Selective Photoexcitation Research Articles

Step-by-step self-assembled hybrids that feature control over energy and charge transfer

In the current work, we have documented the use of two complementary supramolecular motifs, namely multipoint hydrogen bonding and metal complexation, as a means to control the step-by-step assembly of a panchromatically absorbing and highly versatile solar energy conversion system. On one hand, two different perylenediimides (1a/1b) have been integrated together with a metalloporphyrin (2) by means of the Hamilton receptor/cyanuric acid hydrogen bonding motif into energy transduction systems 1a•2 or 1b•2. Steady-state and time-resolved measurements corroborated that upon selective photoexcitation of the perylenediimides (1a/1b), an energy transfer evolved from the singlet excited state of the perylenediimides (1a/1b) to that of the metalloporphyrin (2). On the other hand, fullerene (3) and metalloporphyrin (2) form the electron donor-acceptor system 2•3 via axial complexation. Photophysical measurements confirm that an electron transfer prevails from the singlet excited state of 2 to the electron-accepting 3. The correspondingly formed radical ion pair state decays with a lifetime of 1.0 ± 0.1 ns. As a complement to the aforementioned, the energy transduction features of 1a•2 were combined with the electron donor-acceptor characteristics of 2•3 to afford 1a•2•3. To this end, time-resolved measurements reveal that the initially occurring energy-transfer interaction (53 ± 3 ps) between 1a/1b and 2 is followed by an electron transfer (12 ± 1 ps) from 2 to 3. From multiwavelength analyses, the lifetime of the radical ion pair state in 1a•2•3-as a product of a cascade of light-induced energy and electron transfer-was derived as 3.8 ± 0.2 ns.

Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+•)-An(-•), and An(-•) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+•)-An(-•)-FN(n)-NI → DMJ(+•)-An-FN(n)(-•)-NI → DMJ(+•)-An-FN(n)-NI(-•). The charge-shift reaction from An(-•) to NI(-•) exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å(-1), which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN(n)(-•) radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D(+•)-B(-•)-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.

Luminescence and electronic excitations in Li6Gd(BO3)3: Ce3+ crystals

This paper reports on a study of the luminescence emitted by Li6Gd(BO3)3: Ce3+ crystals under selective photoexcitation to lower excited states of the host ion Gd3+ and impurity ion Ce3+ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd3+ ions and vibrational energy relaxation between the 6Ij and 6Pj levels. It has been shown that radiative transitions in Ce3+ ions from the lower excited state 5d1 to 2F5/2 and 2F7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.

Electron Transfer within Self-Assembling Cyclic Tetramers Using Chlorophyll-Based Donor–Acceptor Building Blocks

The synthesis and photoinduced charge transfer properties of a series of Chl-based donor-acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll a was converted into zinc methyl 3-ethylpyrochlorophyllide a (Chl) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis(dicarboximide) (NDI) secondary electron acceptors to give Chl-PI-NDI and Chl-PI-NDI(2). The pyridine ligand within each ambident triad enables intermolecular Chl metal-ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene-1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from (1*)Chl to PI to form Chl(+•)-PI(-•)-NDI and Chl(+•)-PI(-•)-NDI(2). This initial charge separation is followed by a rapid charge shift from PI(-•) to NDI and subsequent charge recombination of Chl(+•)-PI-NDI(-•) and Chl(+•)-PI-(NDI)NDI(-•) on a 5-30 ns time scale. Charge recombination in the Chl-PI-NDI(2) cyclic tetramer (τ(CR) = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τ(CR) = 10 ± 1 ns in toluene-1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems.

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K2Al2B2O7 (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21eV) measured at 7K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5eV were detected and assigned to charge-transfer absorption from O2− to Fe3+ ions; the intrinsic PL band at 3.28eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54eV were separated; the strong PL band at 1.72eV was revealed and attributed to a radiative transition in Fe3+ ion.

