Charge resonance character in the charge transfer state of bianthryls: effect of symmetry breaking on time-resolved near-IR absorption spectra

We study the effects of symmetry breaking on the photogenerated intramolecular charge transfer (CT) state of 9,9'-bianthryl (BA) with femtosecond time-resolved near-IR spectroscopy. The time-resolved near-IR spectra are measured in acetonitrile for a symmetric substituted derivative of 10,10'-dicyano-9,9'-bianthryl (DCBA) and asymmetric substituted derivatives of 10-cyano-9,9'-bianthryl (CBA) and 9-(N-carbazolyl)anthracene (C9A), as well as nonsubstituted BA. The transient near-IR absorption spectrum of each compound at 0 ps has a locally excited (LE) absorption band, which agrees with the transient absorption band of the corresponding monomer unit. At 3 ps after the photoexcitation, the symmetric compounds show a broad charge transfer (CT) absorption band, whereas no absorption peak appears in the spectra of the asymmetric compounds. The broad CT absorption at 1250 nm only observed for the symmetric compounds can be attributed to the charge resonance transition associated with two equivalent charge separated states.     

Ultrafast excited-state dynamics of rhenium(I) photosensitizers [Re(Cl)(CO)3(N,N)] and [Re(imidazole)(CO)3(N,N)]+: diimine effects

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)(3)(N,N)](n) (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650-285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (～100 fs)(-1) than in imidazole (～150 fs)(-1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV-vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N(?-) → Re(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy(?-)) transitions. For phen and dmp, the UV excited-state absorption occurs at ～305 nm, originating from a series of mixed ππ* and Re → CO;N,N(?-) MLCT transitions. UV-vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1-5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (～1 and 6-10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.     

Electronic excited States of polynucleotides: a study by electroabsorption spectroscopy

Electroabsorption spectra were obtained for single-stranded polynucleotides poly(U), poly(C), poly(A), and poly(G) in glycerol/water glass at low temperature, and the differences in permanent dipole moment (Deltamu) and polarizability (Deltaalpha) were estimated for several spectral ranges covering the lowest energy absorption band around 260 nm. In each spectral range, the electrooptical parameters associated with apparent features in the absorption spectrum exhibit distinct values representing either a dominant single transition or the resultant value for a group of a relatively narrow cluster of overlapping transitions. The estimated spacing in energy between electronic origins of these transitions is larger than the electronic coupling within the Coulombic interaction model which is usually adopted in computational studies. The electroabsorption data allow us to distinguish a weak electronic transition associated with a wing in polynucleotide absorption spectra, at an energy below the electronic origin in absorption spectra of monomeric nucleobases. In poly(C) and poly(G), these low-energy transitions are related to increased values of Deltamu and Deltaalpha, possibly indicating a weak involvement of charge resonance in the respective excited states. A model capable of explaining the origin of low-energy excited states, based on the interaction of pipi* and npi* transitions in neighboring bases, is introduced and briefly discussed on the grounds of point dipole interaction.     

Photochromogenic nanosheet crystallites of tungstate with a 2D bronze structure

Layered rubidium tungstate, Rb(4)W(11)O(35), with a two-dimensional (2D) bronze-type tunnel structure was successfully delaminated into colloidal nanosheets via a soft-chemical process involving acid exchange and subsequent intercalation of tetrabutylammonium ions. Characterizations by transmission electron microscopy and atomic force microscopy confirmed the formation of unilamellar 2D nanosheet crystallites with a unique thickness of ～3 nm and an average lateral size of 400 nm. The obtained nanosheets exhibited reversible color change upon UV-light excitation via an optical band gap of 3.5 eV. The ultimate 2D aspect ratio favorable for an adsorption of charge-compensating cations to trapped electrons working as a color center is presumably responsible for highly efficient photochromic behavior. Its coloration mainly consists of a broad band at a wavelength of 1800 nm and longer, which is much different from that of the common tungstate nanomaterials. Thus, the chromogenic nanosheet obtained in this study features the intense UV absorption and optically switchable visible-to-IR absorption, which may be useful for window applications such as cutoff filters and heat-absorbing films.     

