Perhydrotriphenylene Research Articles

Molecular recognition at growing crystal faces revealed by phase sensitive second harmonic microscopy

The absolute orientation of the local average polarization in relation to the main crystal growth direction has been investigated for channel-type inclusions of perhydrotriphenylene (PHTP) and five guests: N,N-dimethyl-3-nitroaniline (DMNA), 1-(4-nitrophenyl)-piperazine (NPP), 4-iodo-4′-nitrobiphenyl (INBP), 4-amino-4′-nitrostilbene (ANS) and N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA). For this we have used phase sensitive second harmonic generation microscopy (PH-SHM) and a crystal of 2-cyclooctylamino-5-nitropyridine (COANP) as a NLO reference material for which we know the absolute polarity from X-ray anomalous scattering. Bi-polar states of all five inclusion compounds have been confirmed. Application of PS-SHM revealed that the nitro groups preferably point into the direction of growth. The attributed molecular orientation is further on confirmed (PHTP/DNAA and PHTP/INBP) by scanning pyroelectric microscopy. Results from Monte Carlo (MC) simulations allow us to conclude that the fundamental energy parameter ΔEf driving stochastic polarity formation is larger than zero.

Directional exciton transport in supramolecular nanostructured assemblies

Supramolecular nanostructured host-guest compounds provide an intriguing strategy for improved materials in optoelectronic devices. Here, conjugated organic guest molecules are embedded in channel-forming organic or inorganic hosts, like perhydrotriphenylene (PHTP) and zeolites, leading to weakly coupled, highly ordered, and brightly emissive materials. In-depth material characterization allows for a full understanding of the exciton transport mechanism in the weak coupling regime through steady-state time-resolved fluorescence studies combined with quantum-chemical based Monte-Carlo simulations without adjustable parameters. Despite weak chromophore coupling and dilution by the host, the exciton diffusion length exceeds 50 nm similar to molecular crystals, and might be tuned to the μm range by the proper choice of host and guest materials.

Delayed Fluorescence in Perhydrotriphenylene–Oligothiophene Inclusion Compounds: Evidence for Molecular Oxygen-Related Excited States

Delayed emission from α-terthiophene (3T) and α-quinquethiophene (5T) in a perhydrotriphenylene (PHTP) host is investigated. Delayed fluorescence for the lowest singlet excited state of 3T and 5T is detected at both low (80 K) and room temperatures. In addition, at low temperature, phosphorescence from 3T is observed with a lifetime of ∼100 μs. Comparison of the dependence of delayed fluorescence and phosphorescence on excitation intensity and time shows that delayed fluorescence does not originate from triplet--triplet annihilation. A dependence of the delayed fluorescence on atmospheric pressure indicates that it originates, at least partially, from complexes of photoexcited oligothiophene and molecular oxygen O(2).

One-Hundred Eighty Degree Domain Detection by Surface Phase Sensitive Second Harmonic Generation Microscopy of Polar Materials

Phase sensitive second harmonic microscopy (PS-SHM) is applied to different nonlinear optical crystals such as N,N-dimethyl-2-acetamido-4-nitroaniline (DAN), 2-cyclo-octylamino-5-nitropyridine (COANP), a channel-type inclusion compound of perhydrotriphenylene (PHTP)/N,N-dimethyl-3-nitroaniline (DMNA), potassium dihydrogen phosphate (KH2PO4) crystals stained by amaranth, and the zeolite AlPO4-5 loaded by 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). Present experiments show that phase sensitive experiments can be performed using a sufficiently flat surface of samples irrespective of their thickness. The new application of PS-SHM reveals a bipolar state in stained KH2PO4 and dye-loaded AlPO4-5.

Alignment of PHTP–DNAA inclusion crystals by zone casting

Inclusion compound of perhydrotriphenylene (PHTP) and polar N,N-dimethyl-4(4-nitrophenylazo)-aniline was grown from solution into oriented crystalline layers using a zone casting technique. The parallel orientation of channel axes in the direction of the casting was investigated by means of a combination of optical absorption, polarized visible light and a pronounced SHG response. The maximum of the SHG signal of an oriented needle-like crystal was observed along the channel axis. A phase sensitive second harmonic generation (PS-SHG) experiment has demonstrated a bi-polar state of PHTP/DNAA growth from solution. Experimental evidence is provided, that unipolar layers can be obtained.

Polarized Fluorescence from Single Stopcock Molecules at Channel Entrances of an All-Organic Host−Guest Compound

Novel porphyrin-based stopcock molecules close the nanochannels of all-organic perhydrotriphenylene (PHTP) host−guest compounds as demonstrated by polarized single molecule fluorescence microscopy.