Hierarchical Assembly of Nanostructures to Decouple Fluorescence and Photothermal Effect

Hybrid nanoparticles are designed and produced by a two-step procedure with the aim to obtain optically controlled multifunctional nanomaterials. In particular, using a sol–gel method in alcoholic/water media, silica particles doped with 9-aminoacridine molecules are prepared with a mean diameter of 31 nm, which preserves the fluorescent properties of the dye. In a second step, these nanoparticles are capped with a thin (7 nm-size) gold shell whose growth does not quench the emission of the dye as proven by steady-state and time-resolved fluorescence measurements. The careful choice of the organic dye and the control of the metal layer growth make possible to completely uncouple the fluorescence and the plasmon bands of gold. The selective photoexcitation of fluorescence or plasmon absorption, leading to heat release, has been tested on phospholipidic membranes loaded with the prepared hybrid particles. Under 400-nm irradiation fluorescence is activated, which is used to image the membranes; upon 650-nm i...

Photoinduced Singlet Charge Transfer in a Ruthenium(II) Perylene-3,4:9,10-bis(dicarboximide) Complex

Elucidation of photoinduced charge transfer behavior in organic dye/metal hybrids is important for developing photocatalytic systems for solar energy conversion. We report the synthesis and photophysical characterization of a perylene-3,4:9,10-bis(dicarboximide) (PDI)-ruthenium(II) complex, bis-PDI-2,2'-bipyridineRu(II)Cl(2)(CN(t)butyl)(2), which has favorable energetics, ΔG(CS) ≈ -1.0 eV, for singlet electron transfer from the Ru complex to PDI. Time-resolved optical spectroscopy reveals that upon selective photoexcitation of PDI, ultrafast charge transfer (<150 fs) from the Ru complex to (1*)PDI generates the Ru(III)-PDI(-•) ion pair. The resulting vibrationally hot Ru(III)-PDI(-•) ion pair exhibits fast relaxation (τ = 3.9 ps) and charge recombination (τ(CR) = 63 ps). Our experimental and computational (DFT and TDDFT) studies show that energy-preserving photodriven singlet electron transfer can dominate in properly designed organic dye/metal complexes, making them of particular interest for use in artificial photosynthetic systems for solar fuels formation.

Optical and Structural Properties of Zn–Cd–Mn–Se Double Quantum Well Systems

Double quantum well (DQW) structures consisting of a ZnCdSe well and a ZnCdMnSe well separated by a ZnSe barrier are grown with molecular beam epitaxy (MBE). The DQW structures are characterized by using X-ray diffraction measurement and simulation. Thickness of each well layer is designed so that the lowest energy level of ZnCdMnSe well is close to the excited level of the ZnCdSe well. Optical properties of the DQWs are studied with photoluminescence (PL) and reflection spectra in external magnetic fields up to 8 T in the Faraday geometry. Exciton transfer from ZnCdMnSe well to ZnCdSe well is observed in magneto PL with energy selective photoexcitation. Exciton energies in ground and excited states are estimated from PL excitation spectra and reflection spectra.

Photoinitiated multistep charge separation in ferrocene–zinc porphyrin–diiron hydrogenase model complex triads

