Photoinduced Color Change and Photomechanical Effect of Naphthalene Diimides Bearing Alkylamine Moieties in the Solid State

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical‐anion species, which may be formed through a photoinduced electron‐transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed‐valence stacks and radical‐anion stacks, as shown by the broad absorption bands in near‐IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed‐valence stacks, and radical‐anion stacks.     

A matrix isolation study of the C(s) symmetric OCNO(X 2A') radical

Here we report the first experimental detection of the C(s) symmetric nitroformyl radical, OCNO(X 2A') in a nitrogen-carbon dioxide matrix at 10 K using a Fourier transform infrared spectrometer (FTIR). The nu1 vibrational frequency was observed at 2113 cm(-1). This assignment was confirmed by follow-up experiments using isotopically labeled reactant molecules (15N, 18O, 13C). To synthesize this radical, we irradiated solid nitrogen-carbon dioxide ice mixtures with energetic electrons at 10 K. Suprathermal nitrogen atoms in their electronic ground and/or first electronically excited state were generated via the radiation induced degradation of molecular nitrogen; these atoms could then react with carbon dioxide to eventually yield the nitroformyl radical. We also investigated the kinetics of the formation of the nitroformyl radical and support the arguments with computations on the doublet and quartet OCNO potential energy surfaces (PESs).     

Synthesis, structure and physical properties of the first one-dimensional phenalenyl-based neutral radical molecular conductor

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-step stack. This particular mode of 1-D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the pi-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm(-1). Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10(-3) S/cm and is best described as a Mott insulator.     

Self‐Assembled Architectures with Segregated Donor and Acceptor Units of a Dyad Based on a Monopyrrolo‐Annulated TTF–PTM Radical

An electron donor–acceptor dyad based on a polychlorotriphenylmethyl (PTM) radical subunit linked to a tetrathiafulvalene (TTF) unit through a π‐conjugated N‐phenyl–pyrrole–vinylene bridge has been synthesized and characterized. The intramolecular electron transfer process and magnetic properties of the radical dyad have been evaluated by cyclic voltammetry, UV/Vis spectroscopy, vibrational spectroscopy, and ESR spectroscopy in solution and in the solid state. The self‐assembling abilities of the radical dyad and of its protonated non‐radical analogue have been investigated by X‐ray crystallographic analysis, which revealed that the radical dyad produced a supramolecular architecture with segregated donor and acceptor units in which the TTF subunits were arranged in 1D herringbone‐type stacks. Analysis of the X‐ray data at different temperatures suggests that the two inequivalent molecules that form the asymmetric unit of the crystal of the radical dyad evolve into an opposite degree of electronic delocalization as the temperature decreases.     

Hierarchical supramolecular structuring and dynamical properties of water soluble polyethylene glycol-perylene self-assemblies

The structural and dynamical properties of dilute aqueous solutions of poly(ethylene glycol)-perylene diimides (PEG(n)-PDI) have been investigated by means of static and dynamic light scattering, TEM microscopy, and small-angle X-ray scattering experiments. The amphiphilic PEG(n)-PDI molecules first self-assemble into stable and compact primary stacks of a few units of planar PDI through hydrophobic and π-π interactions. These primary stacks subsequently arrange in large and globular aggregates of typically 100-250 nm via weak PEG chain interpenetration. Surprisingly, the scattered electric field autocorrelation function g((1))(q,t) measured by dynamic light scattering evolves over very long periods of times (several months) and up to a bimodal distribution. The fast relaxation mechanism is associated to the diffusion of free primary stacks, whereas the slower relaxation still indicates the presence of large self-assemblies. Kinetic experiments show that the large supramolecular aggregates slowly release the free primary stacks whose proportion increases with time. This dissociation depends on several parameters such as PEG side chain length, total concentration, and shaking.     

NC-(CF2)4-CNSSN radical containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting-solidification: structural, spectroscopic, and magnetic characterization

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC-(CF2)4-CNSSN radical by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric Sdelta+...N(delta-) interactions of ca. 3.2 angstroms, and the dimeric units are linked by electrostatic -C triple bond N(delta-)...Sdelta+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 microB per dimer (1.3 microB per monomer) at 300 K and a good fit to the Bleaney-Bowers model in the temperature range 2-300 K with 2J = -1500 +/- 50 cm(-1), g = 2.02(5), rho = 0.90(3)%, and TIP = 1.25(4) x 10(-3) emu mol(-1). The [NC-(CF2)4-CNSSN radical]2 dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = -1730 +/- 100 cm(-1), |D| = 0.0278(5) cm(-1), |E| = 0.0047(5) cm(-1)]. The value of the singlet-triplet gap has enabled us to estimate the "in situ" dimerization energy of the radical dimer as ca. -10 kJ mol(-1). The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305-335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown (paramagnetic liquid).     

