Evidence for excited electronic state interference in resonance raman scattering

The complex resonance Raman spectra of molecular bromine have been analyzed quantitatively and a clear demonstration of interference in the Raman intensity from the B(³11₀⁺_u) and ¹17_1u excited states has been found.     

Polarized Raman spectra IX. Low Frequency Vibrations in Crystallne Trans-Stilbene

Raman spectrum of powdered crystalline trans-stilbene is recorded by a laser Raman spectrometer. The intensity of a low frequency skeleton bending vibration at 220 cm⁻¹ is observed to be very weak. Its Raman intensity, after corrected for the photocathode response and for the v⁴ scattering factor, agrees within 10% of the theoretically calculated value. It is therefore proved that the intensity of a Raman active vibration is proportional to the square of its vibrational frequency, if only one excited state is involved in the scattering process. Lattice vibrations in trans-stilbene crystal are also reported.     

Diamagnetic Raman Optical Activity of Chlorine,Bromine, and Iodine Gases

Magnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012 , 51, 11058; Angew. Chem. 2012 , 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground‐state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10^?4. These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.     

Diamagnetic Raman Optical Activity of Chlorine,Bromine, and Iodine Gases

Magnetic Raman optical activity of gases provides unique information about their electric and magnetic properties. Magnetic Raman optical activity has recently been observed in a paramagnetic gas (Angew. Chem. Int. Ed. 2012 , 51, 11058; Angew. Chem. 2012 , 124, 11220). In diamagnetic molecules, it has been considered too weak to be measurable. However, in chlorine, bromine and iodine vapors, we could detect a significant signal as well. Zeeman splitting of electronic ground‐state energy levels cannot rationalize the observed circular intensity difference (CID) values of about 10⁻⁴. These are explicable by participation of paramagnetic excited electronic states. Then a simple model including one electronic excited state provides reasonable spectral intensities. The results suggest that this kind of scattering by diamagnetic molecules is a general event observable under resonance conditions. The phenomenon sheds new light on the role of excited states in the Raman scattering, and may be used to probe molecular geometry and electronic structure.     

Combination of Resonance and Non-Resonance Chiral Raman Scattering in a Cobalt(III) Complex

Resonance Raman optical activity (RROA) spectra with high sensitivity reveal details on molecular structure, chirality, and excited electronic properties. Despite the difficulty of the measurements, the recorded data for the Co(III) complex with S,S-N,N-ethylenediaminedisuccinic acid are of exceptional quality and, coupled with the theory, spectacularly document the molecular behavior in resonance. This includes a huge enhancement of the chiral scattering, contribution of the antisymmetric polarizabilities to the signal, and the Herzberg-Teller effect significantly shaping the spectra. The chiral component is by about one order of magnitude bigger than for an analogous aluminum complex. The band assignment and intensity profile were confirmed by simulations based on density functional and vibronic theories. The resonance was attributed to the S₀→S₃ transition, with the strongest signal enhancement of Raman and ROA spectral bands below about 800 cm⁻¹. For higher wavenumbers, other excited electronic states contribute to the scattering in a less resonant way. RROA spectroscopy thus appears as a unique tool to study the structure and electronic states of absorbing molecules in analytical chemistry, biology, and material science.     

Raman spectroscopy from a picosecond-lived excited state of methyl orange

Vibrational Raman scattering from a picosecond-lived excited state of methyl orange in 9 N H₂SO₄ is reported. The vibrational frequencies of normal modes in ground and electronic excited states are separated by ≈ 10 cm^?1 but rather large differences exist in their intensities. In particular, the intensity of a mode at ≈ 1180 cm^?1, due to the NN stretch, is sensitive to the frequency of the nanosecond pulsed tunable laser. A bandwidth comparison between ground- and excited-state spectra reveals that the widths of bands of the latter, like that of the former are due to dephasing and other effects associated with interaction of molecules in the liquid phase.     