Two covalently linked linear electron donor–acceptor triads Fc-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (1) and Fc-Ph-ZnTPP-[NMI-FeI-FeI-S2(CO)6] (2) consisting of a zinc meso-tetraphenylporphyrin (ZnTPP) chromophore, a naphthalene monoimide diiron hydrogenase active site model [NMI-FeI-FeI-S2(CO)6], and a ferrocene (Fc) secondary electron donor have been synthesized along with their corresponding dyad reference molecules ZnTPP-[NMI-FeI-FeI-S2(CO)6] (3), Fc-ZnTPP (4), and Fc-Ph-ZnTPP (5). Time-resolved transient absorption and emission studies in CH2Cl2 show that selective photoexcitation of ZnTPP in triads 1 and 2 results in two competing quenching pathways for 1*ZnTPP: electron transfer from 1*ZnTPP to [NMI-FeI-FeI-S2(CO)6] and energy transfer from 1*ZnTPP to low-lying Fc excited states. Our studies on reference dyads 4 and 5 show that the majority of 1*ZnTPP produced by the laser pulse decays rapidly by energy transfer to Fc in triad 1 (τ < 10 ps), while electron transfer to [NMI-FeI-FeI-S2(CO)6] dominates in triad 2, allowing the second rapid electron transfer step from Fc to ZnTPP+˙ to proceed. Quantum yields of the fully charge separated states Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] and Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] are 0.13 and 0.71, respectively. Charge recombination in Fc+-ZnTPP-[NMI-Fe0-FeI-S2(CO)6] occurs with τCR = 9 ± 1 ns and τCR = 67 ± 2 ns for Fc+-Ph-ZnTPP-[NMI-Fe0-FeI-S2(CO)6]. By incorporating a secondary electron donor, the lifetime of the reduced diironhydrogenase mimic was extended by a factor of >450. Studies of photochemical hydrogen evolution using 1 and 2 reveal that the hydrogen generation efficiency depends on the lifetime of the final charge separated state. The ability to execute a multi-electron proton-coupled electron transfer mechanism in a stepwise manner will allow us to investigate the structural and electronic requirements for each step aiding in overall system optimization. Thus, it is possible to use the same multi-step electron transfer strategy that has been employed to extend the lifetime of charge-separated states in photodriven donor–acceptor systems to extend the lifetime of the reduced states of metal complexes of potential use in catalytic proton reduction.

Selective photoexcitation of confined exciton-polariton vortices

We resonantly excite exciton-polariton states confined in cylindrical traps. Using a homodyne detection setup, we are able to image the phase and amplitude of the confined polariton states. We evidence the excitation of vortex states, carrying an integer angular orbital momentum $m$, analogous to the transverse $TE{M}_{{01}^{\ensuremath{\ast}}}$ ``donut'' mode of cylindrically symmetric optical resonators. Tuning the excitation conditions allows us to select the charge of the vortex. In this way, the injection of singly charged $(m=1\text{ }\text{and}\text{ }m=\ensuremath{-}1)$ and doubly charged $(m=2)$ polariton vortices is shown. This work demonstrates the potential of in-plane confinement coupled with selective excitation for the topological tailoring of polariton wave functions.

Ultrafast Photodriven Intramolecular Electron Transfer from a Zinc Porphyrin to a Readily Reduced Diiron Hydrogenase Model Complex

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe(2)(mu-R)(CO)(6-n)(L)(n)], where commonly R = alkyl or aryl dithiolate and L = CO, CN(-), or PR(3), are a promising class of catalysts for use in photodriven H(2) production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe(2)S(2)(CO)(6), 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe(2)S(2)(CO)(6) donor-acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (-0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at -0.78 V and a new reduction wave at -1.4 V. As the acid concentration was increased, the current at -0.78 V remained constant while the current at -1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH(2)Cl(2) yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe(2)S(2)(CO)(6)](-*) on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded tau(CS) = 12 +/- 1 ps and tau(CR) = 3.0 +/- 0.2 ns in toluene and tau(CS) = 24 +/- 1 ps and tau(CR) = 57 +/- 1 ps in CH(2)Cl(2). Photoexcitation of the disulfide precursor to 2 in both toluene and CH(2)Cl(2) produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H(2).

Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO−Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical−Triplet Interaction

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3*)PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1*)PDI --> TEMPO(+*)-PDI(-*) occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3*)PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized (3*)PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3*)PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to (3*)PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3*)PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

Electroluminescence induced by photoluminescence excitation in GaInN/GaN light-emitting diodes

Optical emission resulting from 405 nm selective photoexcitation of carriers in the GaInN/GaN quantum well (QW) active region of a light-emitting diode reveals two recombination channels. The first recombination channel is the recombination of photoexcited carriers in the GaInN QWs. The second recombination channel is formed by carriers that leak out of the GaInN QW active region, self-bias the device in forward direction, induce a forward current, and subsequently recombine in the GaInN active region in a spatially distributed manner. The results indicate dynamic carrier transport involving active, confinement, and contact regions of the device.

