Excimer luminescence of 9-cyanoanthracene under pressure

The excimer emission of 9-cyanoanthracene has been measured at room temperature as a function of pressure. Near 8 kbar the luminescence undergoes a drastic shift to lower energies corresponding to a change of the emitting excimer from the trans, head-to-tail, to the cis, head-to-head, molecular pair. The latter excimer deactivates the photodimerization reaction due to the absence of the topochemically preformed geometry in the excited state which would favour the trans-photoproduct.     

Exciton-induced lattice distortion and excimer formation

The considerations necessary for the development of a complete theory of excimer formation in crystals are analysed. A simple model which incorporates exciton-generated lattice distortion into the travelling exciton is presented, a distortion type related to excimer formation defined and the role it plays in establishing a route for excimer formation discussed. The consequences of a large change in pair molecule separation in going from crystal to excimer are emphasised and possible competition with photodimerization as an alternative end result mentioned. Referring to experimental evidence for a travelling excimer in 9-cyanoanthracene, it is shown here that a strong exciton-phonon coupling term peculiar to this crystal would be favourable. Mechanisms suggested elsewhere for migration of excimer excitation in pyrene are discussed in terms of the ideas developed here.     

The spectroscopy and photochemistry of the excimer-emitting 9-cyanoanthracene crystal; The importance of multiphonon processes

A study of the spectroscopy and photochemistry of crystalline 9-cyanoanthracene is reported. This crystal gives rise to excimer emission; at 77 K the excimer has a lifetime of 160 ±5 ns and the emission spectrum is of gaussian shape. There is, prior to reaction, transfer of excitation energy by a process occurring at the excimer—exciton level and involving an activation energy of about 420 ^cm?1. Radiationless deactivation of the excimer involves an activation energy of 660 cm^?1. These experimental observations are in good agreement with the predictions of theories based on the model of strong exciton—phonon coupling. The photoreaction occurs at specific sites which are believed to be head-to-tail defects.     

Molecular Architecture with Nucleobases,Metal Ions and Water Molecules: Mixed Adenine,Hypoxanthine Quartet Containing trans -(NH 3 ) 2 Pt II and Ag + and Harboring a Water Hexamer in Its Chair Conformation

The formation and X-ray crystal structure of a molecular rectangle of 14.25(2) Å ‐ 10.36(2) Å, comprised of two neutral 9-methyladenine (9-MeA) and two anionic 9-methylhypoxanthinate (9-MeHx) model nucleobases as well as two trans -(NH 3 ) 2 Pt II and two Ag + entities, and further cross-linked intermolecularly by Ag + ions, is described: trans -[{(NH 3 ) 2 Pt(9-MeA)(9-MeHx)Ag(NO 3 ) (H 2 O)} 2 Ag](NO 3 ) 3 6H 2 O ( 4 ). The water molecules are located between adjacent purine quartets and adopt a cyclic water hexamer structure in a chair conformation. In addition, the X-ray crystal structure of the precursor of 4 , trans -[(NH 3 ) 2 Pt(9-MeA)(9-MeHxH)](NO 3 ) 2 H 2 O ( 2 ), is reported. 4 is discussed in terms of its relationship to proposals in the literature concerning possible structures of metalated forms of purine quartets.     

Electronic energy trapping in a crystal due,to a dopant of higher excitation energy than the host

An excimer emitting crystal (9-cyanoanthracene) doped with a guest molecule (9-methoxyanthracene) having its first singlet level ca. 2000 cm^?1 above the host singlet exciton band exhibits efficient energy trapping as demonstrated by host sensitized, red-shifted emission and hetero-photodimerization. It is considered that the trapping is due to exciplex formation between host and guest molecules.     

Luminescent Radical‐Excimer: Excited‐State Dynamics of Luminescent Radicals in Doped Host Crystals

The excited‐state dynamics of the photostable luminescent organic radical (3,5‐dichloro‐4‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl (PyBTM) doped in a host crystal was investigated by using optically detected magnetic resonance (ODMR) and time‐resolved emission spectroscopies. In the radical system, the unpaired electron can be used as the probe for studying the electronic state and its dynamics. The mixed crystal with a high concentration of the radical showed excimer emission, together with the monomer emission. The ODMR signals were observed with opposite signs for monitoring the monomer and the excimer emissions. Based on their temperature and concentration dependencies, the excited‐state dynamics on the doped crystal and the mechanism of the excimer formation and the ODMR signal generation are discussed with the help of the quantum mechanical simulation of the excited‐state spin dynamics. The initial process of excimer formation has been clarified for the first time from the viewpoint of the spin‐dynamics.     