IR,Raman, and Surface‐enhanced Raman Spectroscopic Study on Triruthenium Dipyridylamide Diruthenium Nickel Dipyridylamide Family: Metal‐metal Bonding and Structures

We report the infrared, Raman, and surface‐enhanced Raman scattering (SERS) spectra of triruthenium dipyridylamido complexes and of diruthenium mixed nickel metal‐string complexes. From the results of analysis on the vibrational modes, we assigned their vibrational frequencies and structures. The infrared band at 323–326 cm^?1 is assigned to the Ru₃ asymmetric stretching mode for [Ru₃(dpa)₄Cl₂]^0–2+. In these complexes we observed no Raman band corresponding to the Ru₃ symmetric stretching mode although this mode is expected to have substantial Raman intensity. There is no frequency shift in the Ru₃ asymmetric stretching modes for the complexes with varied oxidational states. No splitting in Raman spectra for the pyridyl breathing line indicates similar bonding environment for both pyridyls in dpa^– , thus a delocalized structure in the [Ru₃]^6–8+ unit is proposed. For Ru₃(dpa)₄(CN)₂ complex series, we assign the infrared band at 302 cm^?1 to the Ru₃ asymmetric stretching mode and the weak Raman line at 285 cm^?1 to the Ru₃ symmetric stretching. Coordination to the strong axial ligand CN^– weakens the Ru‐Ru bonding. For the diruthenium nickel complex [Ru₂Ni(dpa)₄Cl₂]^0–1+, the diruthenium stretching mode ν_Ru‐Ru is assigned to the intense band at 327 and 333 cm^?1 in the Raman spectra for the neutral and oxidized forms, respectively. This implies a strong Ru‐Ru metal‐metal bonding.     

Substituent effects on the resonance Raman spectra of bis(dithiobenzil)nickel

The influence of substituents on the resonance Raman spectra of bis(p-substituted dithiobenzil)nickels has been examined. The assigned sulfur—nickel stretching vibrations in the complexes appeared in the range 390–410 cm⁻¹ with a shift to higher frequency being observed for the electron-donating substituent. It was found that Raman intensities at vibrations of the benzene ring for ligands excited with a 457.9 nm laser line are about 1.5–3.0 times larger than with a 514.5 nm laser line. The assignments of electronic transitions in the visible region of the nickel complexes were made on the basis of observed resonance Raman intensity patterns.     

Electronic raman transition from an optically pumped excited state

From the intensity behaviour of a 29 cm^?1 Raman shift of α-Al₂O₃:Cr³⁺ as a function of the incident power, it is concluded that the shift is due to an electronic Raman transition between the components of the excited state ²E. Population of the excited states is obtained through a pumping mechanism induced by the laser radiation λ_$\text{o}$ = 514.5 nm which at the same time serves as a Raman probe.     

Contrasting picosecond photokinetics of CoII and CoIII porphyrins verify excited-state predictions

Using picosecond transient absorption techniques, we have different low-lying excited states for several Co^II and Co^III porphyrins. These differences are interpreted in terms of theoretical calculations that predict the lowest-energy excited states to be ²(π·d) in Co^II and ³(d, d) in Co^III.     

Interpretation of resonance cars and shpolskii spectra with calculated molecular geometries,vibrational frequencies and relative intensities: Chrysene in it's lowest excited singlet and triplet state

Coherent anti-Stokes Raman scattering (CARS) spectra of excited molecules as well as Shpolskii spectra provide information about geometry changes between ground and excited states. Vibrational frequencies and relative intensities from recently obtained CARS spectra of the chrysene S₁ and T₁ state and earlier observed Shpolskii spectra are interpreted in terms of molecular geometry and force-field changes by means of quantum-chemical consistent force field (QCFF) and Franck-Condon factor calculations. The comparison of observed and calculated relative intensities indicates a coupling between the S₁ and S₂ state enhancing some of the vibrational radiative singlet transitions both in absorption and fluorescence spectra whereas within the phosphorescence spectra proportionality to calculated Franck-Condon factors is obeyed. The T₁ state is the more loosely bound state and its geometry change is different from that of the S₁ state. The resonance CARS transitions in the S₁ state are assigned to totally symmetric vibrations getting their intensity by a coupling scheme analogous to the A term of the resonance Raman effect: the relative intensity of a transition is shown to be proportional to the Franck-Condon factor to the higher excited state and to the squared vibrational frequency. Using this relation this state can be identified by means of its finger-print-like intensity pattern.     