Effects of organic and aqueous solvents on the electronic absorption and fluorescence of chloroaluminum (III) tetrasulphonated naphthalocyanine

The effects of various solvents on the ground and excited states of chloroaluminum (III) tetrasulphonated naphthalocyanine (AlNcS(4)) were studied. Both the absorbance and fluorescence spectra were found to be influenced by the hydrogen bond donating ability of various solvents. As the hydrogen bond donating ability of the solvent increased, hypsochromic and bathochromic shifts in the absorbance and fluorescence spectra were observed in protic and aprotic solvents respectively. Plots of the absorbance and fluorescence maxima versus the E(T)(30) solvent parameter showed linear relationships in binary mixtures of protic-protic (methanol-H(2)O) and aprotic-protic (DMSO-H(2)O) solvents. Aggregation was indicated by a broad band in the ground state absorption spectra and a low quantum efficiency 0.04 relative to the efficiency observed in organic solvents. A face-to-face conformation of the monomeric subunits of the dimer is suggested due to the red-shifted absorbance band. The acid-base properties of the dye were studied and were indicative of a multi-step process. In acidic conditions (pH 1), protonation of the bridging nitrogen atoms was identified by a broad band appearing red-shifted to those obtained at higher pH values. Under slightly acidic conditions a pKa value of 6.7 was determined for one of the meso-nitrogen. In alkaline conditions a pKa of 11.5 was determined for another meso-nitrogen and a second fluorescence band emerged at 804 nm, red-shifted to the emission maxima.     

Spectroscopic measurements of biphenylene singlet states

We report the observation of fluorescence from higher singlet levels of biphenylene. The molecule was excited by either two- or three-photon absorption and the resulting fluorescence to the ground state was observed in a broad band centered near 265 nm. We have measured the decay time of the lowest excited singlet state to be 240 ± 20 ps in hexane at 300 K.     

Effect of strong coupling on interfacial electron transfer dynamics in dye-sensitized TiO2 semiconductor nanoparticles

Dynamics of interfacial electron transfer (ET) in ruthenium polypyridyl complex [{bis-(2,2′-bpy)-(4-[2-(4′-methyl-[2,2′]bipyridinyl-4-yl)-vinyl]-benzene-1,2-diol)}ruthenium(II) hexafluorophosphate] (Ru-cat) and 5,10,15-tris phenyl-20-(3,4-dihydroxy benzene) porphyrin (TPP-cat)-sensitized TiO₂ nanoparticles have been investigated using femtosecond transient absorption spectroscopic detection in the visible and near-infrared region. We have observed that both Ru-cat and TPP-cat are coupled strongly with the TiO₂ nanoparticles through their pendant catechol moieties. We have observed a single exponential and pulse-width limited (<100 fs) electron injection from nonthermalized-excited states of Ru-complex. Here electron injection competes with the singlet-triplet manifold relaxation due to strong coupling of catecholate binding, which is a unique observation. Optical absorption spectra indicate that the catechol moiety interacts with TiO₂ nanoparticles showing the characteristic pure catechol-TiO₂ charge-transfer (CT) band in the visible region. Transient absorption studies on TPP-cat/TiO₂ system exciting both the Soret band at 400 nm and the Q-band at 800 nm have been carried out to determine excitation wavelength-dependence on ET dynamics. The reaction channel for the electron-injection process has been found to be different for both the excitation wavelengths. Excitation at 800 nm, is found directly populate directly the excited CT state from where diffusion of electrons into the conduction band takes place. On the other hand, excitation at 400 nm light excites both the CT band of cat-TiO₂ and also Soret band of TPP-cat.     

Femtosecond time-resolved photophysics of 1,4,5,8-naphthalene diimides

The photophysical properties of a tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds were investigated. The absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500 and 650 nm. This band is sensitive to the polarity of the solvent and is attributed to a CT state. The absorption spectra and lifetime (10+/-2 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5-1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran glass.     

Spectroscopic effects of excited-state coupling in a tetragonal chromium(III) complex

Detailed low-temperature single-crystal polarized absorption and luminescence spectra of Cs2[CrCl2(H2O)4]Cl3 are reported. The luminescence spectrum is a broad band with a maximum at 11,800 cm (-1), indicating that the trans-[CrCl2(H2O)4]+ complex emits from a quartet excited state. The resolved vibronic structure reveals a progression in a nontotally symmetric 445 cm (-1) b1g mode, a manifestation of a Jahn-Teller effect in the emitting state. The absorption spectrum shows completely linearly polarized, magnetic-dipole-allowed electronic origins, defining the tetragonal splitting of the states originating from 4T2g (Oh). An energy gap of approximately 800 cm (-1) is observed between the electronic origins of the emitting state and the onset of the pi-polarized absorption spectrum. Both Jahn-Teller and spin-orbit couplings in the orbitally degenerate 4Eg (D4h) state are necessary to account for the spectroscopic observations.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Fluorescence of the DNA double helix (dA)20 x (dT)20 studied by femtosecond spectroscopy--effect of the duplex size on the properties of the excited states