Effects of molecular structure and environment on the photodimerization of substituted anthracenes

Photochemical and photophysical studies of a number of 9-substituted anthracene sandwich pairs in their corresponding photodimer crystal matrices have been carried out. Corresponding studies have also been made in methylcyclohexane matrices at 6 K. Photodimerization of the 9-methyl, 9-chloro and 9-cyano derivatives in the photodimer matrices occurs at 6 K with unit quantum yield. The presence of excimer fluorescence from sandwich pairs indicates the lack of a perfect topochemical orientation while activation processes, leading to photodimerization, involve molecular re-orientation from more stable ground state configurations which are achieved within the constraints imposed by the solvent or crystalline cage.     

Polarization of the monomer and excimer fluorescence of pyrene derivatives in liquid-crystalline hosts

The monomer and excimer fluorescence polarization of 4,9-disubstituted pyrenes with molecular long axes nearly perpendicu- lar to the pyrene skeleton were measured in a nematic liquid crystal. By comparison with unsubstituted pyrene it is found that the excimer transition moment is oriented parallel to the sandwich plane and perpendicular to the pyrene long axis.     

Mixed-crystal packing calculations. 9-methoxyanthracene in host 9-cyanoanthracene

Packing calculations by the atom-atom interaction method are reported for 9-methoxyanthracene (9MeOA) as a guest in host crystalline 9-cyanoanthracene (9CNA). The lower-energy local packing has approximately parallel anthracene nuclei, and substituents in head-to-head (“cis”) registry, in agreement with the structure found experimentally for the photochemical 9MeOA-9CNA complex. Electronic excitation of a host 9CNA adjacent to the guest 9MeOA causes an impulsive, short-lived, displacement of 9CNA*, giving dynamic preformation of either (9CNA)₂ excimer, or 9CNA-9MeOA exciplex. Over longer times, the structural response to excitation is followed by calculations including the successive relaxation of neighbour molecules, converging to an equilibrium structure for the locally excited crystal.     

Theoretical investigation of perylene dimers and excimers and their signatures in X-ray diffraction

The structures of the ground and excimer states of perylene pairs are calculated [using density functional theory (DFT) and time-dependent DFT techniques] in a free as well as a crystal environment, and their spectroscopic properties are studied for the most stable configurations. The vertical transition energies for the absorption and emission bands are obtained, and they are in good agreement with experimental data. In these calculations, up to six excited states are considered. With the calculated structures of the ground and excimer states, the scattering factors are analyzed as a function of the concentration of excimers in a crystal. The intensity of the 110, 005, and 0 10 0 reflections are found to be fairly sensitive to the presence of excimers in the crystal. The finite (nanosecond) lifetime of the excimer may make it possible to observe this state using time-resolved X-ray diffraction techniques.     

Dynamic instabilities in excited molecular crystals. Packing calculations in anthracenic systems

Lattice energy calculations on excited molecules in crystals, carried out using a simple centre-of-mass attractive excitation potential, are reported for two phases of anthracene, 9-cyanoanthracene and 9-methylanthracene. Calculations with a fixed environment can show whether or not short-term instability is expected, predisposing the structure to excimer formation or photochemical change, whereas calculations including relaxation show what the topochemical possibilities are for a lattice equilibrium structure in the vicinity of the excited molecule. In neither P2₁/a nor P$\text{1}$ anthracene crystal is there a significant change in position or orientation of the excited molecule or its neighbours. For 9-cyanoanthracene there is instability within the fixed environment, in which a movement of the excited molecule over distances around 0.1 nm occurs, preforming an excimer for short periods followmg excitation. With inclusion of lattice relaxation, simulating events at longer times, there is lattice contraction about the excited site and disappearance of the asymmetric displacement. For 9-methylanthracene, however, there is a local lattice instability for an excited molecule both in a fixed environment and one allowing full relaxation.     

Theoretical evaluation of potential surfaces,equilibrium geometries and vibronic transition intensities of excimers: the pyrene crystal excimer

A general method for evaluation of equilibrium geometries and vibronic transition intensities of crystalline excimers is described. The method provides realistic potential surfaces which include, in addition to the α electron surfaces, also the repulsive hard core of the α electrons and the effect of the crystal potential. The method is applied to the study of the potential surfaces of the pyrene excimer, both in the crystal and in the gas phase. The method is also used for a direct evaluation of the vibronic structure of the excimer emission. The calculations reproduce the main experimentally observed properties including the structureless envelope of the emission band and the temperature dependence of its line width.     

trans–cis and trans–cis–trans Microstructure Evolution of Azobenzene Liquid‐Crystal Polymer Networks

