Optical spectroscopy on uniaxial assemblies of partially aligned molecules is surveyed. Orientation factors are defined and some examples of applications are mentioned: electric dipole absorption of linearly polarized light, magnetic circular dichroism, and spectroscopy based on simultaneous or successive two-photon processes. New simple expressions are given for magnetic circular dichroism of uniaxial samples and for the orientation-averaged interaction of a molecule in a uniaxial sample with two circularly polarized photons. These are of particular interest in connection with Raman scattering and two-photon absorption.

Spectroscopy with linearly polarized light on aligned samples (LD spectroscopy) is a useful technique that may greatly enhance the possibilities for determining sample structure, making spectral assignments, and several other tasks, generally based on a determination of transition moment directions. However, the technique does not provide the same amount of information in all cases; this depends strongly on the molecular symmetry. The almost ideal case is that of D-2h, D-2, or C-2v symmetry, when only 3 different (perpendicular) transition moment directions are possible. When the molecular symmetry decreases, the information that may be obtained becomes less precise. If the molecule has a plane of symmetry left, the transition moment may be located perpendicular to the symmetry plane or at any direction in the plane, and much useful information may still be extracted. In the case of a molecule with no symmetry at all the information that can be obtained often becomes highly qualitative, unless special information. happens to be available. Unfortunately, this severe limitation is sometimes overlooked when low-symmetry molecules are studied, and the spectra are evaluated the same way spectra for symmetrical molecules are. This is an obvious source of error, and,conclusions based on such an analysis are hardly reliable. In this paper, the spectroscopic technique, assumptions commonly made, and the mathematical treatment of IR and UV spectra are briefly summarized for different molecular symmetries. This is illustrated by a few examples, including new IR LD spectra of limonene, a very difficult, low symmetry case.

Dept. of Natural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark, E-mail: pwt@kvl.dk Received 11-11-2005Linear Polarization 371Transition Moment Directions 373Aligned samples 374LD Spectroscopy 376Mathematical descriptions of aligned samples 376Recording and Analyzing LD Spectra 377Reduced spectra – symmetrical molecules 379Low-symmetry molecules 379Conclusion and Perspectives 382References 382

The electronic states of dibenzothiophene are investigated by UV Linear Dichroism (LD) spectroscopy on samples partially aligned in uniaxially stretched polyethylene. The LD spectra reveal previously unobserved spectral features, leading to experimental identification and symmetry assignments of at least seven electronic singlet states in the region below 50,000 cm−1. The observed transitions are well reproduced by Time-Dependent Density Functional Theory by using the B3LYP/6-31+G* functional, and by calculations in the semiempirical LCOAO method. The assignment of the observed absorption bands is further supported by comparison with previously published Magnetic Circular Dichroism spectra, and with the spectrum of the closely related compound dibenzofuran.

The electronic structures of 3,6-diphenyl-s-tetrazine (I), 3,6-bis(4-pyridyl)-s-tetrazine (II), 3,6-bis(3-pyridyl)-s-tetrazine (III), and 3,6-bis(2-pyridyl)-s-tetrazine (IV) were investigated by linear dichroism (LD) UV–Vis absorption spectroscopy using stretched polyethylene matrices, by magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The LD spectra contain information on the transition moment directions of the observed electronic transitions, and here in combination with the MCD spectra, they lead to resolution of the otherwise hidden transitions. The electronic transitions predicted by time dependent density functional theory (TD-B3LYP/6-31G*), using molecular geometries determined by X-ray crystallography, were in excellent agreement with the observed transitions. The combined experimental and theoretical evidence lead to a consistent assignment of electronic states. In particular, the second observed transition of I around 27000 cm−1 can be assigned to an in-plane short-axis polarized π–π* transition, in contrast to a previous assignment.

Abstract- In studies of polycyclic aromatic hydrocarbon carcinogenicity three dibenzopyrenes have been named as the strongest mutagens together with the frequently studied benzo[a]pyrene. A detailed study of the electronic structure of one of the three compounds, dibenzo [a, i] pyrene, was performed several years ago. Here we present a similar study of the two remaining compounds, dibenzo [a, h] pyrene and dibenzo [a, e] pyrene. The studies include electronic linear dichroism spectra, fluorescence polarization spectra and quantum mechanical calculations for both molecules, as well as vibrational linear dichroism spectra for the former of the two.

New linear dichroism spectra in the mid-IR region of fluorene aligned in stretched polymers and nematic liquid crystals are compared with earlier experimental and theoretical investigations. Linear dichroism spectra of molecules aligned in such anisotropic solvents are simple to obtain, especially for small and medium sized molecules. Most molecules obtain a satisfactory alignment by these methods and the spectra are simple to interpret. The information obtained from these spectra about vibrational transition moment directions is often crucial for vibrational assignments in molecules like fluorene. Even high quality calculations are still unable to provide safe assignments for all fundamentals in fluorene as long as they are only compared with traditional spectra. Linear dichroism spectra provide a separation of the experimental information according to symmetry classes (particularly striking for molecules of C2v and similar symmetries); this reduces the assignment puzzle drastically. In the present case the result is an almost complete and safe assignment of all symmetry allowed invibrational fundamentals of fluorene. A number of earlier assignments, which had been questioned or were doubtful, have been confirmed or corrected.

