Polymorphism-dependent fluorescence of 9,10-bis(pentafluorobenzoyloxy)anthracene

Anthracene derivatives possessing pentafluorobenzoyloxy moieties at 9- and 10-positions showed polymorphism affording two types of fluorescent crystals with blue and bluish green fluorescence in their crystalline state, respectively. Their single crystal X-ray structures showed that the degree of overlap of anthracene moieties was responsible for the difference in fluorescence. Fluorescence in the crystalline state originated in the dimer emission deduced from their excitation spectra.     

Time resolved fluorescence of anthracene in mixed crystals at 2K

An optical Kerr shuttered spectrograph has been used to time resolve the spontaneous fluorescence of aromatic mixed crystal systems at low temperature with moderate resolution. Transient effects on the fluorescence of anthracene in naphthalene excited with 614 cm^?1 vibrational excess energy in ¹B_2u have been observed that may signal measurable vibrational relaxation pathways. A model consistent with these observations is presented: it implicates a strong interaction between the intramolecular Franck—Condon and non-Franck—Condon modes in the relaxation process for specific excitation in the region of large excess lattice energy. Examination of the fluorescence for several aromatic systems integrated over the interval 0 to 30 ps following excitation high in the S₁ vibrational manifold failed to reveal evidence of non-Boltzmann vibrational distributions, although other largely unexplained effects have been observed.     

Singlet exciton trapping and heterofission in tetracene doped anthracene crystals

In tetracene doped anthracene, the magnetic field modulation of prompt tetracene fluorescence following excitation into the anthracene singlet manifold has been measured as a function of the magnetic field orientation and optical excitation energy. The results show that this modulation with low energy excitation is caused by singlet heterofission into one anthracene triplet exciton and one tetracene triplet. With higher excitation energies this modulation is due to both the singlet heterofission and also singlet homofission into a pair of anthracene triplet excitons. Heterofission occurs mainly from anthracene molecules next to a tetracene and competes with the singlet trapping. From the singlet trapping rate and from the magnetic modulation of tetracene prompt fluorescence the heterofission rate is estimated as ≈10^?11s^?1.     

Femtosecond fluorescence dynamics imaging using a fluorescence up-conversion microscope

Femtosecond fluorescence dynamics imaging microscopy was performed. Femtosecond fluorescence dynamics images were constructed based on the "mean" fluorescence decay or rise time constants that were evaluated by the time-resolved intensity sampling using a fluorescence up-conversion microscope. This dynamics imaging microscopy was carried out for the organic microcrystals alpha-perylene and tetracene-doped anthracene microcrystal, and ultrafast dynamics in the organic microcrystals were clearly imaged in the two-dimensional manner. For the alpha-perylene microcrystal, the obtained dynamics images showed that the crystal edges exhibited relatively shorter free exciton and the Y-state lifetimes compared to the crystal center, reflecting the higher concentration of defects. For the tetracene-doped anthracene microcrystal, the image was constructed based on the time constant of excitation energy transfer from anthracene to tetracene. By experiments changing the doping ratio of tetracene in anthracene, it was concluded that the inhomogeneity observed in the dynamics image arises from the difference in the local concentration of tetracene in the mixed crystal.     

The 4000 Å transition of crystal anthracene

New low temperature determinations of ?₂(ω) for crystal anthracene locate the 0-0 transition origin at 25096 ± 6 cm^?1 for the b-polarization. There is no Stokes shift of the fluorescence origin. The exciton bands which are optically accessible by probing the (ab) crystal face with normal incidence light polarized either parallel or perpendicular to the b-axis have a temperature dependent splitting. The Davydov splitting at 77°K is 267 ± 12 cm^?1. The temperature dependence of the k = 0 polariton splitting and the exciton bandwidths (b-polarization and a-polarization) are also given and together with the Davydov splitting provide a test of the adequacy of theories of exciton interactions. The polarization ratio measured as the ratio of the oscillator strengths of the two transitions (f_b/f_a) is quite temperature dependent for both the region of the (0-0) transition and over the total vibronic transition. At 77°K, the value of f_b/f_a is 8.0 for the origin region close to that expected of an oriented gas but is 4.5 over the total transition. Some spectral structure observed only in the b-polarized reflectance spectrum at temperatures less than 77°K is related to the existence of defects localized near the surface.     