The photomechanics of azobenzene LCNs is modeled using a nonlinear continuum mechanics approach that couples photoisomerization of liquid crystal domain structures with light absorption and deformation of a glassy polymer network. The effects during UV‐stimulated trans–cis photomechanical deformation versus blue‐green light (trans–cis–trans) photomechanical deformation are simulated. Different bending deformation is predicted by assuming liquid‐crystal order/disorder behavior during trans–cis photoisomerization in comparison to light‐polarization‐driven reorientation of the trans phase during potential trans‐cis‐trans photoisomerization. Light‐controlled deformation mechanisms offer support for improved control of photo‐responsive morphing structures with a single blue‐green polarized light source.

     

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. II. relaxation processes

It is shown that by takin into account anhormonicity it is possible to explain why, in ground-to-excimer absorption in pyrene, the non-phonon peak can be resolved whereas the other peaks are overlapped. The same treatment applied to emission gives an explanation of the lack of structure in excimer emission. A possible model for vibrational relaxation of the excimer in pyrene crystal is considered and the corresponding time for relaxation is estimated.     

On the relationship between fluorescnece and molecular geometry. The effect of intermolecular shift on the excimer emission of crystalline dianthronylidene ethane.

The existence of two luminescent excimer geometries in polymorphic dianthronylidene ethane has been verified by X-ray structure determinations of three crystal modifications. The different excimer emission energies are interpreted in terms of variations in transannular π-orbital interaction resulting from an intermolecular shift of adjacent molecules across the log axis of the anthronylidene moiety.     

CuCl_2对聚乙二醇(PEG)构象和结晶行为的影响

用傅里叶变换红外光谱法(FT-IR)、差示扫描量热法(DSC)、广角X射线衍射法(WAXD)研究了少量过渡金属盐氯化铜(CuCl_2)对聚乙二醇(PEG)构象和结晶行为的影响.Cu~(2+)与PEG分子链中的氧原子有配位键生成,通过溶液电导率的测定,发现一个Cu~(2+)大约与3个PEG链节单元形成配位结构,从而使PEG的构象和结晶行为发生变化,随着体系中CuCl_2含量的增加,PEG分子链中OC-CO旁式构象相对反式构象含量逐渐增加,在CuCl_2含量(w)为7%左右时达到极大值,在CuCl_2含量(w)为9%时,反而是反式构象相对旁式构象含量多些.混合物中PEG的结晶度随着体系中CuCl_2含量的增加逐渐降低,含量(w)为9%时,又有所提高.     

Effect of pressure on the luminescence of 9,10-dichloroanthracene crystals

Luminescence spectra of β-9,10-dichloroanthracene have been studied at room temperature in the pressure range ≈ 0–10 GPa. The initial shift to lower energies of the excimer emission is linear (≈500 cm⁻¹/GPa) up to ≈3 GPa. Above this pressure a sharper emission grows at the high-energy side at the expense of the excimer luminescence. The suppression of the excimer state at pressures above ≈ 7 GPa is attributed to the strong repulsive interactions between the molecular pair. The new emission is thought to arise from a more loosely bound configuration in the excited state which could originate from a crystal phase change around 3 GPa.     

Observation of the diffusion-free intermolecular excimer of 9-methyladenine in aqueous solution by picosecond time-resolved spectroscopy

Diffusion-free intermolecular excimer formation in liquid solution is observed for an aqueous solution of 9-methyladenine using the synchroscan streak camera technique. The excimer fluorescence (330 ps lifetime) appears at 380 nm, at longer wavelength than the monomer fluorescence (5 ps lifetime). The excimer is formed via a weakly coupled stacking dimer composed of ground-state monomers.     

The use of doped crystals as a medium for the study of excited state bimolecular interactions

A molecular crystal is an attractive medium for the study of excited state intenmolecular interactions since the ground state positions of interacting molecules may be accurately determined. Besides excimer interactions in pure crystals it is possible to study (in doped crystals) exciplex interactions between host and guest molecules, and even excimer formation between two adjacent guest molecules. This approach has been utilized to observe excimer formation by several anthracene derivatives introduced as dopants in pyrene crystals. Photochemical reactions of some of these exciplexes and excimers are discussed. Excimer forming crystals are considered to be good candidates for excited state structure determination by means of pulsed x-ray diffraction.     

1.1](3,3')-azobenzenophane: novel crystal structure and cis-trans isomerization of distorted azobenzene

[formula: see text] [1.1](3,3')-Azobenzenophane, in which two azobenzenes are cyclically connected by -CH2- chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.