Photoinduced Electron Transfer in Porphyrin- and Fullerene/Porphyrin-Based Rotaxanes as Studied by Time-Resolved EPR Spectroscopy

Photoinduced intramolecular electron-transfer (ET) and energy-transfer (EnT) processes in two rotaxanes, one containing both zinc porphyrin and C(60) fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)(2)-Cu(I)(phen)(2)-C(60)] and a second complex lacking the fullerene [(ZnP)(2)-Cu(I)(phen)(2)], were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) combined with a selective photoexcitation of the rotaxane moieties. The experiments were carried out in isotropic toluene and ethanol and in anisotropic nematic liquid-crystal (E-7) media over a wide range of temperatures corresponding to the different states of the solvents. The TREPR results are compared with those obtained previously by optical methods in dichloromethane at room temperature. It is demonstrated that the efficiencies and pathways of the light-driven ET and EnT processes in both rotaxanes strongly depend on the properties of their microenvironment, resulting in the formation of distinct charge-separated states under different experimental conditions. The complementary results revealed by the optical and TREPR techniques are attributed to the relatively high conformational mobility of the mechanically interlocked rotaxane systems. Because of the solute-solvent interactions, the rotaxanes are able to change conformation in different microenvironments, which affects the parameters of the photoinduced processes occurring in these systems.

Fluorescence/photoisomerization competition in trans-aza-1,2-diarylethenes

The effect of the presence and position of the nitrogen heteroatom on the photobehaviour of a series of symmetric and asymmetric 1,2-diarylethenes, where one or both the one-, two- or three-membered aryl groups, linked to the ethene bridge, contain(s) nitrogen heteroatoms, has been investigated by fluorimetric and photochemical techniques. The yield of the radiative pathway is generally reduced with respect to the corresponding hydrocarbons, due to a smaller energy barrier for trans-->cis (E-->Z) photoisomerization in the singlet state, which often can be slightly favoured in the aza-compounds. All compounds investigated exist in solution as mixtures of two or more conformers, which can have different spectral and kinetic properties. Attempts to separate the intrinsic properties by selective photoexcitation, at a wavelength where the light is mainly absorbed by one conformer, allowed in some favourable cases the spectra and decay times of the largely prevalent (quasi-pure) conformer to be obtained. In two cases, the formation of intramolecular (hydrogen-bond-type) interactions between the nitrogen atom and the nearest hydrogen atom of the ethene bridge (proved by NMR measurements and confirmed by theoretical calculations) were found to stabilize a specific conformer thus affecting a selective relaxation of the excited molecule.

Guided locally optimal control of quantum dynamics in dissipative environments

We present ideas for controlling quantum dynamics in a dissipative environment using short, intense laser pulses. The proposed algorithm is an extension of the well-established time-local pump-dump control scheme to the density matrix formalism, using time-dependent targets. This provides a natural framework for guiding the reaction along a desired path. As a test case, the selective photoexcitation of charge transfer and semi-charge-transfer states of the LiCN molecule in a strong dissipative environment is studied.

Photoluminescence of Si nanocrystals under selective excitation

Photoluminescence of Si nanocrystals passivated by different alkanes (hexane, octane, and tridecane) was studied at room temperature. It is shown that the emission band shape is not affected by the length of the carbon chain in the alkanes used for passivation. A pronounced fine structure of the photoluminescence band consisting of peaks separated by 150–160 meV was observed under resonant excitation. The structure is interpreted by predominant contribution from Si nanocrystal groups with particular stable size/shape existing in addition to the previously reported nanocrystals with “magic” numbers of Si atoms. The contribution of these stable nanocrystals is revealed using selective resonant photoexcitation to the higher energy states in the discrete energy spectrum of such nanocrystals.