Theoretical study of conformational aspects, excited states and directional energy transfer: The Terphenylene, Diphenylhexatriene and Quinquethiophene triad included in PHTP nanochannels

The limited one-dimensional space of nanochannels of the supramolecular Perhydrotriphenylene (PHTP) matrix host induce changes on the structure of included molecules that affect some of their properties. A combination of Density Functional Theory and semi-empirical methodologies is used to study the structural changes, the electronic excited states and the energy transfer processes between the triad of oligomers Terphenylene (P3), Diphenylhexatriene (DPH) and Quinquethiophene (T5) included in PHTP. Details of the conformational aspects of the oligomeric guest species have been obtained assuming a rigid matrix approach for the PHTP host. Optimal geometries were considered to study excited states and processes for the P3 → DPH → T5 chain of two-steps transfer of energy from the UV to the visible. Trends for the dominant directional mechanism for the energy transfer have been resolved from a theoretical point of view.

Alignment and Relaxation Dynamics of Dye Molecules in Host−Guest Inclusion Compounds As Probed by Dielectric Spectroscopy

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Channel forming organic crystals: guest alignment and properties

Purely organic solid state materials providing nanometre sized parallel channels are suited for physical property design. In this tutorial review, key materials in this field are reviewed along with new directions forming porous covalent and polymeric networks. Among many known host materials, perhydrotriphenylene (PHTP), tris-(o-phenylenedioxy)-cyclotriphosphazene (TPP) and members of the 2,4,6-tris-(4-halo-phenoxy)-1,3,5 triazine (4-X-POT) family are prime materials for hosting guest molecules featuring diverse physical functionality.

Highly Emissive Nanostructured Thin Films of Organic Host–Guests for Energy Conversion

All-organic nanostructured host-guest systems, based on dyes inserted in the nanochannels of perhydrotriphenylene (PHTP) and deoxycholic acid (DCA), show enhanced fluorescence properties with quantum yields even higher than those of the dyes in solution, thanks to the high concentration of emissive molecules with controlled spatial and geometrical organization that prevents aggregation quenching. Both host molecules crystallize, growing with the long axis oriented along the direction of the nanochannels where the linear-chain dyes are inserted, to yield crystals emitting well-polarized light. For the DCA-based host-guests, homogeneous thin films suitable for several applications are obtained. Colour emission in such films can be tuned by co-inclusion of two or three dyes due to resonant energy-transfer processes. We show that films obtained by low-cost techniques, such as solution casting and spin-coating, convert UV light into visible light with an efficiency much higher than that of the standard polymeric blends.

Dynamics of guest molecules in PHTP inclusion compounds as probed by solid-state NMR and fluorescence spectroscopy

Partially deuterated 1,4-distyrylbenzene () is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of is investigated over a wide temperature range by (13)C, (2)H NMR as well as fluorescence spectroscopy. Simulations of the (2)H NMR spectral shapes reveal an overall wobble motion of in the channels with an amplitude of about 4 degrees at T = 220 K and 10 degrees at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 180 degrees flips of the terminal phenyl rings with a frequency of 10(6) Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol(-1) which imply a strong sterical hindrance by the surrounding PHTP channels. The experimental vibrational structure of the fluorescence excitation spectra of is analyzed in terms of small amplitude ring torsional motions, which provide information about the spatial constraints on by the surrounding PHTP host matrix. Combining the results from NMR and fluorescence spectroscopy as well as of time-dependent density functional calculations yields the complete potential surfaces of the phenyl ring torsions. These results, which suggest that intramolecular mobility of is only reduced but not completely suppressed by the matrix, are corroborated by MD simulations. Unrealistically high potential barriers for phenyl ring flips are obtained from MD simulations using rigid PHTP matrices which demonstrate the importance of large amplitude motions of the PHTP host lattice for the mobility of the guest molecules.

Photoexcited triplets of dyes in zeolitic nanostructured channels. A time resolved EPR study

Two dyes (4-nitrostilbene, NST and 4-N,N-dimethylamino-4'-nitrostilbene, DANS) included in zeolites with nanometric channels and different Si : Al ratios have been photoexcited and their triplet state studied by time resolved EPR (TR-EPR). This is the first time that a TR-EPR spectrum of photoexcited triplet states of dyes in zeolites has been observed. The zeolites used were ZSM-5 and mordenite, with either protons or lithium as charge compensating ions, and the aluminium-free porosil. The ZFS parameters and the polarized spin populations have been obtained, and compared with those obtained in glassy solutions and in the organic nanostructured matrix perhydrotriphenylene (PHTP). For (3)DANS in neutral solvents and in PHTP the dimethylamino group is pi conjugated, whereas only its acid form is detected in all the zeolites. In the latter the spectra of the radical cations formed by spontaneous oxidation have also been observed both by cw-EPR and TR-EPR, the last spectra being spin polarized in emission. The mobility of the triplets is discussed, taking into account the spin polarization of the radical cations indicating a strong radical-triplet interaction.

