The role of the excited electronic states in the C+ + H2O reaction

The electronic excited states of the [COH2]+ system have been studied in order to establish their role in the dynamics of the C+ + H2O-->[COH]+ +H reaction, which is a prototypical ion-molecule reaction. The most relevant minima and saddle points of the lowest excited state have been determined and energy profiles for the lowest excited doublet and quartet electronic states have been computed along the fragmentation and isomerization coordinates. Also, nonadiabatic coupling strengths between the ground and the first excited state have been computed where they can be large. Our analysis suggests that the first excited state could play an important role in the generation of the formyl isomer, which has been detected in crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], but could not be explained in quasiclassical trajectory computations [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003); J. R. Flores, J. Chem. Phys. 125, 164309 (2006)].     

Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2?μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S(1) state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S(0)→S(1) transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T(1)→T(N) absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene's photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.     

A peculiar dependence of intersystem crossing of p-nitro-2,5-distyrylfuran on the dielectric properties of the solvent

The excited state behaviour of EE-2-(4'-nitrostyryl),5-styryl-furan (NSF) has been studied with different stationary and pulsed spectrometric techniques in solvents of different polarity and polarizability. The interpretation of previous results on the fluorescence and intersystem crossing of NSF [Phys. Chem. Chem. Phys., 2011, 13, 4519] was found to be complicated by an uncommon effect of solvent polarity on the competitive relaxation pathways. To answer the open questions, the photobehaviour was revisited in solvents with a restricted range of the dielectric constant, also under conditions of constant polarizability, and in a large temperature range. The results thus obtained, supported by parallel quantum-mechanical calculations on the singlet/triplet properties, allowed a reasonable interpretation of the photobehaviour to be reached. This includes the role of an activated photoisomerization above room temperatures and the play of small changes of dielectric properties of the solvent in favouring ICT, thus affecting the efficiency of the ISC process, where an "inverse" energy gap trend was found to be operative.     

Probes of the microenvironments of the cationic copolymers of styrene and vinylbenzenetrialkylammonium halides

The microenvironments of the cationic copolymers of styrene and vinylbenzenetrialkylammonium halides were explored by use of fluorescence spectroscopy. 5-Dimethylamino-1-naphthalenesulfonate (DANS) and 1-pyrenebutyrate (PB) were the fluorescent probes selected to bind to the polymers. The fluorescence energy of the former responds to the polarity or hydrophobicity of the microenvironment, whereas the absorption and fluorescence of the latter reveal the extent of ground-state and excited-state interactions. Polyelectrolyte coiling occurs in proportion to the fraction of binding sites occupied with charge-neutralizing, probe molecules. The bound DANS probe shows that coiling makes the binding-site environment more hydrophobic, and the bound PB probe shows that coiling facilitates excimer formation not only with nearest-neighbor pyrene moieties, but also with non-nearest neighbors. With methyl groups at the quaternary nitrogen binding sites, pyrene moiety interactions preceding excimer fluorescence occur in both ground and excited states. When the methyl groups are replaced with butyl or pentyl groups, pyrene excimers still form in the excited state, but the weak, hydrophobic interactions of the pyrene ground state decrease, because the longer alkyl groups serve as hosts for the hydrophobic pyrene moieties.     

Synthesis, characterization, and fluorescence behavior of twisted and planar B2N2-quaterphenyl analogues

A series of planar and twisted heteroaromatic quaterphenyl analogues containing BN ring linkages has been synthesized using primarily difunctional Lewis acidic diborabiphenyl moieties as molecular cores. Crystal structure analyses indicated the presence of large twist angles between adjacent aromatic rings in 1 and 3, which were also observed to possess nonfluorescent behavior due to a lack of molecular rigidity and insufficient B=N character in the excited state. In contrast, the incorporation of one or two bridging ethylene groups between the adjacent rings (installed via an ethynyl cycloisomerization) was found to afford planar phenanthrene or pyrene moieties, which resulted in weak fluorescence behavior (Phi F = 0.02-0.16) for the n-Bu and Ph derivatives 5-12. Emission colors ranged from green (lambda em = 521 nm) to red (lambda em = 630 nm) and depended primarily on the conformation (2,2'- vs 4,4'-), the extent of chromophore conjugation (phenanthrene vs pyrene), and the type of exocyclic substituent present (n-Bu vs Ph). Communication between the two phenanthrene or pyrene moieties was observed in some cases, which was characterized by bathochromically shifted emission bands relative to that of monomeric phenanthrene or pyrene species. Unique excited-state dimer (excimer) fluorescence was observed for the 2,2'-isomer 8, which was characterized by broad, low-energy emission bands bathochromically shifted from that of the corresponding monomer.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