2-[5-(2-Pyridyl)-1,2,4-triazol-3-yl]phenol 7,7,8,8-tetracyanoquinodimethane, PTP-TCNQ, synthesis, structure and physical properties

A new charge-transfer complex PTP-TCNQ (PTP=2[5-(2-pyridyl)-1,2,4-triazol-3-yl]phenol, TCNQ=1,1,8,8-tetracyanoquinodimethane) has been synthesized and crystallized in the form of black needles. Single crystal X-ray diffraction shows that the solid contains stacks of alternating PTP and TCNQ molecules. The localized charge-transfer responsible for the colour of the material is confirmed by low conductivity of the pelletised powder form (<10⁻⁶ Ω⁻¹ cm⁻¹ at 298 K). The complex is paramagnetic: this is interpreted as a free electron localization on the acceptor molecule of the complex. The mid-IR region of the spectrum of the material confirms the presence of a charge-transfer interaction.     

C3-symmetrical supramolecular architectures: fibers and organic gels from discotic trisamides and trisureas

Hydrogen bonded C(3)-symmetrical molecules that associate into supramolecular stacks are described. Structural mutation on these molecules has been performed to elucidate the contribution of the different secondary interactions (hydrogen bonding, pi-pi stacking) to the self-assembly of the disks into chiral stacks. Twelve C(3)-symmetrical molecules have been investigated, six of which contain three central amide functionalities (1a-f) and six of which contain three central urea groups (2a-f). Peripheral groups of the disks are "small", "medium", or "large", half of them being achiral and the other half being chiral, to enable investigation of the supramolecular architectures with CD spectroscopy. In all of the cases, elongated, helical stacks are formed in apolar solution, except for the "medium" amide disks 1c/d. The elongated stacks of the C(3)-symmetrical disks form gels, which are visualized by AFM and SANS, and this confirms the directionality of the interactions. For the "large" urea disk, 2f, fibers with a length of up to 2 microm are observed. Temperature dependent and "sergeants-and-soldiers" CD measurements reveal that the urea stacks are much more rigid than the corresponding amide ones. In case of the "medium" urea disks, 2c/d, a true rigid rod, is formed. Where amide disks immediately reach their thermodynamic equilibrium, kinetic factors seem to govern urea aggregation. In a number of experiments aimed at reversibility with the urea stacks, hysteresis is observed, implying that these urea disks initially form a poorly defined stack, which subsequently transforms slowly into a well-defined, chiral architecture.     

7:8,9:10-dibenzo-1,2,3,4-tetrafluoro- triphenylene: synthesis, structure, and photophysical properties of a novel [5]helicene

A simple and efficient synthesis and crystallographic analysis of the partially fluorinated 7:8,9:10-dibenzo-1,2,3,4-tetrafluorotriphenylene, 1, in 59% yield is reported. Compound 1 is a blue luminescent [5]helicene in both solution and the solid state. It spontaneously forms enantiomorphous single crystals from racemic samples and exhibits slipped, face-to-face F...C(pi) contacts within the stacks of molecules.     

Photochemistry of ethyl chloride caged in amorphous solid water

Caging and photo-induced decomposition of ethyl chloride molecules (EC) within a layer of amorphous solid water (ASW) on top of clean and oxygen-covered Ru(001) under ultra-high vacuum (UHV) conditions are presented. The caged molecules were estimated to reside 1.5 +/- 0.2 nm above the solid surface, based on parent molecule thermal decomposition on the clean ruthenium. Dissociative electron attachment (DEA) of the caged molecules following 193 nm laser irradiation, result in initial fragmentation to ethyl radical and chloride anion. It was found that photoreactivity on top of the clean ruthenium surface (Ru) is twenty times faster than on the oxygen-covered surface (O/Ru), with DEA cross sections: sigma(Ru) = (3.8 +/- 1) x 10(-19) cm(2) and sigma(O/Ru) = (2.1 +/- 0.3) x 10(-20) cm(2). This difference is attributed to the higher work function of oxygen-covered ruthenium, leading to smaller electron attachment probability due to mismatch of the ruthenium photo-electron energy with the adsorbed EC excited electron affinity levels. EC molecules fragmented within the cage, result in post-irradiation TPD spectra that reveal primarily C(4)H(8), C(3)H(5) and C(3)H(3), without any oxygen-containing molecules. Unique stabilization of the photoproducts has been observed with the first layer of water molecules in direct contact with the substrate, desorbing near 180 K, a significantly higher temperature than the desorption of fully caged molecules. This study may contribute for understanding stratospheric photochemistry and processes in interstellar space.     