The fluorescence of the DNA double-stranded oligomer (dA)20 x (dT)20 is studied at room temperature by fluorescence up-conversion at times shorter than 10 ps. The profile of the up-conversion spectra is similar to that of the steady-state fluorescence spectrum, showing that the majority of the photons are emitted within the probed time scale. At all the probed wavelengths, the fluorescence decays are slower than those of the monomeric chromophores dAMP and TMP. The fluorescence anisotropy decays show strong wavelength dependence. These data allow us to conclude that energy transfer takes place in this double helix and that this process involves exciton states. The spectral and dynamical properties of the oligomer are compared to those of the polymer poly(dA) x poly(dT), composed of about 2000 base pairs, reported previously. The oligomer absorption spectrum is characterized by a smaller hypsochromic shift and weaker hypochromism compared to the polymer. Moreover, the fluorescence decays of (dA)20 x (dT)20 are twice as fast as those of poly(dA) x poly(dT), and its fluorescence anisotropy decays more slowly. These differences are the fingerprints of a larger delocalization of the excited states induced by an increase in the size of the duplex.     

Electronic and vibrational spectra of diphenylmethane

From the critical analyses of Raman and infrared spectra, different normal modes of vibration of diphenylmethane (DPM) have been identified. The near ultraviolet absorption spectra of the molecule are found to consist of two band systems, one around 220 nm and the other around 270 nm with respective f-values 5.23 x 10(-2) and 6.44 x 10(-3). The first system is broad and shows few diffuse structures, whereas the later one exhibits very well-resolved structure. They are respectively assigned as 1L(a) and 1L(b) bands. The Raman excitation profiles of several normal modes have been analyzed to get structural and other information of different excited electronic states.     

Photophysical properties of ligand localized excited state in ruthenium(II) polypyridyl complexes: a combined effect of electron donor-acceptor ligand

We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)(2)(L(1)), (2) Ru(II)(bpy)(2)(L(2)) and (3) Ru(II)(bpy)(L(1))(L(2)), where bpy = 2,2'-bipyridyl, L(1) = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L(2) = 4-(N,N-dimethylamino-phenyl)-(2,2'-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe(2) functionality in L(2) and electron withdrawing catechol fragment in L(1) ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the (3)MLCT state by the low-emissive (3)LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited (3)MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the (3)MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ～200 ps and ～700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes.     

Photocurrent generation from semiconducting manganese oxide nanosheets in response to visible light

Unilamellar nanosheet crystallites of manganese oxide generated the anodic photocurrent under visible light irradiation (lambda < 500 nm), while the nanosheets themselves were stable as revealed by in-plane XRD and UV-visible absorption spectra. The band gap energy was estimated to be 2.23 eV on the basis of the photocurrent action spectrum. The molecular thickness of approximately 0.5 nm may facilitate the charge separation of excited electrons and holes, which is generally very difficult for strongly localized d-d transitions. The monolayer film of MnO2 nanosheets exhibited the incident photon-to-electron conversion efficiency of 0.16% in response to the monochromatic light irradiation (lambda = 400 nm), which is comparable to those for sensitization of monolayer dyes adsorbed on a flat single-crystal surface. The efficiency declined with increasing the layer number of MnO2 nanosheets, although the optical absorption was enhanced. The recombination of the excited electron-hole pairs may become dominant when the carriers need to migrate a longer distance than 1 layer through multilayered nanosheets.     