The structure of 5-cyanoindole in the ground and the lowest electronically excited singlet states, deduced from rotationally resolved electronic spectroscopy and ab initio theory

The structure and electronic properties of the electronic ground and the lowest excited singlet states of 5-cyanoindole (5CI) were determined using rotationally resolved spectroscopy of the vibrationless electronic origin of 5CI. In contrast to most other indole derivatives, the lowest excited state of 5CI is determined to be of L(a) character. The conventional approximate coupled cluster singles and doubles model (CC2) fails to describe the geometry of the excited state correctly. Nevertheless, scaling the spin components of equal and opposite spins within the CC2 model as proposed by Hellweg et al. (Phys. Chem. Chem. Phys., 2008, 10, 1159) resulted in very good geometry parameters for the excited state.     

Photoreactivity and emission properties of liquid-crystalline twin dimers containing cinnamic acid moieties at both ends of ethylene glycol spacers. II. Emission properties

Emission properties of liquid-crystalline twin dimers containing 4-methoxy-cinnamic acid moieties at both ends of various ethylene glycol derivatives (nEGMC), where n denotes the number of the ethylene glycol units in the spacers, have been explored by steady-state and time resolved fluorescence spectroscopy in relation to the morphology of the system. Characteristic emissions were observed in the twin dimers, particularly in 2EGMC and 5EGMC, which were ascribed to excimer emission, and the energy gap between the locally excited state (monomer emission) and the excimer state and could be correlated to the difference in the spatial orientation of the mesogenic 4-methoxycinnamate groups adopted in nEGMC in the ground state. The picosecond time resolved fluorescence spectroscopy revealed no observed rise in the excimer emission in any sample at any temperature, indicating that the excimer is most probably formed by direct excitation of the pair wise arrangement of the 4-methoxycinnamate moieties in the ground state; thus, the excimer formation behaviour reflects well the ground state morphology of the twin dimers.     

ELECTRONICALLY EXCITED PRODUCTS IN THE PHOTODISSOCIATION OF Di-9-METHYLANTHRACENE

Abstract— Although the vast majority of photochemical reactions in condensed phase lead directly to product molecules in the ground states it might be expected that in favorable cases a certain fraction of the reacting species would escape deactivation long enough to attain the product configuration adiabatically in an electronically excited state. In this communication, we report that di-9-methylanthracene, in addition to its normal diabatic course, also leads to small but finite amounts of both excited singlet monomer and excimer species. Oxygen quenching experiments seem to indicate that the monomer and excimer are derived from excited di-9-methylanthracene with little mutual interconversion. The fluorescence yields were determined for these processes in aerated solution at room temperature to be φ₁= 0·00040 ± 0·00005 and φ₂= 0·00037 ± 0·00005 for monomer and excimer species, respectively.     

Fluorescence Emission from 2,6‐Naphthylene‐Bridged Mesoporous Organosilicas with an Amorphous or Crystal‐Like Framework

We report that 2,6‐naphthylene‐bridged periodic mesoporous organosilicas exhibit unique fluorescence behavior that reflects molecular‐scale periodicities in the framework. Periodic mesoporous organosilicas consisting of naphthalene–silica hybrid frameworks were synthesized by hydrolysis and condensation of a naphthalene‐derived organosilane precursor in the presence of a template surfactant. The morphologies and meso‐ and molecular‐scale periodicities of the organosilica materials strongly depend on the synthetic conditions. The naphthalene moieties embedded within the molecularly ordered framework exhibited a monomer‐band emission, whereas those embedded within the amorphous framework showed a broad emission attributed to an excimer band. These results suggest that the naphthalene moieties fixed within the crystal‐like framework are isolated in spite of their densely packed structure, different from conventional organosilica frameworks in which only excimer emission was observed for both the crystal‐like and amorphous frameworks at room temperature. This key finding suggests a potential to control interactions between organic groups and thus the optical properties of inorganic/organic hybrids.     

Comment on "Excitations in photoactive molecules from quantum Monte Carlo" [J. Chem. Phys. 121, 5836 (2004)

It is shown that the qualitative differences between high-level ab initio calculations and restricted open-shell Kohn-Sham (ROKS) results for the lowest singlet excited electronic state of formaldimine along a particular isomerization path found by Schautz, Buda, and Filippi [J. Chem. Phys.121, 5836 (2004)] play a minor role in molecular dynamics simulations of photoisomerization at room temperature. In fact, ROKS yields, within its well-known limitations, a good representation of the physically relevant isomerization pathway.     

Do coupling exciton and oscillation of electron-hole pair exist in neutral and charged pi-dimeric quinquethiophenes?