From Monomers to π Stacks, from Nonconductive to Conductive: Syntheses, Characterization, and Crystal Structures of Benzidine Radical Cations

Salts that contain radical cations of benzidine (BZ), 3,3',5,5'-tetramethylbenzidine (TMB), 2,2',6,6'-tetraisopropylbenzidine (TPB), and 4,4'-terphenyldiamine (DATP) have been isolated with weakly coordinating anions [Al(OR(F) )(4) ](-) (OR(F) =OC(CF(3) )(3) ) or SbF(6) (-) . They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of benzidine or its alkyl-substituted derivatives in CH(2) Cl(2) . The salts were characterized by UV absorption and EPR spectroscopy as well as by their single-crystal X-ray structures. Variable-temperature UV/Vis absorption spectra of BZ(.) (+) [Al(OR(F) )(4) ](-) and TMB(.) (+) [Al(OR(F) )(4) ](-) in acetonitrile indicate an equilibrium between monomeric free radical cations and a radical-cation dimer. In contrast, the absorption spectrum of TPB(.) (+) SbF(6) (-) in acetonitrile indicates that the oxidation of TPB only resulted in a monomeric radical cation. Single-crystal X-ray diffraction studies show that in the solid state BZ and its methylation derivative (TMB) form radical-cation π dimers upon oxidation, whereas that modified with isopropyl groups (TPB) becomes a monomeric free radical cation. By increasing the chain length, π stacks of π dimers are obtained for the radical cation of DATP. The single-crystal conductivity measurements show that monomerized or π-dimerized radicals (BZ(.) (+) , TMB(.) (+) , and TPB(.) (+) ) are nonconductive, whereas the π-stacked radical (DATP(.) (+) ) is conductive. A conduction mechanism between chains through π stacks is proposed.     

Bistability and the phase transition in 1,3,2-dithiazolo[4,5-b]pyrazin-2-yl

The molecular radical 1,3,2-dithiazolo[4,5-b]pyrazin-2-yl (PDTA) exhibits magnetic bistability just above room temperature, undergoing a well-defined hysteretic phase change with TC downward arrow = 297(1) K and TC upward arrow = 343(1) K. The crystal structures of the two phases of PDTA have been determined by single-crystal X-ray diffraction at 323(2) K. LT-PDTA consists of diamagnetic (S = 0) nearly superimposed pi-dimer stacks, while that of HT-PDTA comprises slipped stacks of pi-radicals (S = 1/2). The structural interconversion is suggested to proceed via the cooperative breaking and making of intermolecular S- - -N interactions and an inversion symmetry-preserving "domino cascade" of the pi-stacked rings.     

Cyclic tetra- and hexaynes containing 1,4-donor-substituted butadiyne units: synthesis and supramolecular organization

Cyclic bis(1,3-butadiynes) with sulfur centers placed in the alpha-position to the 1,3-butadiyne units (2(n)) were synthesized by Glaser coupling of the corresponding open chain dithia-alpha,omega-diynes 1(n). In a second protocol we applied a four-component cyclization by reacting alpha,omega-dithiocyanatoalkanes 6(n) or alpha,omega-diselenocyanatoalkanes 7(n) with dilithium-1,3-butadiynide. This concept afforded either the cyclic dimers (S, 2(n); Se, 9(n)) or the cyclic trimers (S, 8(n); Se, 10(n)). Most of the molecular structures of 2(n) and 9(n) adopt chairlike conformations in the solid state. Tubular structures in the solid state with short distances between the chalcogen centers of neighboring stacks were encountered for 2(5), 9(5), 8(4), 10(4), and 10(5). Recrystallization of 10(5) from various polar and nonpolar solvents yielded inclusion of the solvent guest molecules. The solvent-accessible volume was calculated to vary from 19% (n-hexane) to 25% (mesitylene). The elastic properties of our cycles are due to the flexible methylene chains and the easily variable torsional angles between the rigid 1,3-butadiyne rods.     

