SOLVENT-INDUCED ENHANCEMENT OF WEAKLY ALLOWED VIBRONIC TRANSITIONS OF AROMATIC HYDROCARBONS

Abstract— The enhancement of weakly allowed vibronic bands in the fluorescence and absorption spectra of 1,12-benzoperylene, 1,2,3,4-dibenzanthracene, naphthalene and 1-methylnaphthalene has been observed in polar solvents at room temperature. No satisfactory correlation has been found between the extent of solvent effect and the solvent dielectric constant, dipole moment or the Kosower's Z -value. It is proposed that, as we previously found for pyrene, the enhancement is the result of complex formation between the hydrocarbons and the polar solvents which reduces the molecular symmetry and gives, in turn, rise to the intensification of weakly allowed vibronic transitions. The finding that the 2-methyl, 1,3-dimethyl and 1,6-dimethyl derivatives of naphthalene, which have in inert solvents more strongly allowed transitions than naphthalene, do not show the solvent effect to any appreciable extent is consistent with this interpretation.     

MICROENVIRONMENT OF AROMATIC HYDROCARBONS EMPLOYED AS FLUORESCENT PROBES OF LIPOSOMES

Abstract— The enhancement of weakly allowed vibronic transitions in the fluorescence spectra of pyrene, 1,12-benzoperylene and naphthalene, which we previously found to be the result of the reduction of the molecular symmetry caused by ground state complex formation with polar solvents, has been employed in the present work to study the microenvironment of these aromatic hydrocarbons when incorporated as fluorescent probes into egg phosphatidylcholine and dipalmitoyl phosphatidylcholine liposomes. The probes are found to interact and form ground state molecular complexes with polar groups of the liposomes. For egg phosphatidylcholine the degree of enhancement is matched by that in butanol, methanol and dioxane for the three probes, respectively. For dipalmitoyl phosphatidylcholine the matching solvents are ethanol, dimethylformamide and dioxane below the phase transition, while they are ether, methanol and dioxane above it. Thus, above the phase transition pyrene and 1,12-benzoperylene penetrate further into the liposomes, while naphthalene retains approximately the same location.     

Zero kinetic energy photoelectron spectroscopy of jet cooled benzo[a]pyrene from resonantly enhanced multiphoton ionization

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[a]pyrene (BaP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of the first excited state (S(1)) and those of the ground cationic state (D(0)). Similar to pyrene, another peri-condensed polycyclic aromatic hydrocarbon we have investigated, the first two electronically excited states of BaP exhibit extensive configuration interactions. However, the two electronic states are of the same symmetry, hence vibronic coupling does not introduce any out-of-plane modes in the REMPI spectrum, and Franck-Condon analysis is qualitatively satisfactory. The ZEKE spectra from the in-plane modes observed in the REMPI spectrum demonstrate strong propensity in preserving the vibrational excitation of the intermediate state. Although several additional bands in combination with the vibrational mode of the intermediate state are identifiable, they are much lower in intensity. This observation implies that the molecular structure of BaP has a tremendous capability to accommodate changes in charge density. All observed bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far infrared bands for astrophysical applications.     

Electronic and vibrational spectroscopic investigation of phenylacetylene-amine complexes. Evidence for the diversity in the intermolecular structures

Shifts in the electronic transitions for the complexes of phenylacetylene with ammonia, methylamine, and triethylamine clearly indicate the variation in the intermolecular structures of the three complexes. The infrared spectrum of phenylacetylene in the acetylenic C-H stretching region shows Fermi resonance bands, which act as a sensitive tool to probe the intermolecular structures. The IR-UV double resonance spectra of the three complexes are disparate and signify the formation of distinct structures. The formation of C-H...N hydrogen-bonded complex with ammonia and two distinct types of pi complexes with methylamine and triethylamine can be inferred from the analysis of electronic and vibrational spectra in combination with ab initio calculations. These complexes clearly point out the fact that marginal changes in the interacting partner can significantly alter the intermolecular structure.     