Piezomodulated reflection spectra of anthracene single crystals

The first piezomodulation spectra of Frenkel exciton states of a molecular crystal are reported for the (001) face of anthracene from 25 000 cm^?1 to 45 000 cm^?1. The piezoreflection spectra show structure at 300 K which may be correlated with that observed in the specular reflection spectra at 2 K. Davydov splittings at 300 K of 165 cm^?1 for 0–0 and 50 cm^?1 for 0–1 are observed.     

Fluorescence as probe for transport and lateral order in vapor grown Langmuir-Blodgett(LB) multilayer-type single crystals of anthracene chromophores with fatty acid tails

Fluorescence spectra of anthracene moieties with fatty acid tails attached at the 2-position are reported for Langmuir-Blodgett(LB)-type multilayer configurations obtained as single crystals in the gas phase. These systems are considered the best ordered experimental model systems for chromophores with fatty acid tails arranged in the LB multilayer configuration. Closest agreement of the fluorescence spectrum with that of the unsubstituted anthracene crystal is found for the 4-(2-anthryl)-1-butanoic acid (2A4) crystal. Packing of the anthracene moieties is very similar in both crystals. Low-temperature spectra (10 K) reveal strong broadening of the peaks due to spatial disorder in the 2A4 crystal. 7-(2-Anthryl)-1-heptanoic acid (2A7) crystals and even more so actual 2A7 LB multilayers have less structured fluorescence spectra that extend further to lower energies. Picosecond time resolved fluorescence decay curves show exciton transport and trapping in low energy traps. Compressed ground state pairs with excimer-type spatial configurations are excited directly in the red (476 nm) to emit structureless excimer-type fluorescence.     

Energy transfer in unsymmetrical phenylene ethynylene dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.     

Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

Electric-field modulated absorption spectra of polycrystalline anthracene layers delineate existence of a series of five charge-transfer bands that can be assigned to transitions within the ab crystal plane. The energy versus distance relationship is coulombic (e = 3.2) yielding an optical band gap E_g^opt = 4.4±0.05 eV. Absorption coefficients are about one order of magnitude lower than calculated by Bounds et al. and indicate a coupling constant A ～ 0.15 for interaction between CT and Frenkel exciton states. Previous data for intrinsic free-carrier production, in particular the energy dependence of the “thermalization” distance, can be consistently interpreted in terms of dissociation of CT pairs if the assumption is made that the vibrational CT energy (0.3 eV) can fully or in part be used for additional separation of the electron-hole pair. The adiabatic (electrical) band gap is E_g^e1 = 4.1±1 eV.     

Nonlinear polariton effects in the phenanthrene crystal

Two-photon fluorescence excitation (TPE) and second harmonic generation (SHG) spectra of the origin region of the 3500 Å system of the phenanthrene crystal at 2 K are presented and compared for both melt-grown and sublimed crystals. The dependence of the upper Davydov component position on the photon polarization and propagation direction reported previously is confirmed for both TPE and SHG- This behavior is considered to be a manifestation of phase matching to the upper branch of the b-polariton dispersion curve and is intrinsic to the two-photon process for transitions which are allowed in both one- and two-photon absorption. Both TPE and SHG are understood as resulting from the fundamental process of polariton fusion. The appearance of the dipole-forbidden lower Davydov component in these spectra is commented on in terms of misalignment and excitation of a longitudinal exciton.     