Quantum chemical studies on structures and spectra of 2,5-distyrylpyrazine (DSP) laser dye

Semiempirical (MNDO and PM3) molecular orbital calculations have been undertaken to study the structures of the ground and excited states of 2,5-distrylpyrazine dye to assess its activity as a laser dye. In the ground and first excited singlet states, the trans-trans structure of C_2h symmetry is the most stable structure in the gas phase and in DMSO, which agrees with the experimental findings. Upon excitation, the flexibility of the molecule decreases, leading to a subsequent decrease in the radiationless deactivation pathway and this increases the fluorescence efficiency of DSP. The absorption, excitation, and emission spectra have been calculated at the MNDO level using the PM3 optimized geometries in DMSO. At this level the agreement between theory and experiment is quite good. An estimated absorption band at 377 nm (expt 380 nm) is assigned to the S₀→S₁ transition. The excited state absorption band at 457 nm (expt 460 nm) is assigned to the S₁→S₁₂ transition. The emission band at 458 nm (expt 460 nm) is assigned to the S′₁→S′₀ transition. The overlap between the emission and the excited-state absorption spectra is presumably the main reason behind the reduced laser activity of the investigated dye. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 585–592, 1998     

Electronic band structure of titania semiconductor nanosheets revealed by electrochemical and photoelectrochemical studies

Electrochemical and photoelectrochemical studies were conducted on self-assembled multilayer films of titania nanosheets on a conductive ITO substrate. Cyclic voltammogram (CV) curves indicated that the titania nanosheet electrode underwent insertion/extraction of Li(+) ions into/from the nanosheet galleries, associated with reduction/oxidation of Ti(4+)/Ti(3+). These processes accompanied reversible changes in UV-vis absorption of the titania nanosheet electrodes. Applying a negative bias of -1.3 V (vs Ag/Ag(+)) and lower brought about absorption reduction where the wavelength is shorter than 323 nm, and vice versa, indicating a flat-band potential of (approximately) -1.3 V and a band gap energy of 3.84 eV. Photocurrents were generated from the titania nanosheet electrodes under a positive bias. The onset potential for photocurrent generation from the titania nanosheet electrodes was around -1.27 V, and the band gap energy estimated from the photocurrent action spectra was 3.82 eV, in excellent agreement with the values obtained from the spectroelectrochemical data. The lack of difference in the band gap energies for titania nanosheet electrodes with different numbers of layers suggests that a nanosheet is electronically isolated in multilayer assemblies without affecting the electronic state of neighboring nanosheets. Similar measurements on the anatase-type TiO(2) electrode revealed that the lower edge of the conduction band for the titania nanosheet is approximately 0.1 V higher than that for anatase, while the upper edge of the valence band is 0.5 V lower.     

Exploring concerted effects of base pairing and stacking on the excited‐state nature of DNA oligonucleotides by DFT and TD‐DFT studies

We have taken (dA)₅, (dT)₅, and (dA)₅·(dT)₅ as model systems to study concerted effects of base pairing and stacking on excited‐state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial A → A charge‐transfer nature in the A·T oligonucleotides. The T → T charge‐transfer transitions produce dark states, which are hidden in the energy region of the steady‐state absorption spectra. This is different from the previous assignment that the T → T charge‐transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The A → T charge‐transfer states were predicted to have relatively high energies in the A·T oligonucleotides. The present calculations predict that the T → A charge‐transfer states are not involved in the spectra and excited‐state dynamics of the A·T oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the ¹nπ* and ¹ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Photoexcitation of dimethyl sulfide and dimethyl disulfide in the vacuum ultraviolet region: Rydberg states and photofragment emissions

Photoabsorption cross sections and fluorescence excitation spectra of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) vapors have been studied in the 110–220 nm region using synchrotron radiation. For DMS, a new Rydberg series originating from the n_s orbital is identified. A number of broad bands from DMDS are assigned as Rydberg transitions. Emissions from DMS and DMDS are assigned as the CH₃( - ) band. For DMDS, another emission which is attributable to the S₂(B-X) band appears in the excitation below 125 nm. Photodissociation processes forming the excited fragments are discussed.     

Excitation and emission spectra of polyethylene terephthalate and polyethylene 2,6‐naphthalate films

With excitation by the light of the wavelengths longer than 320 nm, fluorescence spectra of polyethylene terephthalate (PET) films had somewhat different shapes from those excited below 300 nm through the intrinsic absorption of PET molecules. Also, in the measurements taken with a polarizer in front of the receiving monochoromator but none before the sample, the intensity ratio of parallel (to the draw axis of the film) and perpendicular components of the emission spectra was different if excited above 320 nm or below 300 nm. Discussion was made about the first step of pumping photon energy at the wavelengths above 320 nm. Fluorescence spectra of polyethylene 2,6‐naphthalate films showed a mirror image of their absorption spectra, consisting of one broad band having the same polarization as their absorption spectra. Their fluorescence occurred from the lowest excited level, conforming to the Kasha law. Copyright © 1999 John Wiley & Sons, Ltd.