Synthesis, structure, and physical properties of a partial π-stacked phenalenyl-based neutral radical molecular conductor

We report the synthesis, crystallization, and solid-state characterization of the 3,7-ethoxy-substituted spirobiphenalenyl-boron neutral radical 22. The radical is distinguished by its low disproportionation energy and one-dimensional structure. We show that our strategy of substitution of OEt group at the active positions of the phenalenyl units changes the crystal packing from its previously known OMe analogue and the solid-state properties are dictated by the partial π-stack structure and the oxygen atoms at the 3,7-positions and can be best rationalized in terms of the resonating valence bond model. Magnetic susceptibility measurements show that in the solid state the radical remains paramagnetic but there is significant spin-spin interaction between the molecules. Band structure calculations reflect efficient overlap between the molecules along the π stack and show evidence of interactions between the spin-bearing oxygen atoms. The room temperature electrical conductivity (σ(RT)=2.0×10(-2) S cm(-1)) of 22 is higher than that observed in previously known one-dimensional phenalenyl radicals.     

Novel fused D-A dyad and A-D-A triad incorporating tetrathiafulvalene and p-benzoquinone

Novel fused donor-acceptor dyad (TTF-Q or D-A) and acceptor-donor-acceptor triad (Q-TTF-Q or A-D-A) incorporating the donor tetrathiafulvalene (TTF) and the acceptor p-benzoquinone (Q) have been synthesized. The solution UV-vis spectra of these molecules display a low-energy absorption band that is attributed to an intramolecular charge transfer between both antagonistic units. The presence of reversible oxidation and reduction waves for the donor and acceptor moieties was shown by cyclic voltammetry, in agreement with the ratio TTF/quinone(s) units. The successive generation from these compounds of the cation radical and anion radical obtained upon (electro)chemical oxidation and reduction, respectively, was monitored by optical and ESR spectroscopies. The anion radical Q-TTF-Q(-.) triad was demonstrated to be a class II mixed-valence system with the existence of a temperature-dependent intramolecular electron transfer. The crystallographic tendency of these fused systems to overlap in mixed stacks of alternating A-D-A units is also discussed.     

Energy-transfer efficiency in stacked oligo(p-phenylene vinylene)s: pronounced effects of order.

The supramolecular structure of two types of oligo(p-phenylene vinylene) (OPV) building blocks in dodecane solution is studied. Monofunctional chromophores (MOPV) form well-defined helical assemblies, whereas bifunctional molecules (BOPV) aggregate into so-called frustrated stacks, lacking any higher helical order. This difference in organization has a major influence on the transfer of excitation energy through the assemblies. Energy transfer to supramolecularly incorporated guests (MOPV with lower bandgap) is used to probe the intrinsic differences in exciton mobility in these two types of mixed aggregates. From the observed donor fluorescence quenching, it can be concluded that the helically ordered nature of the MOPV stacks facilitates the transfer of excitation energy, yielding evidence for higher exciton mobility in the well-ordered assemblies than in the frustrated stacks. Finally, the concept of energy transfer in supramolecular assemblies is extended to the solid state by the successful implementation in a light-emitting diode (LED).     

Singlet and triplet excited-state interactions and photochemical reactivity of phenyleneethynylene oligomers

The rigid rodlike character of phenyleneethynylenes and their ability to communicate charge/excitation energy over long distances have made them useful as molecular linkers in the light energy harvesting assemblies and molecular electronics devices. These linker molecules themselves possess rich photochemistry as evident from the relatively large yields of the excited singlet (0.5-0.66) and triplet (0.4-0.5) states of two model oligomers, 1,4-bis(phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-1) and 1,4-bis((4-phenylethynyl)phenylethynyl)-2,5-bis(hexyloxy)benzene (OPE-2). In particular, the long-lived triplet excited state is capable of undergoing deactivation by self-quenching processes such as ground-state quenching and triplet-triplet (T-T) annihilation. The T-T annihilation occurs with a nearly diffusion-controlled rate (approximately 2 x 10(9) M(-1) s(-1)), and ground-state quenching occurs with a rate constant of approximately 6 x 10(7) M(-1) s(-1). The electron transfer from the excited OPE-1 and OPE-2 to benzoquinone as characterized from the transient absorption spectroscopy illustrates the ability of these molecules to shuttle the electrons to acceptor moieties. In addition, pulse radiolysis experiments confirm the spectroscopic fingerprint of the cation radical (or "trapped hole") with absorption bands in the 500-600 nm region.     