Self-trapping limited exciton diffusion in a monomeric perylene crystal as revealed by femtosecond transient absorption microscopy

Self-trapping and singlet-singlet annihilation of the free excitons in a monomeric (beta) perylene crystal were studied by using femtosecond transient absorption microscopy. The free exciton generated by the photo-excitation of the beta-perylene crystal relaxed to the self-trapped exciton with a rate constant of 7 x 10(10) s(-1). The singlet-singlet annihilation of the free exciton observed under the high excitation density conditions was competed with the self-trapping of the free exciton; we estimated the annihilation rate constant for the free exciton to be 1 x 10(-8) cm(3) s(-1) from the excitation density dependence of the free exciton decay. After self-trapping of the free exciton, no annihilation was observed in the 100 ps time range, suggesting that the diffusion coefficient was reduced drastically by self-trapping. The results show that the major factor limiting the exciton diffusion in the beta-perylene crystal is a relaxation of the free exciton to the self-trapped exciton, and not the lifetime of the exciton. Though the singlet-singlet annihilation rate constants and fluorescence lifetime of the beta-perylene crystal are similar to those of the anthracene crystal, the estimated exciton diffusion length (2 nm) in the beta-perylene crystal is much smaller than that (100 nm) in the anthracene crystal as a result of the exciton self-trapping.     

Eosin y triplet state as a probe of spatial heterogeneity in microcrystalline cellulose

The photophysical behavior of eosin Y adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy, steady-state fluorescence spectroscopy and laser induced time-resolved luminescence. On increasing the concentration of the dye, small changes in absorption spectra, fluorescence redshifts and fluorescence quenching are observed. Changes in absorption spectra point to the occurrence of weak exciton interactions among close-lying dye molecules, whereas fluorescence is affected by reabsorption and excitation energy trapping. Phosphorescence decays are concentration independent as a result of the negligible exciton interaction of dye pairs in the triplet state. Lifetime distribution and bilinear regression analyses of time-resolved phosphorescence and delayed fluorescence spectra reveal the existence of two different environments: long-lived, more energetic triplet states arise from dyes tightly entrapped within the cellulose chains, while short-lived, less-energetic states result from dyes in more flexible environments. Stronger hydrogen bond interactions between the dye and cellulose hydroxyl groups lead in the latter case to a lower triplet energy and faster radiationless decay. These effects, observed also at low temperatures, are similar to those encountered in several amorphous systems, but rather than being originated in changes in the environment during the triplet lifetime, they are ascribed in this case to spatial heterogeneity.     

Fluorescence spectra of naphthalene-perdeuteronaphthalene mixed crystals - energy transfer via guest excitons

The fluorescence spectra of the mixed crystal system naphthalene in perdeuteronaphthalene at concentrations between 0.1 and 50% have been measured with high spectral resolution. Up to 10% the spectra are superpositions of the spectra of monomers, pairs, trimers and some higher aggregates. The relative intensities of these individual aggregate spectra given evidence for trap to trap energy transfer without thermal excitation into the host exciton band. The monomers as the most abundant traps form a dilute exciton band 50 cm^?1 below the host band. By thermal activation into this dilute band energy is transferred between the aggregate states. In the 50% crystal emission from a mixed guest-host exciton band without individual clusters is observed.     

Excited state properties of novel p- and n-type organic semiconductors with an anthracene unit

Photoinduced dynamics of novel p- and n-type organic semiconductors with an anthracene unit are theoretically investigated with quantum chemistry methods. The calculated vertical absorption and fluorescence frequencies of them are consistent with the experimental data. The changing tendencies of the dihedral angles between anthracene unit and trifluoromethylphenyl (or thiophene) in the photoinduced dynamics processes (vertical absorption and vertical fluorescence) are examined from the geometries of optimized ground and excited states. To study the influence of the individual units of the derivatives to the excited state properties of the derivatives, the energies and densities of frontier orbital HOMOs and LUMOs of the individual unit and the derivatives are studied in the processes of vertical absorption and fluorescence. The excited state properties of the two derivatives in the processes of vertical absorption and fluorescence are studied with 2D and 3D real space analysis methods, which are employed to study the electron–hole coherence and the excitation delocalization (with transition density matrix method), and charge and energy transfer (with transition and charge difference density method). Overall, the computed results remain in good agreement with the relevant experimental data, and the theoretical results promote deeper understanding to the optical and electronic properties of the semiconductor in the process of photoinduced dynamics.     

