Molecular Spectroscopy—Experiment and Theory

Spectroscopy investigates the interaction of electromagnetic radiation with matter. Along with the development of theoretical methods, increasingly effective numerical algorithms and computational methods as well as computer technologies and resulting growing computer power available for scientists, the so-called in silico experiments—computer simulations of materials and their properties in computer—have become an irreplaceable tool supporting experimental research, often allowing a better understanding of phenomena taking place during these interactions, and associated material properties. As a result, it becomes possible in growing number of cases to effectively design new materials with desired properties and to modify existing ones, to improve their properties. This chapter is devoted to a brief introduction to issues related to theoretical foundations of quantum mechanics and density functional theory, both in stationary and time-dependent form. The key assumptions of these theories are presented, together with the description of various approximations and simplifications necessary for their practical application to the calculation of properties examined by spectroscopic methods. The most important practical problems encountered during calculations, resulting from the complexity of real materials and typical ways of dealing with these problems by means of various simplifications, idealizations, and abstractions in designed structural models corresponding to real materials, are also presented.

This chapter contains a brief review of the up-to-date scaling procedures that are used in the computational vibrational spectroscopy to improve agreement between the calculated harmonic frequencies and the observed fundamentals. Initially, the basics of vibrational spectroscopy are reminded. This includes the concept of potential energy surface, harmonic approximation, and a basic quantum chemistry treatment of the anharmonicity for a diatomic molecule. Brief description of the Wilson–Decius–Cross method for polyatomic molecules is also presented. Then the commonly used scaling procedures are discussed. The distinction between single- and multi-parameter scaling procedures is made. Four scaling procedures are reviewed. First, Pople’s uniform scaling is presented. Second, Yoshida’s wavenumber linear scaling method is discussed. Both methods are simple single-parameter frequency scaling methods. Then basics of two multi-parameter scaling methods, which are much more accurate but less straightforward to use, are given. Thus, Pulay’s scaled quantum mechanical force field method, in which scaling factors are applied directly to the calculated force constants is reviewed. Finally, introduction to quite recently proposed multi-parameter frequency scaling method, called effective scaling frequency factor method, is provided. The relevant sections start with a short description of the theory for a given method. Then a brief literature review on the historical background of methodology development is given.

This review paper is focused on the research of molecular mechanisms occurring in porphyrin-like systems such as porphyrins, phthalocyanines, and corroles as well as in chromophore-semiconductor quantum dot (QD-CdSe/ZnS) or corrole-fullerene (C₆₀) as electron-donor-acceptor unites. The basic spectroscopic investigations describe properties of materials in organic solutions in the ultraviolet, visible, and infrared ranges and in a form of Langmuir and Langmuir–Blodgett molecular nanolayers to get knowledge on photophysics of dyes and the influence of QD and C₆₀ on the electron redistribution within the molecular structures. The studies also allowed to explain the impact of solvent on the spectroscopic properties of corroles and on the redistribution of the π-electrons in the excited state. The fluorescence studies very evidently showed strong interaction between chromophores and C₆₀ or QD and clearly demonstrated the strong donor-acceptor nature of the phthalocyanines-quantum dot and the corrole-fullerene dyad. In addition, spectroscopic studies in polarized light allowed determining molecular arrangement of the chromophore molecules in the Langmuir–Blodgett layers with respect to solid substrates. The computer calculations (TD-DFT theory) confirmed the experimental results, in particular the redistribution of the π-electrons in the excited state and the location of HOMO and LUMO levels. The DFT calculations let also to evaluate the reorganization energy values for the set of free-base corroles and C₆₀ fullerene. In this review, it was shown the electron-donor-acceptor character of the systems composed of: porphyrin-quinone, phthalocyanines-QD, corroles-C₆₀ dyads. It has been demonstrated potential capabilities of the photoactive organic materials with QD and fullerene in the future applications in many areas of optoelectronic and in the process of converting solar energy into electric energy in solar cells.

A comparative Raman spectroscopic study of single-walled carbon nanotubes (SWCNT), special multi-walled carbon nanotube (MWCNT) material, and graphite is presented. Their Raman spectra have been recorded using different excitation wavelengths, 532, 785, and 1064 nm, in the region of 1800–1200 cm⁻¹. The G-bands of SWCNTs observed at all excitation wavelengths were fitted taking bands into account with Breit-Wigner-Fano (BWF) and Lorentzian line shape functions. The contribution of both BWF and Lorentzian line shapes to the asymmetric G-band shows a mixture of semiconducting as well as metallic CNTs in the SWCNTs. For graphite and MWCNTs, only Lorentzian line shape functions were used for band deconvolution. The variation in wavenumber position of component bands of G-band with laser lines may be due to the resonance Raman effect, where the energy of laser lines matches with the electronic transition energy of CNTs with different diameters and chirality. The apparent Young’s modulus of SWCNT and MWCNT materials was determined using the integrated band intensity ratio of G- and D-bands, I_D/I_G, and it was found that the SWCNTs have a larger value of the apparent Young’s modulus compared to that of the highly aligned MWCNT material.