Synthesis,structural investigation,and solid-state properties of iodine-doped zirconium diphthalocyanine, [ZrPc2]I3.I2

Crystals of iodine-doped zirconium(IV) diphthalocyanine, [ZrPc(2)]I(3).I(2) (where Pc = C(32)H(16)N(8)), were grown directly in the reaction of pure zirconium powder with phthalonitrile under a stream of iodine at 260 degrees C. [ZrPc(2)]I(3).I(2) crystallizes in the space group P2(1)/m (No. 11) of the monoclinic system with lattice parameters of a = 6.735(1), b = 25.023(5), and c = 17.440(3) A, beta = 99.43(3) degrees, and Z = 2. The crystals of [ZrPc(2)]I(3).I(2) are built up from two pseudo-monodimensional aggregates: one-electron-oxidized [ZrPc(2)](+) units; weak interacting triiodide I(3)(-) ions with neutral diiodine molecules. The I(3)(-) ions and neutral I(2) molecules in the crystal of [ZrPc(2)]I(3).I(2) have been also detected by Raman spectroscopy. The [ZrPc(2)](+) units form stacks along the a axis, while the polymeric...I(3)(-)...I(2)...I(3)(-)...I(2)(-)... zigzag chains are located in the crystal along the b axis, so both pseudo-monodimensional aggregates are perpendicular to each other. This arrangement is different from that found in the tetragonal crystals of [ZrPc(2)](I(3))(2/3) in which both monodimensional aggregates, i.e., the stacks of partially oxidized [ZrPc(2)](2/3+) units and chains of symmetric triiodide ions, are parallel. EPR experiment together with the X-ray single-crystal analysis clearly shown that oxidation of the diamagnetic ZrPc(2) complex by iodine is ligand centered and homogeneously affecting both phthalocyaninato rings of ZrPc(2); thus, the formal oxidation state of both Pc rings in [ZrPc(2)]I(3).I(2) is nonintegral (-1.5). The UV-vis spectrum of [ZrPc(2)] I(3).I(2) is very similar to the spectrum of unoxidized ZrPc(2) complex in the B Soret and Q spectral region. However, in the spectrum of [ZrPc(2)] I(3).I(2) one additional band at approximately 502 nm is observed, which indicates the existence of the one-electron-oxidized phthalocyaninato(-) radical ligand and is assigned to the electronic transition from a deeper level to the half-occupied HOMO level. The single-crystal electrical conductivity data show anisotropy and nonmetallic character in conductivity (d sigma/dT > 0). The charge transport mainly proceeds along the pseudo-monodimensional stacks of [ZrPc(2)](+) units. The relatively high conductivity along the stacks of one-electron-oxidized [ZrPc(2)](+) units results from the staggering orientation of Pc rings (rotation angle 45.0(2) degrees ) that leads to the short inter-ring C(alpha)(pyrrole)[bond]C(alpha)(pyrrole) contacts (2.839(3)-3.024(3) A). These C(alpha)-pyrrole atoms make appreciable contribution to the partially occupied pi-molecular orbital of Pc macrocycle and the greatest overlap of the HOMO orbitals that form the conduction band of partially oxidized molecular crystals.     

Complex mechanism of the gas phase reaction between formic acid and hydroxyl radical. Proton coupled electron transfer versus radical hydrogen abstraction mechanisms

The gas phase reaction between formic acid and hydroxyl radical has been investigated with high level quantum mechanical calculations using DFT-B3LYP, MP2, CASSCF, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction has a very complex mechanism involving several elementary processes, which begin with the formation of a reactant complex before the hydrogen abstraction by hydroxyl radical. The results obtained in this investigation explain the unexpected experimental fact that hydroxyl radical extracts predominantly the acidic hydrogen of formic acid. This is due to a mechanism involving a proton coupled electron-transfer process. The calculations show also that the abstraction of formyl hydrogen has an increased contribution at higher temperatures, which is due to a conventional hydrogen abstraction radical type mechanism. The overall rate constant computed at 298 K is 6.24 x 10(-13) cm3 molecules(-1) s(-1), and compares quite well with the range from 3.2 +/- 1 to 4.9 +/- 1.2 x 10(-13) cm3 molecules(-1) s(-1), reported experimentally.     

Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers

We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 x 10(-3) cm2/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.