition of organic electron acceptors to solutions of RTILs with
SCN<sup>â</sup> or DCA<sup>â</sup>, charge-transfer
(CT) absorption bands due to the formation of donorâacceptor
complexes between the anion and the electron acceptor were observed.
Time-resolved measurements from the femtosecond to the microsecond
regimes were used to investigate the nature and the excited-state
dynamics of these complexes upon excitation in the CT band. We show
that even though the RTILs are seemingly inert according to their
electrochemical properties, the dicyanamide and thiocyanate based
RTILs can actively participate in photochemical reactions in oxidizing
environments and therefore differ from the behavior expected for an
inert solvent. This has not only important implications for the long-term
stability of RTIL-based systems but can also lead to misinterpretation
of photochemical studies in these solvents
Driving Force Dependence of Charge Recombination in Reactive and Nonreactive Solvents
This study addresses the free energy dependence of charge
recombination following photoinduced bimolecular electron transfer
in three different solvents of either inert (acetonitrile and benzyl
acetate) or reactive (<i>N</i>,<i>N</i>-dimethylaniline)
character. Femtosecond time-resolved fluorescence and transient absorption
have been used to determine the time scales for charge recombination.
In pure <i>N</i>,<i>N</i>-dimethylaniline, charge
recombination is found to be substantially slower than charge separation
in a range of driving forces covering 1.5 eV. In all three solvents,
the so-called Marcus inverted region is clearly observed for charge
recombination. Additionally, the charge recombination step is found
to be influenced by the solvent relaxation dynamics. A diffusion-reaction
equation approach using an electron transfer model accounting for
solvent relaxation is used to rationalize the experimental results
Bimolecular Photoinduced Electron Transfer Beyond the Diffusion Limit: The RehmâWeller Experiment Revisited with Femtosecond Time Resolution
To
access the intrinsic, diffusion free, rate constant of bimolecular
photoinduced electron transfer reactions, fluorescence quenching experiments
have been performed with 14 donor/acceptor pairs, covering a driving-force
range going from 0.6 to 2.4 eV, using steady-state and femtosecond
time-resolved emission, and applying a diffusion-reaction model that
accounts for the static and transient stages of the quenching for
the analysis. The intrinsic electron transfer rate constants are up
to 2 orders of magnitude larger than the diffusion rate constant in
acetonitrile. Above âŒ1.5 eV, a slight decrease of the rate
constant is observed, pointing to a much weaker Marcus inverted region
than those reported for other types of electron transfer reactions,
such as charge recombination. Despite this, the driving force dependence
can be rationalized in terms of Marcus theory
Excited-State Dynamics of Wursterâs Salts
The excited-state dynamics of a series of Wursterâs
salts
(<i>p</i>-phenylenediamine radical cations) with different
subtituents on the nitrogen atoms was investigated under a variety
of experimental conditions using a combination of ultrafast spectroscopic
techniques. At room temperature, the lifetime of the lowest excited
state of all radical cations is on the order of 200 fs, independently
of the solvent, that is, water, nitriles, alcohols, and room-temperature
ionic liquid. On the other hand, all cations, except that with the
bulky nitrogen substituents, become fluorescent below 120 K. The observed
dynamics can be accounted for by the presence of a conical intersection
between the D<sub>1</sub> and D<sub>0</sub> states. For the cations
with a small nitrogen substituent, this conical intersection could
be accessed through a twist of one amino group, as already suggested
for Wursterâs Blue. However, this coordinate cannot be invoked
for the cation with bulky nitrogen subtituents, and more probably,
pyramidalization of the nitrogen center and/or deformation of the
phenyl ring play an important role. Consequently, the excited-state
dynamics of these structurally very similar Wursterâs salts
involves different decay mechanisms
Ultrafast Long-Distance Excitation Energy Transport in DonorâBridgeâAcceptor Systems
The excited-state dynamics of two energy donorâbridgeâacceptor
(DâBâA) systems consisting of a zinc tetraphenylporphyrin
(ZnP) and a free base tetraphenylporphyrin (FbP) bridged by oligo-<i>p</i>-phenyleneethynylene units with different substituents
has been investigated using ultrafast spectroscopy. These systems
differ by the location of the lowest singlet excited state of the
bridge, just above or below the S<sub>2</sub> porphyrin states. In
the first case, Soret band excitation of the porphyrins is followed
by internal conversion to the local S<sub>1</sub> state of both molecules
and by a S<sub>1</sub> energy transfer from the ZnP to the FbP end
on the 10 ns time scale, as expected for a center-to-center distance
of about 4.7 nm. On the other hand, if the bridge is excited, the
energy is efficiently transferred within 1 ps to both porphyrin ends.