Optical physical properties of neutral and charged quinquethiophene monomer, and neutral and cationic pi-dimeric quinquethiophenes were investigated with density functional theory as well as the two dimensional (2D) site (transition density matrix) and three dimensional (3D) cube (transition density and charge difference density) representations, stimulated by the recent experimental report [T. Sakai et al., J. Am. Chem. Soc. 127, 8082 (2005)]. Transition density shows the orientation and strength of the transition dipole moment of neutral and charged quinquethiophene monomer, and charge difference density reveals the orientation and result of the charge transfer in neutral and charged quinquethiophene monomer. To study if coupling exciton and oscillation of electron-hole pair exist in neutral and cationic pi-dimeric quinquethiophenes, the coupling constants J (coupling exciton of electron-hole pair) and K (coupling oscillation of electron-hole pair) were introduced to the exciton coordinate and momentum operators, respectively, and the 2D and 3D analysis methods were further developed by extending our previous theoretical methods [M. T. Sun, J. Chem. Phys. 124, 054903 (2006)]. With the new developed 2D and 3D analysis methods, we investigated the excited state properties of neutral and cationic pi-dimeric quinquethiophenes, especially on the coupling exciton and oscillation of electron-hole pair between monomers. The 2D results show that there is neither coupling exciton (J=0) nor oscillation (K=0) of electron-hole pair in neutral pi-dimeric quinquethiophenes. For some excited states of cationic pi-dimeric quinquethiophenes, there is no coupling exciton (J=0), but there is coupling oscillation (K not equal0); while for some excited states, there are both coupling exciton and coupling oscillator simultaneously (J not equal0 and K not equal0). The strength of transition dipole moments of pi-dimeric quinquethiophenes were interpreted with 3D transition density, which reveals the orientations of their two subtransition dipole moments. The 3D charge transition density reveals the orientation and result of intermonomer and/or intramonomer charge transfer. The calculated results reveal that excited state properties of neutral pi-dimeric quinquethiophene are significantly different from those of the cationic pi-dimeric quinquethiophenes.     

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.     

Syntheses and photophysical studies of cyclodextrin derivatives with two proximate anthracenyl groups

A series of permethylated cyclodextrin derivatives, cyclodextrin dimers doubly bridged with two anthracene moieties (An2CD2) and singly bridged with one (AnCD2) and the monomer bearing two anthracene moieties (An2CD), were newly synthesized. For An2CD2, the two isomeric forms are also identified. All compounds are soluble in both aqueous and various organic solvents. The bisanthracene systems, An2CD2 and An2CD, show the thermal equilibrium in an aqueous solution between the intramolecularly interacting (closed) and less-interacting (open) states of the anthracene moieties, which results in the temperature-dependent absorption changes. These systems also show the characteristic excimer emission that is enhanced in water and weakened in organic solvents. The excitation spectra for the monomer and excimer fluorescence are found to be quite different from each other and similar to the absorption spectra of the open and the closed forms, respectively. The observed unique parallelism between excitation and absorption spectra for the present excimer systems indicates the dual ground state-dual excitation scheme where the excitation state formed from the closed ground state mainly gives excimer. The fluorescence lifetime analyses reveal that the rates of the conversion from the excited state of the open form to that of the closed one (6.0 x 10(6) s(-1) for An2CD2-2) are largely retarded compared with that of the ethyleneoxy linked bisanthracene system (8.8 x 10(7) s(-1)).     

Intramolecular exciplex and intermolecular excimer formation of 1,8-naphthalimide-linker-phenothiazine dyads

Steady-state fluorescence spectra were measured for 1,8-naphthahlimide-linker-phenothiazine dyads (NI-L-PTZ, where L = octamethylenyl ((CH2)8) and 3,6,9-trioxaundecyl ((CH2CH2O)3C2H4)), NI-C8-PTZ and NI-O-PTZ, as well as the NI derivatives substituted on the nitrogen atom with various linker groups without PTZ as the reference NI molecule in n-hexane. Normal fluorescence peaks were observed at 367-369 nm in all NI molecules together with a broader emission around 470 nm, which is assigned to the excimer emission between the NI in the singlet excited state (1NI*) and the NI moiety of another NI molecule (1[NI/NI]*). In addition, a broad peak around 600 nm was observed only for NI-L-PTZ, which is assigned to an intramolecular exciplex emission between donor (PTZ) and acceptor (NI) moieties in the excited singlet state, 1[NI-L-NI]*. The formation of an intramolecular exciplex corresponds to the existence of a conformer with a weak face-to-face interaction between the NI and PTZ moieties in the excited state because of the long and flexible linkers. The excited-state dynamics of the NI molecules in n-hexane were established by means of time-resolved fluorescence spectroscopy.     