Photophysical hole burning in the resonance Raman excitation profile of the chlorophyll a dimer in solution

The Raman excitation profiles of normal modes of chlorophyll a dimers in hexane exhibit sharp minima between 427 and 450 nm. In this spectral range, a non-linear relationship between the Raman intensifies and the intensity of the pulsed laser is observed. These non-linearities indicate population of lower-lying excited states of the chlorophyll a dimer. The Raman scaterring from these excited states is weaker than the rigorous resonance-enhanced Raman scattering from the ground states, which leads to the observed minima.     

Time-resolved electronic Raman spectrum of terbium aluminum garnet excited with visible and UV laser sources

Excitation profiles for the intensities of electronic Raman transitions between crystal field components of the ⁷F₆ and ⁷F₅ manifolds of terbium aluminum garnet are recorded for excitation in the spectral region where absorption bands due to levels of the ⁵D₄ manifold occur. The intensities of the electronic transitions are not enhanced which is thought to be caused by the small values of electric dipole matrix elements of the resonating electronic states in comparison to the values of such elements to other intermediate states which occur in the expression for the scattering tensor. Fluorescence from the ⁵D₄ levels is induced and resonance fluorescence are time resolved with respect to the Raman transitions. We report electronic Raman transitions excited with the 308.0 nm line of an XeCl excimer laser. As opposed to excitation with visible laser sources, transitions are recorded which terminate on all the crystal field levels of the ⁷F_5…0 levels. In addition, fluorescence from ⁵D₃ to the ground state of terbium aluminum garnet is also observed.     

A quantitative method for determination of Co(II) based on the inner filter effect of reagents on the Raman scattering signals of water

It is known that Raman scattering signals are one of main interference sources leading up to determination errors in spectrofluorometry, and thus the signals can be easily detected with a common spectrofluorometer. In this contribution, we propose a quantitative method based on the inner filter effect (IFE) of reagents on the Raman scattering signals of solvent by taking the complexation of divalent cobalt ion with 4-[(5-chloro-2-pyridyl)azo]-1,3-diaminobenzene (5-Cl-PADAB) as a model system. By adjusting the excitation wavelength of the spectrofluorometer, we could easily detect the Raman scattering signals of water at 424 nm where the maximum absorption of 5-Cl-PADAB reagent is located. In a solution of 5-Cl-PADAB, the Raman scattering signals of water are decreased owing to the IFE of 5-Cl-PADAB. If Co(II), which could form the binary complex of Co(II)/5-Cl-PADAB and consumes the 5-Cl-PADAB reagent, is present in such a case for a given amount of 5-Cl-PADAB solution, recovered Raman scattering signals could be observed and measured with a spectrofluorometer. It was found that the intensity of the enhanced Raman scattering signals is proportional to the Co(II) concentration over the range from 2.0 × 10⁻⁷ mol L⁻¹ to 1.0 × 10⁻⁵ mol L⁻¹, and the detection limit could reach 1.2 × 10⁻⁷ mol L⁻¹. With that, Co(II) in samples could be detected with R.S.D. values lower than 2.6% and recoveries over the range of 97.2-104.7%.     

Resonance Raman spectra of β-carotene and its molecular structure in the excited electronic state

The resonance Raman spectra of β-carotene have been obtained at low temperature. The excitation profiles of ν₁ (1525 cm^?1) and 2ν₁ (3043 cm^?1) are analysed in terms of the Albrecht theory. The overlap integrals between the vibrational wavefunctions of the ground and the first excited electronic states are shown to be the most important factor in determining the resonance Raman intensities of this molecule. Information on the structure of the electronically excited state has been obtained.     

One-pot synthesis of nickel porphyrins with 2-pyridine-acetyl substituents: the unexpected Sonogashira cross-coupling products

Nickel porphyrins with 2-pyridine-acetyl substituents were synthesized in one step by the Sonogashira cross-coupling method. The structures of the products were determined by elemental analysis, ¹H NMR, UV-Vis, and X-ray spectroscopic techniques. It is suggested that cross-coupling bromonated nickel porphyrins with 2-pyridine-ethyne first yielded nickel porphyrins with 2-pyridine-ethynyl substituents, followed by in situ hydrolysis to the final products, nickel porphyrins with 2-pyridine-acetyl substituents.     