Functional ligand-protected noble metal cluster nanomaterials with enhanced two-photon absorption and two-photon excited emission may lead to new technologies for bio-imaging applications. In this article, I review experimental and theoretical methodologies allowing detailed investigation of two-photon absorption/emission properties of ligand-protected silver and gold metal clusters. This includes femtosecond two-photon excited fluorescence experimental setups and quantum chemical methodologies based on time-dependent density functional theory. I thoroughly analyze physical phenomena and trends leading to large two-photon absorption/emission responses of model nanoclusters focusing on the effects of the relaxation pathways in the linear and nonlinear optical regime, as well as strategies aiming at enhancing their two-photon emission responses.

This chapter presents selected techniques of Raman spectroscopy, i.e. Raman imaging, Raman optical activity (ROA), and surface-enhanced Raman spectroscopy (SERS), and gives an overview on their biomedical applications. The current state of the art in the research on chiroptical compounds of biomedical importance, as well as the study on early apoptosis and inflammation processes occuring in the endothelium, is presented. The pathophysiology of the endothelium is discussed based on the example of Raman imaging results for primary cells and cell cultures. Moreover, the comparison of classical Raman imaging, application of optical fiber probes, and immuno-SERS nanosensors in detection of marker proteins in ex vivo studies is discussed.

In this chapter, we highlight the power of vibrational spectroscopy as central technique to investigate the structure and reactivity of two relevant families of nitrogen-containing heterocyclic molecules: hydantoins and mercaptoimidazoles. Infrared spectroscopy is used in connection with the matrix isolation technique to investigate the structures of the isolated molecules and their photochemistry, while both infrared and Raman spectroscopies, supplemented by thermodynamics, microscopy, and diffraction techniques, are used to investigate neat condensed phases of the compounds and transitions between these phases. The experimental studies are supported by extensive computational studies, which include several approaches for detailed analysis of the electron density.

Over the last years, a rapid development in the material science, which is an answer to an increasing demand for functional, smart systems, has taken place. The recent progress in design, synthesis and characterization of stimuli-responsive polymer systems (SRPS) fits in this trend very well. However, extensive experiments, simulations as well as theoretical works are still conducted to deepen the knowledge about these systems, their complexity and diversity result in still insufficient understanding of some crucial phenomena. One of them is intermolecular interactions which change during swelling/deswelling processes, phase transitions (commonly leading to the phase separation) and loading or a release of various additives. Since the vibrational spectroscopy is considered to be the most powerful tool to study molecular interactions, this chapter presents various aspects related to the usage of vibrational spectroscopy in the field of SRPS.

Magnetoelectrics have been one of the most widely studied materials in recent years. They are very interesting from the fundamental point of view due to joining magnetic and electric orderings in the same phase, which tends to exclude each other. On the other hand, the orderings are coupled each other what makes them promising material to be applied in many electronic devices. This phenomenon is strongly related to the structure and its changes which can be tested by Mössbauer spectroscopy. In the chapter, we look closer to this technique in application to magnetoelectric perovskite solid solutions of BiFeO₃–Pb(Fe_0.5Nb_0.5)O₃. The possibility of confirmation of random cation distribution, magnetic ordering temperature, and iron magnetic properties will be presented and discussed. The presented experimental hyperfine interaction parameters will be compared to those theoretically calculated using ab initio methods.

In this chapter, the ab initio calculations have been used to analyze the structural properties and vibrational spectra of selected zeolites. The spectra obtained as a result of theoretical calculations along with their interpretation were used to describe the experimental spectra of real zeolite structures. Presented results show that in the experimental spectra of zeolites one can distinguish the bands associated with characteristic vibrations of a bigger element of the structure, composed of tetrahedra, the primary building blocks. It was also shown that the composite envelopes of particular bands are significantly affected by component bands associated with characteristic vibrations of building units that form zeolite structures.