尼龙1010红外光谱的研究

本文研究了尼龙1010的红外光谱。观测了不同热处理样品的红外光谱变化和密度,并测定了拉伸样品的偏振红外光谱。应用因子群解析、简正振动类比,得到许多振动的对称类型、偏振光学性质及选律,进而对尼龙1010的红外光谱谱带做了初步归属,还用电子计算机分峰的方法得到一些新谱带,并就若干谱带展开了讨论.     

WEAK COMPLEX FORMATION BETWEEN PYRENE AND POLAR SOLVENTS

Abstract— Evidence of complex formation between pyrene and polar solvent molecules has been obtained by monitoring the S₁,← S₀ and S₂← S₀ absorption spectra of pyrene. The solvents employed are propionitrile, valeronitrile, dichloromethane, and dioxane. The stabilization energy is found to be small relative to general hydrogen bond energies, but somewhat high relative to the Boltzmann energy. These are in good accord with the findings of Lianos and Georghiou [ Photochem. Photobiol. 30 ,355 (1979)] that pyrene forms 1:1 molecular complexes with several dipolar solvents in the ground state.     

Symmetry‐Breaking Charge Transfer and Hydrogen Bonding: Toward Asymmetrical Photochemistry

Symmetry‐breaking charge transfer upon photoexcitation of a linear A‐π‐D‐π‐A molecule (D and A being electron donating and accepting groups) could be visualized using ultrafast time‐resolved infrared spectroscopy by monitoring the CN stretching modes on the A units. Whereas in apolar solvents, the S₁ state remains symmetric and quadrupolar, symmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands, instead of one in apolar solvents, with a splitting that increases with polarity. In protic solvents, symmetry breaking is significantly amplified by H‐bonding interactions, which are the strongest at the CN group with the highest basicity. In strongly protic solvents, the two CN bands transform in about 20 ps into new bands with a larger splitting, and the lifetime of the S₁ state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H‐bond complex.     

Electronic and vibrational spectroscopy of cis-beta-[CrCl(2)(1,4,7,11-tetrazaundecane)chromium(III) perchlorate

The sharp-line absorption spectrum of cis-beta-[CrCl(2)(2,2,3-tet)]ClO(4) (2,2,3-tet: 1,4,7,11-tetraazaundecane) has been measured between 13,000 and 16,000 cm(-1) at 5K. The 77K emission, 298 K infrared and visible spectra have also been measured. The zero-phonon band in the highly resolved absorption spectrum splits into four components. The vibrational intervals of the electronic ground state were determined from the far-infrared and emission spectra. The 10 electronic bands due to spin-allowed and spin-forbidden transitions were assigned. Using the observed transitions, a ligand field analysis has been performed to probe the ligand field properties of coordinated atoms in the title chromium(III) complex. The X-ray crystal structure at low-temperature was determined to help in assigning the sharp-line electronic transitions.     

二氰基二硫纶和4,4-二甲基2,2-联吡啶镍(Ⅱ)配合物的光谱和理论研究

报道了标题配合物Ni(mnt)(dmbpy)溶液的电子光谱、粉末样品部分IR光谱及量子化学理论研究结果.PM3方法的几何优化表明,该配合物分子为平面结构,其对称性属于C2v点群,基态为自旋三重态.结合ZINDO方法的CI计算,解释了实测电子光谱,发现该配合物在可见区450～550nm存在本质上属于配体dmbpy到配体mnt2-的荷移跃迁(LL′CT).建立了解析复杂分子振动光谱的一种新方法:根据理论计算所得三维动态图象,对于每一个正则模,先给出固定不动的点,再给出关键性的振动类型.在本方法中,用符号η(X)定义了一种新的沿给定方向起伏或跳动式的振动类型.     