Complete infrared linear dichroism spectra were recorded between 50 and 3200 cm−1 of acenaphthylene aligned in stretched polyethylene and polyethylene-d4 as well as Raman solution spectra. The information obtained from the infrared spectra made it possible to separate the electric dipole allowed transitions into three groups, corresponding to three different excited state symmetries. The separation is possible for the C2v symmetric molecule, since the three groups of transitions correspond to three mutually perpendicular transition moment directions, which in the stretched polymer obtain different average alignments. This information makes a safe assignment of all allowed fundamental vibrational transitions, belonging to three of the four C2v symmetry classes, possible. Similarly, the Raman spectra permitted an assignment of vibrations belonging to the fourth symmetry class. The assignments were supported by ab initio quantum mechanical modelling calculations at various levels. A comparison of the present assignments with those from recent linear dichroism studies of acenaphthylene aligned in a nematic liquid crystal matrix confirmed most assignments, but also led to new assignments and reassignment of several transitions. This was possible because of the additional information that becomes available in the stretched polyethylene method, which provide an excellent separation of the excited state symmetries corresponding to the two in-plane transition moment directions.

Transition moment directions of mid-infrared transitions in four isotopomers of propene, CH2=CHCH3, CH2=CDCH3, CD2=CHCD3, and CD2=CDCD3, have been determined from linear dichroism spectra recorded in stretched polyethylene as solvent. The results were compared with expectations based on bond directions and with results of ab initio calculations at levels of approximations ranging up to CCSD/6-311G**. The former procedure is found to be unreliable, even for high frequency vibrations. With rare exceptions, the highest levels of ab initio theory agree with the experimental directions to within about +/-20 degrees, and for intense transitions, to within the experimental uncertainty of about +/-(5-10)degrees.

In this paper we provide an assignment of symmetries for all transitions observed in the infrared spectrum of phthalic anhydride (1a), based on data determined from polarization studies of photooriented 1a. Photoselection, using two electronic transitions in phthalic anhydride 1a, at 34083 and 40650 cm(-1) and of A(1) and B-2 symmetry, respectively, produced two differently oriented uniaxial samples of the anhydride immobilized in neon and other noble-gas matrices. Subsequent polarization measurements in the infrared region, combined with polarized Raman data, allowed a complete determination of transition moment directions for all observed fundamental vibrations. To facilitate definitive spectral assignments, we studied three isotopomers of 1a: 1,2-C-13-labeled phthalic anhydride (1b), the perdeuterated phthalic anhydride (1c), and the alpha-C-13-labeled compound (1d). Absolute infrared absorption intensities were determined for all observed transitions utilizing absolute infrared absorption intensities of CO and CO2, formed in equimolar amounts with o-benzyne (2) in phototransformations of 1, Interpretation of the experimental results and the final assignments were aided by quantum mechanical modeling at the SCF/6-31G** level.

Six symmetrically disubstituted derivatives of s-tetrazine have been studied by linear dichroism spectroscopy in the UV-visible region, magnetic circular dichroism spectroscopy, and theoretical calculations of structure and spectra. The electronic absorption spectra, recorded up to 48 000 cm−1, show three major, characteristic transitions: An out-of-plane polarized nπ∗ transition at lowest energy, an in-plane, short axis polarized ππ∗ transition at higher energies (in some compounds overlapping the first transition), and a third, long axis polarized transition at higher energy (for two compounds above 48 000 cm−1 and not observed in the spectra). The calculated and observed transition moments and energies for these transitions are in good agreement. In addition, several weaker transitions, both of nπ∗- and ππ∗-type are predicted for all compounds and several of these are observed and tentatively assigned. The linear dichroism spectra, recorded on solutions in stretched poly(ethylene) (PE) or, when solubility in PE was low, in poly(vinyl alcohol), demonstrate distinct differences in the degree of alignment of similarly shaped molecules. This is remarkable, since studies of well over 100 aromatic hydrocarbons have shown a fairly simple relation between molecular shape and alignment.

A structural application of linear dichroism (LD) of solutes in oriented polymer is presented. IR-LD absorption spectra of the three similar in structure compounds oriented in stretched polyethylene have been analyzed. For each observed vibration band the orientation factor has been determined. The observed orientation factors for the vibrational transition moment directions provides information on symmetry of the molecule. The results of the conformational study are consistent with the assumption of a rigid C2h groundstate structure for trans-stilbene and trans-azobenzene, whereas a planar C(S) symmetry can not be confirmed for trans-N-benzylidene-aniline.