Selective bridge excitation is not possible with the second system,
because of the overlap of the absorption bands. However, the time-resolved
spectroscopic data suggest a reversible conversion between the D*Â(S<sub>2</sub>)âBâA and DâB*Â(S<sub>1</sub>)âA
states as well as a transition from the DâB*Â(S<sub>1</sub>)âA
to the DâBâA* states on the picosecond time scale. This
implies that the local S<sub>2</sub> energy of the ZnP end can be
transported stepwise to the FbP end, i.e., over about 4.7 nm, within
1 ps with an efficiency of more than 0.2
Photoinduced Electron Transfer between Dipolar Reactants: Solvent and Excitation Wavelength Effects
Electron transfer (ET) quenching
in nonpolar media is not as well
understood as in polar environments. Here, we investigate the effect
of dipoleâdipole interactions between the reactants using ultrafast
broadband electronic spectroscopy combined with molecular dynamics
simulations. We find that the quenching of the S1 state
of two polar dyes, coumarin 152a and Nile red, by the polar N,N-dimethylaniline (DMA) in cyclohexane
is faster by a factor up to 3 when exciting on the red edge rather
than at the maximum of their S1 â S0 absorption
band. This originates from the inhomogeneous broadening of the band
due to a distribution of the number of quencher molecules around the
dyes. As a consequence, red-edge excitation photoselects dyes in a
DMA-rich environment. Such broadening is not present in acetonitrile,
and no excitation wavelength dependence of the ET dynamics is observed.
The quenching of both dyes is markedly faster in nonpolar than polar
solvents, independently of the excitation wavelength. According to
molecular dynamics simulations, this is due to the preferential solvation
of the dyes by DMA in cyclohexane. The opposite preferential solvation
is predicted in acetonitrile. Consequently, close contact between
the reactants in acetonitrile requires partial desolvation. By contrast,
the recombination of the quenching product is slower in nonpolar than
in polar solvents and exhibits much smaller dependence, if any, on
the excitation wavelength
Excited-State Dynamics of Rhodamine 6G in Aqueous Solution and at the Dodecane/Water Interface
The
excited-state dynamics of rhodamine 6G (R6G) has been investigated
in aqueous solution using ultrafast transient absorption spectroscopy
and at the dodecane/water interface using the femtosecond time-resolved
surface second harmonic generation (SSHG) technique. As the R6G concentration
exceeds ca. 1 mM in bulk water, both R6G monomers and aggregates are
excited to a different extent when using pump pulses at 500 and 530
nm. The excited-state lifetime of the monomers is shortened compared
to dilute solutions because of the occurrence of excitation energy
transfer to the aggregates, which themselves decay nonradiatively
to the ground state with a ca. 70 ps time constant. At the dodecane/water
interface, both monomers and aggregates contribute to the SSHG signal
to an extent that depends on the bulk concentration, the pump and
probe wavelengths, and the polarization of probe and signal beams.
The excited-state lifetime of the monomers at the interface is of
the order of a few picoseconds even at bulk concentrations where it
is as large as several nanoseconds. This is explained by the relatively
high interfacial affinity of R6G that leads to a large interfacial
concentration, favoring aggregation and thus rapid excitation energy
transfer from monomers to aggregates
Influence of Solvent Relaxation on Ultrafast Excited-State Proton Transfer to Solvent
A thorough understanding
of the microscopic mechanism of excited-state
proton transfer (ESPT) and the influence of the solvent environment
on its dynamics are of great fundamental interest. We present here
a detailed investigation of an ESPT to solvent (DMSO) using time-resolved
broadband fluorescence and transient absorption spectroscopies. All
excited-state species are resolved spectrally and kinetically using
a global target analysis based on the two-step Eigen-Weller model.
Reversibility of the initial short-range proton transfer producing
excited contact ion pairs (CIP*) is observed unambiguously in fluorescence
and must be explicitly considered to obtain the individual rate constants.
Close inspection of the early dynamics suggests that the relative
populations of the protonated form (ROH*) and CIP* are governed by
solvent relaxation that influences the relative energies of the excited
states. This constitutes a breakdown of the Eigen-Weller model, although
the overall agreement between the data and the analysis using classical
rate equations is excellent