Spectroscopic properties and ligand field analysis of cis-diazido(1,4,8,11-tetraazacyclotetradecane)chromium(III) azide

The emission and excitation spectra of cis-[Cr(cyclam)(N3)2](N3) (cyclam = 1,4,8,11-tetraazacyclotetradecane) taken at 77 K are reported. The infrared and visible spectra at room temperature are also measured. The vibrational intervals due to the electronic ground state are extracted from the far-infrared and emission spectra. The ten electronic bands due to spin-allowed and spin-forbidden transitions are assigned. Using the observed transitions, a ligand field analysis has been performed to determine the bonding property of azido group in the chromium(III) complex. It is found that azide ligand has weak sigma- and pi-donor properties toward chromium(III) ion. The zero-phonon line in the excitation spectrum splits into two components by 249 cm(-1), and the large 2Eg splitting can be reproduced by the ligand field theory.     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

Tetraphenylporphine zinc(II) coordination with primary amines and alcohols in chloroform

Changes in the thermodynamic and kinetic parameters of isoequilibrium coordination of tetraphenylporphine zinc(II) (Zn-TPP) with primary amines in chloroform at 283?C308 K follow expanded correlative Taft relation modified for amines. Dependences of stability constants of complexes on shifts of Zn-TPP absorption bands in electronic spectra caused by the reactions with primary amines and the corresponding alcohols are of a linear character, as well as the relations between rate constants of nucleophilic substitution and of complex formation. The formation of molecular complexes with certain amines is accompanied by the appearance of a new absorption band in the electronic spectra near 630 nm.     

马来腈二硫纶·邻菲咯啉镍(II)配合物的电子光谱和理论研究

测量了标题配合物Ni(mnt)(phen)在多种介质中的电子吸收光谱和发射光谱, 使用密度泛函理论的B3LYP方法和分子轨道理论的PM3方法研究了其气态分子几何构型、电子结构和成键, 用ZINDO/S方法通过多组态的组态相互作用(CI)计算解释了实验光谱. 结果表明: 该配合物分子为平面结构, 对称性属于点群C_2v, 基态为自旋三重态, 配位键Ni—N和Ni—S为典型的共价结合, Ni的3d电子反馈效应较显著; 可见区的吸收带和发射带(对应于基态电子组态到较低能量激发态组态的跃迁)本质上属于配体phen到mnt^2－的荷移跃迁(LL'CT), 紫外区的吸收带本质上属于配体的π→π*跃迁.     

DFT vibrational calculations of rhodamine 6G adsorbed on silver: analysis of tip-enhanced Raman spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.     

INDO/SCF-CI calculations and structural spectroscopic studies of some complexes of 4-hydroxyacetanilide

The electronic energies among different possible structures of 4-hydroxyacetanilide (paracetamol) (PA) molecule, were calculated using INDO method and it has been concluded that its structure has C(s) point group symmetry of the cis-form. The ionization potential, electron affinity, dipole moment and binding energy have been calculated. The calculated electronic transitions of the cis-form of PA using SCF-CI method have good coincidence with the electronic absorption spectrum. The temperature effect on the electronic spectrum of PA confirms the presence of one conformer only. The electronic spectra of PA compound were studied in different polar- and non-polar solvents and the hydrogen bonding as well as the orientation energies of the polar solvents were determined from the mixed solvents studies. Complexes of PA with various metal ions such as, Cu(II), Zn(II) or Fe(II) ions of ratio 2:1, respectively, have been prepared and their structure has been confirmed by elemental analysis, atomic absorption spectra, IR spectra and (1)H NMR spectra and finally it can be concluded that the structure of the complexes has C2h point group symmetry in which two PA molecules are chelated to any one of the metal ions, Cu(II), Zn(II) and Fe(II) ions.     

Vibronic absorption spectra of acenaphthene in solution and its excited state electric dipole moments and polarizabilities

The vibronic spectra of acenaphthene in solution have been studied in detail in the region 27778–50000 cm⁻¹. A vibronic analysis of the two longest-wavelength absorption bands was made to reveal the vibrational modes that contribute to the enhancement of the intensities of these bands. The oscillator strengths of the various electronic transitions and the electric dipole moments and polarizabilities of several excited states were determined, the latter two by the solvent spectral frequency shift method.     