Electric‐Field‐Assisted Acceleration of the Photostimulated Nematic—Isotropic Transition

Photoisomerization is a molecular property in which selective photoexcitation of the molecules causes transformation between isomers that are structurally different. The property has been of tremendous practical implication, for instance, in dye molecules employed in optical data storage devices such as CDs, DVDs, etc. More recently they are being investigated as candidates for molecular devices, like molecular switches, motors, and electronic components. The combination of this property with the self-assembling nature of liquid crystalline systems possessing large birefringence values is valuable from both application and fundamental points of view, especially in photostimulated non-equilibrium phase transitions. Here we report a novel method of an accelerated means of recovering the equilibrium from the photo-driven state. The employed guest-host system consists of the photoactive guest azobenzene and host nonphotoactive molecules exhibiting a nematic mesophase. By irradiating the sample with UV radiation the system is photo-driven from the equilibrium nematic phase to the isotropic phase via an isothermal phase transition. The recovery of the equilibrium phase takes place spontaneously, but is very slow. We demonstrate here that by application of an electric field the recovery occurs at least two orders of magnitude faster. The rapid recovery is associated with the faster reverse isomerization and also observed in the isotropic phase, and therefore is applicable to all systems connected with such isomerization processes. Photonics, in which light can be controlled by light as a stimulus, is a field of growing interest with numerous promising applications. [1‐3] Owing to the large photoinduced birefringence, liquid crystals (LC) are attractive candidates in this field, having promising features for a variety of applications including optical data storage, a topic that has generated a lot of activity. [4‐7] Of particular interest to the work presented in this article is the phenomenon of reversible photo-induced shape transformation of the molecules containing chromophoric azo groups. [8] Upon UV irradiation (around 360 nm,

Photo‐Fries Rearrangement in Polymeric Media: An Investigation on Fully Aromatic Esters Containing the Naphthyl Chromophore

AbstractThe present paper deals with the photo‐Fries rearrangement of fully aromatic esters in polymeric media. Low molecular weight aryl esters bearing the 1‐naphthyl chromophore were investigated with respect to their photochemical reactivity and the yield of hydroxyketones as the photo‐Fries rearrangement product. In particular, the phenyl, 4‐methylphenyl and 1‐naphthyl ester of 1‐naphthoic acid, as well as benzoic acid, 1‐naphthyl ester were dissolved in a polysiloxane matrix and exposed to 313 nm UV light. The progress of the photoreaction was assessed by quantitative FT‐IR spectroscopy. Both 1‐naphthoic acid, phenyl ester and benzoic acid, 1‐naphthyl ester underwent the photoreaction upon 313 nm irradiation. This indicates that in these fully aromatic esters, the initial CO bond scission also occurs upon selective photoexcitation of the acyl unit in the molecule. It was found that phenyl and 4‐methylphenyl esters of 1‐naphthoic acid gave the highest yields of hydroxyketone when irradiated in polymer matrix. Based upon these results, a photoreactive monomer was designed which contains naphthoic acid, phenyl ester units attached to a norbornene ring. This monomer was then polymerized by ring opening metathesis polymerization (ROMP). The polymer showed a high yield in the photo‐Fries reaction upon 313 nm illumination. Moreover, a large increase in the refractive index (Δn up to +0.043 at λ = 370 nm) was observed which makes this polymer interesting for optical applications (e.g., waveguiding and diffractive optics).magnified image

Chloride Anion Controlled Molecular “Switching”. Binding of 2,5,7-Trinitro-9-dicyanomethylenefluorene-C60 by Tetrathiafulvalene Calix[4]pyrrole and Photophysical Generation of Two Different Charge-Separated States

The binding of the snake-like trinitrodicyanomethylenefluorene-C60 derivative (TNDCF-C60) to the dynamic receptor, tetrathiafulvalene calix[4]pyrrole (TTF-calix[4]pyrrole), may be controlled via the use of a chloride anion as an external trigger. Whereas, in the absence of a chloride anion, the TNDCF “tail” of the trinitrodicyanomethylenefluorene-C60 substrate binds to the TTF−calix[4]pyrrole in a 2:1 (substrate/receptor) stoichiometry in CH2Cl2 solution, addition of a chloride anion (yellow) leads the TNDCF tail to be displaced in favor of a bound C60 “head”, a process that leads to the formation of a complex with overall 1:2:2 substrate/receptor/chloride anion stoichiometry. These chemical switching events are reflected in easy-to-visualize color changes, as well as in the production of two different kinds of charge-separated states following selective femtosecond photoexcitation.