Nanometric scale investigation of the nonlinear efficiency of perhydrotriphenylene inclusion compounds

A two-photon polarized microscopy technique has been implemented to investigate the nonlinear efficiency and the molecular organization in organic inclusion compounds. Different chromophores have been included as guest molecules in perhydrotriphenylene (PHTP) channels yielding needle-shaped crystals of either millimetric or submicrometric size depending on the sample preparation. Polarization responses of second harmonic generation (SHG) and two-photon fluorescence (TPF) are shown to provide complete information on the crystalline quality of such structures. In particular, macroscopic crystals exhibit one-dimensional molecular ordering along the needle axis with occasional local orientational defects. Their zero frequency nonlinear efficiency can reach up to 340 pm/V with 4-dimethylamino-4′-nitroazobenzene as a guest, which can be ascribed to about 80% of the guest molecules setting-up the noncentrosymmetric sub-lattice. This order is furthermore preserved in submicrometric structures, which are promising systems towards SHG-active nanoparticles.

Organic host–guest systems for blue emission

The photoluminescence (PL) quantum yields (QYs) and lifetimes of the 1,6-diphenyl-1,3,5-hexatriene (DPH) blue emitter and of the acceptor–donor system, DPH and p-terphenyl (P3), are investigated embedded in the nanochannels of a perhydrotriphenylene (PHTP) matrix. In the co-inclusion system, light emission takes place from DPH, due to the efficient resonant energy transfer (RET) from the P3 donor molecules. The host matrix stabilizes the guest molecules against photooxidation degradation. The largest quantum yield, up to 100%, has been found in the co-inclusion compound (IC), where excitations are localized at the emissive acceptor sites.

Chromophores in nanochannels for energy transfer and conversion

Host–guest organic systems obtained by the inclusion of thiophene and phenylene based molecules in perhydrotriphenylene (PHTP) display efficient resonant energy transfers (RET) from short wavelength to longer wavelength emitters. The quantum efficiency of the longer wavelength emitter is lowered of about 20% when donor–acceptor transfer steps are required. A co-inclusion compound (IC) obtained with three molecules shows an efficient conversion of the near ultra violet (250–400 nm) energy into visible (400–600 nm) light with a high photoluminescence (PL) quantum yield (QY) of 45–60%. A proper design of host–guest materials exploiting energy transfer processes can provide interesting artificial nanostructures for optoelectronic applications.

Alignment of radicals into chains by a Markov mechanism for polarity formation

A theoretical concept for setting up 1D magnetic interactions between organic radicals aligned into chains within channels of inclusion compounds is presented by calculating collinear intermolecular binding energies using ab initio methods, followed by a Markov model to predict the characteristic length of chains. Experimentally, inclusion crystals providing radicals in channels of perhydrotriphenylene (PHTP) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were obtained by co-crystallisation or in-diffusion of the 1,3,5-trithia-2,4,6-triazapentalenyl radical (TTTA).

ChemInform Abstract: Structural and Optical Properties of Conjugated Molecules in Perhydrotriphenylene (PHTP) and in Other Channel‐Forming Inclusion Compounds

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Energy transfer in nanostructured oligothiophene inclusion compounds

Resonant energy transfers from terthiophene (T3) to quinquethiophene (T5) oligomers embedded in the nanochannels of perhydrotriphenylene (PHTP) crystals are studied by cw and femtosecond spectroscopy. The stringent geometry imposed by the host results in a very peculiar supramolecular organization of the guest molecules consisting of parallel linear arrays of chromophores. This provides a unique opportunity to make a quantitative analysis of the energy transfer phenomena and in particular to distinguish between heterotransfers and homotransfers. The experimental data fitted by a suitable theoretical analysis indicate that homotransfers become important in the long time dynamics and that the heterotransfer rate is satisfactorily described by the analytical solution obtained by assuming a continuous intermolecular spacing.

One dimensional coupling of oligophenylenevinylenes in perhydrotriphenylene matrices

Single needle-shaped crystals of oligophenylenevinylene (OPV) / perhydrotriphenylene (PHTP) inclusion compounds (IC) are investigated by polarized absorption and fluorescence spectroscopy. High dichroic ratios are detected, originating from head-to-tail orientations of the OPVs within the host lattice. The spectra of the ICs are red-shifted against solution, indicating the presence of planarization effects and / or head-tail-interactions, whereas side-by-side interactions can be excluded. Time-resolved fluorescence studies of two-component OPV/PHTP-ICs allow the determination of energy transfer (ET) efficiencies. The deactivation kinetics are simulated by Forster-type ET processes.

Phase Diagrams of Perhydrotriphenylene and Aromatic Compounds II. Terthiophene and related thiophene oligomers

Terthiophene included in a perhydrotriphenylene (PHTP) adduct exhibits an UV spectrum different from that observed in the bulk or in solution. The X-ray diffraction pattern also differs significantly from that of pure PHTP. The UV spectrum is interpreted as due to an isolated molecule in a vacuum-like space, in agreement with a model developed from X-ray diffraction data. Knowledge of the phase diagram was essential for an interpretation of the optical properties and of some kinetic aspects of the formation of the inclusion compound phase. In this work, the phase diagrams of terthiophene (T3) are presented, and DSC evidence of the inclusion of quater- (T4), penta- (T5) and α,ω-dihexylquaterthiophene (DH4T) is discussed.