On the aggregative properties of rhodamine 6G in ethanolic solutions

A new experimental technique has been developed whereby the observation of the absorption bands due to the tetrameric species of rhodamine 6G in ethanolic solutions could become possible for the first time. The excitonic treatment of the electronic data backed by the ¹H NMR and vibrational studies suggest a coplanar structure for the dimers whereas the interaction between the parallel planes at a twisted angle of θ = 108° can constitute the tetrameric aggregates. The nature of the transitions involved in the absorption spectra are shown to be vibronic. The symmetry considerations have therefore been taken into account in order to ascertain the validity of the proposed structure for the tetramers.     

新颖的5,15-二(4-(5-乙酰基硫戊氧基)苯基)卟啉化合物的结构和性质的密度泛函理论计算

通过密度泛函理论计算比较性地研究了5,15-二(4-(5-乙酰基硫戊氧基)苯基)自由卟啉及其锌配合物的分子结构、电荷性质、分子轨道、电子吸收光谱和红外光谱.这类化合物具有在卟啉相对的两个中位的苯环上连有5-乙酰基硫戊氧基的新颖结构.模拟得到的这两个化合物的分子结构和电子吸收光谱以及红外光谱都与实验测得的符合得很好.通过与未取代的自由卟啉和卟啉锌的结构和性质进行比较,研究了中位取代基、极性溶剂和中心金属取代对此类卟啉化合物结构和性质的影响规律.对化合物的电子吸收光谱中的电子跃迁本质进行了归属,并通过基于正则坐标分析产生的动画对红外光谱的振动模式进行了指认.目前的工作将对理解此类新颖卟啉化合物的结构和性质以及取代基和溶剂效应提供很大的帮助.     

Spectrophotometric and spectroscopic studies of complexation of 8-hydroxyquinoline with π acceptor metadinitrobenzene in different polar solvents

The complexation of electron donor–acceptor complexes of 8-hydroxyquinoline (8HQ) and metadinitrobenzene (MNB) have been studied spectrophotometrically and thermodynamically in different polar solvent at room temperature. A new absorption band due to charge transfer (CT) transition is observed in the visible region. A new theoretical model has been developed which take into account the interaction between electronic subsystem of 8HQ and MNB. The results indicate the extent of charge transfer complexes (CTCs) formation to be more in less polar solvents. Stoichiometry of the complex was found to be 1:1 by straight line method and ¹H NMR between donor and acceptor at the maximum absorption bands. Ionization potential (I_D) and resonance energy (R_N) were determined from the CT transition energy in different solvents. The formation constants of the complexes were determined in different polar solvents from which ΔG° formation of the complexes was estimated and also extinction coefficient of the charge transfer complex (CTC) was calculated. Oscillator strength, transition dipole strengths and maximum wavelength of the CTC (λ_CT) in various solvents and IR spectra of the CTC have also been discussed. It has been observed that all parameters described above changed with change in polarity and concentration of donor.     

Vibrational and electronic absorption spectroscopy of dibenzo[b,def]chrysene and its ions

The vibrational and electronic absorption spectra of dibenzo[b,def]chrysene (DBC) and its ions in argon matrixes have been recorded. Assignment of the observed infrared (IR) bands has been made by comparison with the density functional theory (DFT) computations of harmonic vibrational frequencies (with 6-31G(d,p) or 6-311+G(d,p) basis sets). Extensive time-dependent (TD) DFT calculations of vertical excitation energies have aided in the assignment of the experimental electronic absorption transitions. In general, the theoretical predictions are in good agreement with the observed ultraviolet and visible bands. By correlating IR and UV-visible band intensities (after UV photolysis), it has been shown that both DBC cations and anions are formed. The IR band intensity distributions of the DBC ions differ markedly from neutral DBC. A synthetic spectrum composed of neutral, cationic, and anionic DBC contributions compares reasonably well with the interstellar features of the "unidentified infrared" (UIR) bands from the reflection nebula NGC 7023. Finally, it is shown that the electronic absorption bands of the DBC ions lie in close proximity to several of the diffuse interstellar visible absorption bands (DIBs).