Influence of high pressure on optical impurity spectra

Expressions are derived for inhomogeneous band shapes of impurity spectra in highly compressed glassy matrices. Intermolecular guest-host interactions are approximated to isotropic two-body Lennard-Jones 6-12 potentials having different parameters in the ground state and the excited state. Calculated shifts and widths are compared to published values for the absorption spectra of phenanthrene, anthracene [B. Y. Okamoto and H. G. Drickamer, J. Chem. Phys. 61, 2870 (1974)], and several polymethine dyes [G. A. Samara et al., J. Chem. Phys. 37, 1482 (1962)] embedded in polymer hosts and subject to pressures up to 140 kbars. The magnitudes of barochromic shifts of the band maxima and the inhomogeneous broadening suggest that the equilibrium coordinates of the excited state are typically less by 5+/-2%.     

Multifunctional red phosphorescent bis-cyclometallated iridium complexes based on 2-phenyl-1,2,3-benzotriazole ligand and carbazolyl moieties

Multifunctional phosphorescent bis-cyclometallated iridium(III) complexes based on the 2-phenyl-1,2,3-benzotriazole moiety and bearing branched hole-transporting carbazole fragments were synthesized. The isolated compounds were found to be amorphous and expressed very good solubility. Introduction of flexible aliphatic chains of various lengths into the iridium complexes enabled manipulation of their glass transition temperature. The iridium complexes exhibited red phosphorescence emission at 650 nm with the lifetime of 5.7 μs and phosphorescence quantum yields of 0.22 and 0.17 in solution and solid state, respectively, at room temperature. The shielding effect of the carbazolyl moieties on the concentration quenching of phosphorescence of the iridium centers was found to result in the increased excited state lifetime and quantum yield due to the suppressed exciton migration. Non-optimized OLED devices, based on the phosphorescent bis-cyclometallated iridium(III) complex without host material were fabricated and their electroluminescence properties were evaluated.     

Theoretical description of femtosecond fluorescence depletion spectrum of molecules in solution

A theoretical model used for calculating the fluorescence depletion spectrum (FDS) of molecules in liquids induced by femtosecond pump-probe laser pulses is proposed based on the reduced density matrix theory. The FDS intensity is obtained by calculating the stimulated emission of the excited electronic state. As an application of the theoretical model, the FDS of oxazine 750 (OX-750) molecule in acetone solution is calculated. The simulated FDS agrees with the experimental result of Liu et al. [J. Y. Liu et al., J. Phys. Chem. A 107, 10857 (2003)]. The calculated vibrational relaxation rate is 2.5 ps(-1) for the OX-750 molecule. Vibrational population dynamics and wave packet evolution in the excited state are described in detail. The effect of the probe pulse parameter on the FDS is also discussed.     

Study on the luminescence and energy level of lanthanide ions in Lu(0.8)Sc(0.2)BO3 host

Emission and excitation spectra of undoped and Ln(3+) (Ln = Ce, Pr, Nd, Yb)-doped Lu(0.8)Sc(0.2)BO(3) were studied by vacuum ultraviolet spectroscopy, and the X-ray excited luminescence spectra were measured as well at room temperature. The undoped specimen presented two different emissions upon excitation at energies in the vicinity of the band gap or with X-ray. The emission at the highest energy side, located at 4.77 eV, was ascribed to self-trapped exciton, which is anchored by electron capture around the Sc site. We also observed three broad emission bands at 4.43, 3.02, and 2.10 eV, which might be attributed to the trapped exciton, defect -related emissions, or both. Energy transfer processes to the doped lanthanide ions in Lu(0.8)Sc(0.2)BO(3) via exciton state or sequential electron-hole capture are discussed. Finally, the energy level diagram for divalent and trivalent lanthanides in Lu(0.8)Sc(0.2)BO(3) was constructed using the obtained spectroscopic parameters and the three parameters method (Dorenbos, P. J. Phys.: Condens. Matter 2003, 15, 8417).     

Direct observation of excimer formation in anthracene and 9,9′-bianthryl

The existence of anthracene excimer in fluid solution was confirmed for the first time by observing the coincidence of the rise time of excimer fluorescence with the decay time of monomer fluorescence with the use of a picosecond laser, a streak camera, and a computer. The continuum model of diffusion theory is found to be applicable to the excimer formation and the encounter distance between anthracene in the excited state and that in the ground state is calculated to be 8 ± 2 Å. The anomalously broad featureless fluorescence observed for 9,9′-bianthryl in glycerol-methanol (9:1) solution was found to be emitted from the species formed only in the excited state and its rise time was found to be coincident with the decay time of fluorescence from anthracene moiety composing 9,9′-bianthryl, α,ω-9,9′-bianthrylpropane, -butane, -hexane, and -dodecane were found not to form excimer in the excited state.