Synthesis of the terminally disubstituted rigid rod molecules, [n]staffane-3,3(n-1)-dithiols, n = 1-4, and their singly functionalized derivatives carrying an acetyl or a pentaammineruthenium(II), [Ru(NH3)5]2+, substituent is described. Self-assembled monolayers of the neat [n]staffane derivatives and of their mixtures with n-alkyl thiols were prepared on polycrystalline gold electrodes. Grazing incidence FTIR spectroscopy and comparison with polarized IR spectra of model compounds oriented in stretched polyethylene revealed that, in the films assembled directly from the dithiols, the average orientation of the [n]staffane rods was halfway between perpendicular and parallel to the surface, whereas, in the films obtained from the singly functionalized derivatives, the rods were nearly perpendicular to the surface. The degree of ordering and packing was also investigated by contact angle measurements with deionized water. The electrode blocking properties of the films were investigated by three independent electrochemical techniques, cyclic voltammetry in a solution containing K4Fe(CN)6, chronoamperometry in a solution of Ru(NH3)6Cl3, and cyclic voltammetry of the gold surface in sulfuric acid. The results from the three methods were in good agreement and showed that the films assembled from the dithiols were not as well organized as those obtained from the monoderivatized analogues. In situ hydrolytic removal of the protective acetyl groups and subsequent functionalization of the resultant film covered with free thiol groups with aquapentaammineruthenium(II), [Ru(NH3)5(H2O)]2+, are possible without disturbing the structure of the film. Electrodes covered with monolayers carrying ruthenium pentaammine groups on their outer surface exhibit oxidation-reduction waves at 0.51 V vs SCE attributable to the Ru(II)-Ru(III) couple, demonstrating electron transfer across the monolayer. The formal potential of the immobilized couple is about 60-mV more positive than that of the dissolved complex, [[(NH3)5Ru]-[n]staffanedithiol]2+, in 1 M HClO4 solution. The surface concentration of Ru is the same in the films assembled from this complex and those produced by in situ functionalization of a film covered with free thiol groups and corresponds to a monolayer of (NH3)5Ru. However, in the former type of film, the surface concentration of the staffane rods is only about half of that in the latter, implying that only about half of the surface thiol groups have been metalated.

The carcinogen benzo[a]pyrene is investigated by UV and IR linear dichroism (LD) spectroscopy in stretched polyethylene, by UV magnetic circular dichroism (MCD) spectroscopy, and by fluorescence polarization spectroscopy. The combined results lead to a detailed experimental characterization of transitions in the near-UV region, with determination of polarization directions for five electronic transitions. The experimental results are compared with theoretical predictions using the PPP, CNDO/S, CNDO-SDCI, and LCOAO quantum chemical models. Excellent agreement with observed values in the low-energy region, including transition moment directions and MCD signs, is obtained within the LCOAO model. The theoretical analysis reveals a strong breakdown of the alternant pairing symmetry for benzo[a]pyrene, compared with those observed in the related hydrocarbons benz[a]anthracene and chrysene.

Transition moment directions were determined for all fundamental vibrations of monodeuterated ethylene by using the infrared absorption linear dichroism method. They were compared with results of high-level ab initio calculations. The comparison shows that dipole transition moment directions are predicted quite accurately for stretching vibrations, and poorly for bending modes.

U.v.-vis linear dichroism spectra of trans- and cis-azobenzene in stretched poly(ethylene) (PE) films are reported. Trans → cis photo-isomerization kinetics were measured at 337 nm using polarized light with the electric vector either parallel or perpendicular to the stretching direction of the matrix. Three types of samples were examined, viz. unoriented, kept at constant elongation stretched to (800%) and relaxed (stretched to 600%). Analysis of the kinetic data reveals that the photo-isomerization is purely exponential in time up to 83% conversion, with nearly the same rate constants in both directions. It is faster in the stretched than in the unstretched sample. This suggest that trans å cis isomerization of azobenzene in nonpolar solid solution occurs according to an inversion mechanism. Cis-azobenzene molecules obtained from phototransformation of the trans species in stretched PE attain globular geometry with the phenyl rings twisted perpendicular to the azo plane.

A graphical procedure is presented which allows a determination of the average alignment of the molecular axes in partially oriented, low-symmetry planar molecules from the results of linear dichroism and fluorescence polarization spectroscopy. When applied to benz[a]anthracene, the method makes it possible to extract information on the previously unknown origin of the second excited electronic state.

The linear dichroism spectra of trans-azobenzene and trans-stilbene aligned in stretched polyethylene films are reported for the u.v.-vis region. On the basis of the results the u.v. transition moment directions with respect to the long molecular axis are determined. Infrared data indicate that trans-azobenzene has a non-planar geometry while trans-stilbene is more planar in the polyethylene matrix. The polarization direction of the nπ* transition in azobenzene is different from that of the first ππ* absorption band, which indicates a vibrational borrowing mechanism corresponding to a nonplanar geometry.

The i.r. absorption spectrum of 1,4-dihydroxy-9,10-anthraquinone (quinizarin) has been studied in a stretched polyethylene matrix using linearly polarized radiation. The resulting two linearly independent spectra show that only three different transition moment directions are present in the 30 observed transitions, and this leads to a classification of the 30 peaks according to the point group symmetry of the excited state involved. This classification makes a complete assignment possible, and settles several questions on the assignment which have been discussed in the literature for several years.