Optical Charge Transfer Transitions Research Articles

Circular Photoinduced Electron Transfer in a Donor-Acceptor-Acceptor Triad.

An electron-donor-acceptor-acceptor (D-A1 -A2 ) triad has been developed that provides the first proof-of-concept for a photoinitiated molecular circuit. After photoexcitation into an optical charge-transfer transition between D and A1 , subsequent thermal electron-transfer from A1.- to A2 is followed by geometric rearrangement in the D.+ -A1 -A2.- charge-separated state to form an ion-pair contact. This facilitates "forward" charge recombination between A2.- and D.+ to complete the molecular circuit with an estimated quantum efficiency of 4 % in toluene at 298 K.

Effect of the donor–acceptor properties of ligands on the spectroscopic and electrochemical properties of mixed-ligand complexes of Pt(II) and Ir(III) with cyclometalated 2-phenylbenzothiazole

The results of the spectroscopic NMR (1H, 13C, and 195Pt), infrared, optical, and voltammetric characteristics of the mixed-ligand complexes of Pt(II) and Ir(III) with metalated 2-phenylbenzothiazole and tert-butylisocyanide (tBuNC), acetonitrile (AN), ethylenediamine (En), O-ethyldithiocarbamate (Exn–), and diethyldithiocarbamate (Dtc–) ions are presented. It is demonstrated that the change in donor–acceptor interaction of ligands tBuNC, AN, En, Exn–, and Dtc– with metal leads to an increase in the energy of the highest occupied molecular orbital of the complexes and is accompanied by a shift of the cathode potential of the metal-centered oxidation, a bathochromic shift of the spin-allowed and spin-forbidden metal-tocyclometalated ligand optical charge transfer transitions, and an increase the degree of mixing of the 1MLCT and triplet intraligand states, responsible for the phosphorescence of the complexes.

Optical charge transfer transitions in supramolecular fullerene and porphyrin compounds

By the DFT method with the planar wave basis set and in the PAW approximation the geometric and electronic structures of four supramolecular compounds of porphyrin and fullerene molecules in the crystalline state are performed: H2TPP·C60·3 toluene, H2TpivPP·C60, H2T3,5-dimethylPP·C70·4 toluene, and NiT4-methylPP·2C70·2 toluene. The geometry is optimized using the PBE functional and the Grimme DFT-D2 dispersion interaction correction. The electronic structure and absorbance spectra are calculated using the HSE functional. It is shown that the H2TPP·C60·3 toluene structure having a sufficiently wide absorbance wavelength range, which results in a photoinduced electron transition from the higher occupied states formed by porphyrin molecules to the lower unoccupied states formed by fullerene molecules, is most promising for the design of photogalvanic elements.

Structure, optical and electrochemical properties of binuclear complexes with platinated 2-phenylbenzothiazol and bridging 2-mercapto-derivatives of pyridine, pyrimidine, benzothiazole, and benzoxazole

Binuclear [Pt(bt)μ-(N^S)]2 complexes (bt–is the deprotonated form of 2-phenylbenzothiazole, (N^S)– = 2- mercaptopyridineate, 2-mercaptopyrimidineate, 2-mercaptobenzothiazoleate, 2-mercaptobenzoxazoleate ions) are characterized by the following methods: X-ray crystallography, 1Н and 195Pt NMR spectroscopy, electronic absorption and emission spectroscopy, cyclic voltammetry. The occurrence of the Pt–Pt chemical bond and complexes in both crystalline state and solution in the form of cis-N(bt),S-binding isomers with the asymmetrical arrangement of two cyclometalated and two bridging ligands is shown. Long-wavе absorption bands and luminescence of the complexes in the red spectral range are due to spin-allowed and spin-forbidden optical charge transfer transitions from the metal–metal bond to the metalated ligand. In cyclic voltammetry, two-electron redox waves of the complexes are attributed to metal- and ligand-centered processes involving σ* HOMO and π*(bt) LUMO of the complexes.

ChemInform Abstract: Substituent Effects on the Electronic Properties of Complexes with Dipyridophenazine and Triazole Ligands: Electronically Connected and Disconnected Ligands

AbstractReview: 73 refs.