This chapter intends to present the classical and modern techniques that are used for in situ characterisation of catalytic materials. Determination of the structure of the catalyst presents three main problems: (1) heterogeneous catalysis phenomena are limited to the outer surface of the material where the molecules adsorb and react, and for this reason, there are only a few methods able to assess catalyst surface structure and composition; (2) the catalyst surface under reaction conditions and upon the influence of the reacting agents is different from that occurring under ambient conditions, which limits the application of the analytical methods to those which operate at normal or elevated pressures and high temperatures, (3) catalytic materials are complex and heterogeneous, so many analytical methods, including surface imaging, should be employed in order to understand the structure–activity relationships. The remedy for the problems is the application of in situ analyses that rely on several complementary spectroscopic methods and utilise surface sensitive probe molecules. Different kinds of probe molecules are described: from universal probes to specific ones that enable the determination of acidic and basic activity. The IR, Raman and UV-Vis methods are presented here and described using examples from the literature. New trends in in situ experimentation involve time-resolved techniques for studying fast reactions, fluorescence methods and coupled techniques for surface in situ imaging.

Organosilicon compounds whose structure is based on the stable Si–O bonds show high chemical and thermal stability. Incorporation of organic groups into their molecules results in the materials which combine the properties of the Si–O skeleton and those of organic moieties. Hydrosilylation, i.e., catalytic addition of Si–H groups to carbon–carbon double bonds, is a convenient route to prepare organofunctional silicon compounds. In this chapter, the work that has been done since the year 2000 on functionalization via hydrosilylation of polysiloxanes, cubic  oligomeric silsesquioxanes, and spherosilicates, i.e., the main classes of organosilicon compounds containing Si–O bonds, is reviewed. The emphasis is put on spectroscopic investigations of the functionalization process or its products.

Polydimethylsiloxane polymers were studied for their suitability in optical waveguides. Optical constants of core and clad materials have been measured within the visible and near-infrared spectral range. The absorption loss of PDMS in the datacom region of 600–1600 nm is mainly caused by scattering loss and attenuation by overtone and combination band vibrations of CH containing groups. Based on observed positions of those fundamental, overtone and combination bands and their vibrational assignment, normal vibration frequencies and anharmonicity constants were determined. An empirical correlation between integral band strength and intrinsic absorption loss was derived based on the vibrational spectra of the two materials, providing also estimates for deuterated and halogenated PDMS materials with even lower absorption losses. Besides intrinsic material attenuation, also extrinsic losses from waveguide fabrication have been investigated. Furthermore, multimode waveguides were fabricated with an insertion loss of 0.05 dB/cm at the most frequently applied data communication wavelength of 850 nm. Effects of accelerated ageing have been assessed for the optical characteristics of PDMS materials.

In this chapter, the visible luminescent properties of antimony-germanate and gallo-germanate glasses, glass-ceramics and optical fibers co-doped with different rare-earth (RE) systems (e.g., Sm³⁺, Eu³⁺, Yb³⁺/Tm³⁺, Yb³⁺/Ho³⁺, Yb³⁺/Tm³⁺/Ho³⁺, Yb³⁺/Tb³⁺, Yb³⁺/Eu³⁺) have been analyzed. Emission was obtained by direct excitation of RE ions or in case of co-doped systems by donor-acceptor energy transfer and upconversion processes. Novel constructions of double-clad, offset core, double-core, and RE triply doped optical fibers have been presented. Moreover, the effect of the structure modification of antimony-germanate glasses on the luminescence shaping has been also presented. Another, presented scientific area is investigation of plasmon effect in Ag⁰/Eu³⁺ co-doped glasses and optical fibers. The optimization of Ag⁰/Eu³⁺ content and formation of silver nanoparticles process (thermal annealing) in order to enhance luminescence has been presented.

In spite of a large amount of the literature on amorphous silicate structure studies and a significantly long time that has passed since the pioneer works of Lebiedev, Zachariasen, and Warren, there are still many doubts concerning the description of their structure. The selection of the correct research method allowing for a description of glass structure, specifically one that yields information on short-range order present in the amorphous state, seems to be especially important. In the framework of this manuscript, vibrational spectroscopy was proposed as a fundamental research method for describing the amorphous structure of glass materials. Interpretation procedures were also presented that enable obtaining the maximum amount of information on glass build on the basis of oscillation spectra.

This chapter is dedicated to the application of selected spectroscopic techniques to investigations on cultural heritage objects. The rapid technical advancement of Raman instrumentation, observed in the recent years, positioned this spectroscopy as an outmost tool in this field. The use of Raman spectroscopy in the analysis of chemical composition is presented for several classes of heritage materials: manuscripts, painting, ceramics, minerals, and amber. In Sect. 16.3, Vis fiber optic reflectance spectroscopy is presented as a tool allowing one to obtain information important for selecting proper preventive measures, in this case, exhibition policies safeguarding artifacts against photodegradation. The technique discussed here—the microfade testing (MFT)—allows monitoring color changes as induced by the action of light on a selected spot on the artifact in real time, thus giving the most direct, empirical clues to a possible future alteration of the objects’ appearance when it is exposed to light on a museum wall.