叠氮化氰的光解离机理

采用多参考态方法,在MRCI+Q//CAS(10,9)/6-311+G(2df)水平上对叠氮化氰(N3CN)的光解离机理进行理论研究.优化得到基态(S0)和低激发态(S1、S2、T1)势能面上的极小点、过渡态、内转换交叉点(IC-S1/S0)和隙间窜跃交叉点(ISC-S1/T1)的结构和能量,构建反应势能面.在MRCI+Q//CAS(10,9)水平上计算N3CN的垂直激发能,并和实验值进行对比.结果表明,在S0、S1、S2和T1态势能面上,N—N键断裂生成N2+NCN是主要解离途径,而C—N键断裂通道是次要通道.实验观测到220 nm处的吸收峰对应分子由S0态到S1态的激发,对应主要光解离产物为NCN[a1△g];而在275 nm处的吸收峰则对应分子被激发到T1态,然后直接生成基态产物NCN[X3Σg-].我们的理论结果与实验测量符合得很好.     

Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors and acceptors on the push–pull porphyrins to produce high-potential charge separated states was investigated. The porphyrins were functionalized at the opposite β,β′-pyrrole positions of porphyrin ring bearing triphenylamine push groups and naphthalimide pull groups. Systematic studies involving optical absorption, steady-state and time-resolved emission revealed existence of intramolecular type interactions both in the ground and excited states. The push–pull nature of the molecular systems was supported by frontier orbitals generated on optimized structures, wherein delocalization of HOMO over the push group and LUMO over the pull group connecting the porphyrin π-system was witnessed. Electrochemical studies were performed to visualize the effect of push and pull groups on the overall redox potentials of the porphyrins. Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in polar benzonitrile, the ability of push–pull porphyrins to produce charge-separated states upon photoexcitation was confirmed and the measured rates were in the range of 10⁹ s⁻¹. The lifetime of the final charge separated state was around 5 ns. This study ascertains the importance of push–pull porphyrins in solar energy conversion and diverse optoelectronic applications, for which high-potential charge-separated states are warranted.     

Photofragmentation of CH3Br in the A band

CH₃Br is photodissociated in the first continuum. Dissociation takes place into ground state CH₃ and Br [ = Br(²P_{$\text{3}\text{2}$}] or Br* [ = Br(*P_{$\text{1}\text{2}$})]. Time of flight and angular distributions of the CH₃ fragments are measured. The Br*/Br ratios upon excitation at 222 and 193 nm are found to be 1.00 and 0.20 respectively. The anisotropy parameters at these wavelengths are β = 0.28±0.04 and β = ?0.23±0.02, respectively. The total absorption cross section is decomposed into partial absorption cross sections of the ¹Q, ³Q₀ and ³Q₁ states. It appears that excitation at 222 nm takes place to the ³Q₀ and ³Q ₁ states whereas at 193 nm the ¹Q and ³Q₀ states are excited. Contrary to CH₃I, the adiabatic curve crossing between the ³Q₀ and the ¹Q states in Ch₃Br is not important. The dissociation energy of the CBr bond is determined to be D₀(CH₃Br) = 2.87±0.02 eV.     

Resonance-enhanced Raman scattering by aggregated 2,2′-cyanine on colloidal silver

The Raman scattering spectrum of 2,2′-cyanine on colloidal silver metal particles is discussed. Preliminary assignments of some of the vibrational Raman bands to the motions of specific chromophoric units are presented and multiplet character of some bands is discussed. Enhanced Raman scattering of 2,2′-cyanine occurs when the laser radiation is tuned to the J-aggregate absorption feature at 575 nm. The enhancement in Raman intensity is the result of a diminution of fluorescence intensity, as well as a quantitative increase in Raman scattering intensity, and is distinct from other types of enhancement phenomena (e.g., resonance Raman of monomeric solution dye, and surface-enhanced Raman scattering (SERS)). The resonance Raman enhancement, due to excitation at the frequency corresponding to the J-aggregate absorption, is found to be 2 × 10⁺³.