Optical and luminescence spectroscopy studies of electronic structure of Li6GdB3O9 single crystals

This article presents the study of electronic structure of Li6GdB3O9 single crystals and radiative relaxation of electronic excitations in them. The investigation was performed by the means of low-temperature optical and luminescence far-ultraviolet spectroscopy upon excitation by synchrotron radiation. On the basis of the low-temperature (T=10 and 30K) spectra of the reflection, recorded in the present research and the dispersions of the complex optical functions of εˆ(E),nˆ(E) and μ(E), calculated in the framework of the oscillator model, we have determined the parameters of the electronic structure of the Li6GdB3O9 crystals as follows. The value of the minimum energy for the interband transitions in the boron–oxygen framework is Eg=9.42eV, the energy position of the first excitonic peak in the excitation spectra for anionic excitons is En=1=7.46eV, the minimum threshold energy for excitation of excitons in linear chains of the Gd3+ cations is Ec=6.80eV. The excitation spectra of an intrinsic luminescence of Li6GdB3O9 crystals, recorded at 10K in the range of the optical charge-transfer transitions O–Gd have a band with the maximum at ECT=6.57eV.

140 H/D Isotopomers Identified by Long-Range NMR Hyperfine Shifts in Ruthenium(III) Ammine Complexes. Hyperconjugation in Ru–NH3Bonding

(1)H NMR spectra of the paramagnetic cyanide-bridged mixed-valence compound [(η(5)-C5H5)Fe(CO)2(μ-CN)Ru(NH3)5](CF3SO3)3 (I) have been obtained in several solvents. When traces of partially deuterated water are present, instead of a single cyclopentadienyl (Cp) resonance shifted by the hyperfine interaction, numerous well-resolved resonances are observed. The spectra were simulated satisfactorily by giving the appropriate statistical weight to 140 possible H/D isotopomers formed by deuteration in the five ruthenium(III) ammine ligands. The proliferation of distinct resonances occurs because (a) the hyperfine shifts (HSs) due to each sequential deuteration in a single ammine are different and (b) while deuteration in an ammine cis to the cyanide bridge causes a downfield shift, in the trans ammine it causes an upfield shift that is nearly twice as large. All of these shifts exhibit a 1/T dependence, but temperature-independent components, due to large second-order Zeeman effects at the Ru(III) center, are also present. Combining the results of density functional theory calculations with data from metal-metal charge-transfer optical transitions and with the effect of solvent-induced NMR HSs, it is argued that Fermi contact shifts at the Cp protons are insignificant compared to those due to the dipolar (pseudocontact) mechanism. Analytical expressions are presented for the dependence of the HS on the tetragonal component of the ligand field at the Ru(III) ion. The tetragonal field parameter, defined as the energy by which the 4d(xy) orbital exceeds the mean t(2g) orbital energy, was found to be 147, 52, and 76 cm(-1), in dimethylformamide, acetone, and nitromethane, respectively. The effects of deuteration show that there is a significant component of hyperconjugation in the Ru-ammine interaction and that ND3 is a weaker π donor than NH3. A single deuteration in an axial ammine increases the tetragonal field parameter (ν) by +2.8 cm(-1), resulting in a HS of -37 ppb in the Cp proton resonance, whereas a single deuteration in an equatorial ammine decreases the field by -1.5 cm(-1) with a HS of +20 ppb, despite a nominal separation of seven chemical bonds. We analyze the origin of this remarkable sensitivity, which relies on the favorable characteristics of the Ru(III) low-spin t(2g)(5) configuration, having a spin-orbit coupling constant ζ ≈ 950 cm(-1).

Optical and electrochemical properties of cyclometalated Rh(III) complexes based on 4,6-diphenylpyrimidine with ethylenediamine, 2,2′-bipyridyl, and 1,10-phenanthroline

The [Rh(Hdp)2(N∧N)]ClO4 complexes (Hdp− is the monodeprotonated form of 4,6-diphenylpyrimidine and (N∧N) is ethylenediamine, 2,2′-bipyridyl, and 1,10-phenanthroline) are synthesized and characterized by 1H and 13C NMR, IR, electronic absorption, and emission spectroscopy, as well as by cyclic voltammetry. The magnetic equivalence of two cyclometalated 4,6-diphenylpyrimidine ligands in the composition of complexes points to the cis position of metalated phenyl rings in the inner sphere. Quasi-reversible one-electron reduction waves are attributed to the ligand-centered electron transfer to the π* antibonding orbital of heterocyclic ligands, while irreversible oxidation waves are associated with electron detachment from the Rh-C σ bonding orbital of the {Rh(Hdp)2} metal-complex fragment. The characteristic long-wave-length absorption bands and the vibrationally structured phosphorescence bands of complexes are assigned to the spin-allowed and spin-forbidden charge-transfer optical transitions between the σRh-C and πHdp* orbitals localized on the {Rh(Hdp)2} fragment of the complex.