Fluorescence from the second excited state of an anthracene crystal observed by two-step excitation

Weak fluorescence from the second excited state of an anthracene crystal has been observed by two-step excitation via the lowest excited state. Relaxation processes of a highly excited state to the lowest state are discussed.     

The spectrofluorqmetric determination of anthracene, fluorene and phenanthrene in mixtures

A procedure is presented for the spectrofluorometric determination of mixtures of anthracene, fluorene and phenanthrene. The determination depends on differences in fluorescence emission spectra and on selective excitation of anthracene fluorescence. Some of the fluorescence and absorption spectra involved overlap, but these difficulties can be overcome by empirical corrections. The average relative error in this method is less than 5 % over the concentration range 0 to 5 ppm.     

Resonance Raman scattering from triplet states of acridine and anthracene

By preparing a population of triplet states through excitation with a laser pulse of 308 nm, we could determine the resonance Raman spectra of acridine and of anthracene in their lowest (π π*) state, T₀. Thirty Raman lines in the region between 200 and 2000 cm^?1 are reported for acridine in the T₀ state. These are attributed to either a₁ or b₂ vibrational modes. Nineteen Raman lines between 500 and 2000 cm^?1 are reported for anthracene in the T₀ state.     

Intramolecular energy migration and transfer in macromolecules. A fluorescence depolarization approach

Intramolecular energy migration and trapping by acceptor species which are an integral part of a macromolecule have been studied in three polymer systems using the techniques of fluorescence depolarization. The effect of energy migration produces depolarization of both donor and acceptor emissions. Copolymers of styrene with 1-vinylaphthalene or 2-phenyl-5-(p-vinyl)phenyloxazole have been studied. In addition, energy transfer to a terminal anthracene species has been studied in a polyvinyltoluene sample. When energy transfer occurs with unit efficiency, the depolarization of acceptor emission reflects the path length available to the migrating exciton. In cases of lesser transfer efficiency, the acceptor emission exhibits depolarization characteristics which reflect the distribution of migration lengths from the site of energy absorption.     

Polariton splittings of deuterated hexamethylenetetramine

The i.r. absorption spectra and the Raman scattering spectra of polycrystalline deuterated hexamethylenetetramine (HMTD) have been recorded. The longitudinal and transverse components of all the i.r. active F₂ modes below 1200 cm⁻¹ are assigned in the Raman spectra (except ω₂₀). The observed polariton splittings have been used to determine the static dielectric constant (2.66 ± 0.035) which is the same as that for the HMT crystal. As expected, isotopic substitution does not change this macroscopic data.     

A study on the photophysical properties of anthracene in zeolite

The photophysical behaviours of anthracene in zeolite Y were investigated using various steady state spectroscopic methods and a picosecond time-resolved fluorescence spectroscopic technique. A ¹²⁹Xe nuclear magnetic resonance (NMR) study shows that anthracene molecules can be optimally intercalated into the supercages of Na⁺- and Cu²⁺-exchanged zeolite Y at 570 and 520 K respectively. Anthracene introduced into the supercage of Na⁺-exchanged zeolite Y shows excimeric fluorescence with a lifetime of several nanoseconds, as well as monomeric fluorescence with a lifetime of 300 ps. The excimer forms immediately on absorption of a photon. Cu²⁺-exchanged zeolite Y with anthracene intercalation shows electron spin resonance (ESR) signals and absorption bands attributable to the stable, ground state anthracene cation radical, the Cu⁺ ion and the trapped electron in the copper ion cluster, indicating charge transfer from anthracene to the Cu²⁺ ion or copper ion cluster. The excitation energy of anthracene is quenched rapidly by the presence of copper ion.