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s and Their Radical Cations: Synthesis, Electrochemistry, and ENDOR Spectroscopy

1,6- and 1,7-bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) were synthesized by reaction of n-octylamine with the corresponding dibromo compounds. These compounds display intense charge-transfer optical transitions in the visible spectrum (approximately 550-750 nm) and fluoresce weakly (Phi(F) < 0.06). Cyclic voltammetry reveals that each chromophore undergoes facile and reversible oxidation and reduction. Spectroelectrochemical studies show that the radical cations of these chromophores are stable and show no signs of deprotonation of the secondary amines. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of the chemically generated radical cations of these chromophores corroborate the spectroelectrochemical data by showing that the radical cations persist for days at room temperature in methylene chloride solution. These experiments and complementary density functional theory (DFT) calculations provide a comprehensive picture of the molecular orbitals, spin density distributions, and geometries of the radical cations. The redox properties and stability of these alkylamino-functionalized perylene compounds make them a valuable addition to the family of robust perylene-based chromophores that can be used to develop new photoactive charge transport materials.

Femtosecond dynamics in directly linked pyrenyl donor–acceptor systems: orbital control of optical charge transfer in the excited state

The ultrafast charge transfer (CT) dynamics in two directly linked pyrenyl donor–acceptor systems has been studied using femtosecond broadband pump–probe spectroscopy. This method allows to capture complex chemical dynamics by measuring the time-resolved spectral evolution of transient absorption and stimulated emission induced by femtosecond optical pumping. After electronic relaxation, an optical CT transition between two excited states was found in both systems. Although the excited state CT occurs adiabatically significant differences in the electronic coupling are deduced from the band intensities. The results are discussed in terms of an adiabatic state model and compared with predictions from orbital topology arguments.

Photoionization related phenomena in doped insulators: The role of inversion site symmetry of impurity centers

The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

Lattice relaxation and charge-transfer optical transitions due to self-trapped holes in nonstoichiometric LaMnO3 crystal

We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO$_3$ lattice associated with holes localized on both Mn$^{3+}$ cation and O$^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn$^{3+}$, O$^{2-}$ and La$^{3+}$ ions in LaMnO$_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO$_3$ lattice by matching the optical energies of La$^{4+}$ and O$^-$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn$^{4+}$ state in LaMnO$_3$ host lattice, but the energy of a hole in the O$^-$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn$^{4+}$ and O$^-$ in LaMnO$_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn$^{4+}$ and/or O$^-$ self-trapped holes in non-stoichiometric LaMnO$_3$ compound.

Picosecond molecular switch based on the influence of photogenerated electric fields on optical charge transfer transitions

A concept for a picosecond molecular switch is demonstrated using a photoinduced electron transfer reaction in a covalently linked, fixed distance donor–acceptor molecule D–A linked to a perylene-3,4-dicarboximide chromophore, C. The chromophore C possesses a strong charge transfer transition in its optical spectrum. Selective excitation of C within D–A–C using 530 nm, 130 fs laser pulses produces1*C, which undergoes singlet–singlet energy transfer to produce1*D, which in turn transfers an electron to A. If the D–A–C system is selectively excited with 416 nm, 130 fs laser pulses to produce D+– A−–C prior to excitation of C with 530 nm, 130 fs laser pulses, a 25% lower yield of1*C is generated. The intense local electric field produced by D+– A−causes a 15 nm electrochromic red shift of the charge transfer absorption of C. Thus, the absorption of C at 530 nm is significantly diminished by the presence of D+– A−. The need to use two laser pulses with different wavelengths to observe these effects, and the resulting picosecond time response makes it possible to consider applications of this concept in the design of molecular switches.

Comment on “Measurement of Ultrafast Photoinduced Electron Transfer from Chemically Anchored Ru−Dye Molecules into Empty Electronic States in a Colloidal Anatase TiO2 Film”

Electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)bis(2,2‘-bipyridyl-4,4‘-dicarboxylate) into empty electronic states in a colloidal anatase TiO2 film was measured as a transient absorption signal of the injected hot electrons with a rise time <25 fs. Optical absorption of the anchored dye molecules led to the excited singlet state of the dye with a small admixture of charge transfer states. The electron transfer reaction reported here did not involve redistribution of vibrational excitation energy and was thus completely different from the well-known Marcus−Levich−Jortner−Gerischer type of electron transfer in the case of weak electronic interaction. It was also not a direct optical charge transfer transition from the donor to the acceptor level but rather an electron transfer reaction with an ultrashort but finite reaction time.

Measurement of Ultrafast Photoinduced Electron Transfer from Chemically Anchored Ru-Dye Molecules into Empty Electronic States in a Colloidal Anatase TiO2 Film

Electron transfer from the excited electronic singlet state of chemisorbed ruthenium(II) cis-di(isothiocyanato)bis(2,2‘-bipyridyl-4,4‘-dicarboxylate) into empty electronic states in a colloidal anatase TiO2 film was measured as a transient absorption signal of the injected hot electrons with a rise time <25 fs. Optical absorption of the anchored dye molecules led to the excited singlet state of the dye with a small admixture of charge transfer states. The electron transfer reaction reported here did not involve redistribution of vibrational excitation energy and was thus completely different from the well-known Marcus−Levich−Jortner−Gerischer type of electron transfer in the case of weak electronic interaction. It was also not a direct optical charge transfer transition from the donor to the acceptor level but rather an electron transfer reaction with an ultrashort but finite reaction time.

Metalloorganic Compounds for Nonlinear Optics: Molecular Hyperpolarizabilities of M(diimine)(dithiolate) Complexes (M = Pt, Pd, Ni)

A series of mixed-ligand transition-metal diimine dithiolate complexes have been prepared, and their nonlinear optical properties have been investigated. Molecular hyperpolarizabilities have been obtained from electric field induced second harmonic generation experiments at 1.9 μm and range in magnitude from 0 to −39 × 10-30 esu. Variation in molecular design of the complexes, through changes in the metal ion and diimine and dithiolate ligands, can be used to affect changes in the charge-transfer-to-diimine absorption band frequency and oscillator strength. Correlation between the optical charge-transfer transition and the nonlinear optical properties appears to agree with the two-state model.

Spectroscopic and photorefractive properties of infrared-sensitive rhodium-doped barium titanate

We have grown and characterized crystals of BaTiO(3) doped with Rh. These crystals have a blue-green color that is probably due to charge-transfer optical transitions involving Rh(4+) and/or Rh(3+). Electron paramagnetic resonance spectroscopy confirmed the presence of Rh(4+) centers. Self-pumped phase-conjugation measurements were performed at wavelengths between 776 and 990 nm. Reflectivities near 80% were obtained throughout this wavelength range. In addition, self-pumped phase-conjugation buildup times were measured as a function of input power at 851 nm and are significantly faster than in undoped or Co-doped crystals studied previously. BaTiO(3):Rh is therefore a promising material for near-infrared photorefractive applications.

Determination of the optical properties of Cu A(II) in Bovine Cytochrome c oxidase using magnetic circular dichroism as an optical detector of p

This paper describes the use of a novel magnetic circular dichroism-microwave double resonance (MCDODMR) experiment to study the optical properties of the EPR detectable copper center, Cu A 2+, in bovine cytochrome c oxidase. By irradiating the sample with a monochromatic microwave beam of appropriate frequency it is possible to quench partially the MCD intensity of the features due to Cu A 2+. In this way the MCD bands from this center have been identified even in the presence of overlapping optical transitions from the haem centers of this enzyme. The resulting spectrum compares well with that reported some years ago from this laboratory and obtained by measuring MCD magnetisation characteristics. In addition the shapes of the MCD-ODMR lines obtained in a plot of MCD intensity against magnetic field strength have been analyzed to yield the relative polarizations of the optical transitions of Cu A 2+ which contribute to the MCD spectrum. All of the bands observed between 450 and 850 nm are predominantly polarized in the xy plane perpendicular to the direction of the g-tensor component of Cu A 2+ at g = 2.18. This suggests that all of the Cu A 2+ ligands that contribute to the optical charge-transfer transitions in the visible region lie approximately in the basal plane. Possible structures for Cu A 2+ can now be suggested.

Valence-band splitting of SrTiO3

Optical charge-transfer transitions from the oxygen valence bands to ${\mathrm{Fe}}^{5+}$ and ${\mathrm{V}}^{5+}$ impurity states at ${\mathrm{Ti}}^{4+}$ sites in SrTi${\mathrm{O}}_{3}$ reveal a 0.42-eV valence-band splitting. Since these transitions are observed with conventional optical-absorption spectroscopy, their resolution is far better than previously obtained by photoelectron spectroscopy.

Flash photolysis of the charge-transfer band of 1-methylpyridinium, 1-methylcollidinium, and 1-methylquinolinium iodides

Optical charge transfer transitions in N-alkyl iodide salts, examining transient absorptions due to flash photolysis