Paths by which nonradiative relaxation can occur include, but are not limited to, such phenomena as collisional energy transfer, electron or proton transfer processes, change of molecular conformation, photochemistry, formation of excited state complexes (e.g., excimers or exciplexes), as well as the classic processes of internal conversion (IC) (e.g., vibrational relaxation) and intersystem crossing (ISC) (e.g., singlet-triplet conversion). While fluorescence is a spin-allowed process and generally occurs rapidly, phosphorescence is spin forbidden and is typically a slower relaxation process. Luminescence from such intermediate states can be defined as fluorescence or phosphorescence, where fluorescence is a process by which electronically excited molecules return to a lower electronic state of the same spin multiplicity (which is often the electronic ground state) by emitting a photon phosphorescence, on the other hand, is the corresponding transition between states with different spin multiplicities. Included among such deactivation processes are emission of radiation, more popularly referred to as luminescence, as well as processes that are nonradiative in nature, where lower energy states can be directly populated without the emission of photons. It is to be noted that molecules that are excited to higher energy than the lowest excited state above the ground electronic state can relax to lower excited levels by a range of intrinsic processes. In particular, since absorption spectra of molecules depend on their energy level structure, absorption spectra are not only useful for identifying isolated molecules, but also can be used to probe intermolecular interactions (e.g., effects of aggregation) that affect energy level structure.
And, ideally the absorbance of a dissolved analyte depends linearly on concentration, thereby resulting in an absorption spectrum providing quantitative measurement of the analyte's concentration in solution, arrived at by applying the Beer-Lambert Law. As it turns out, optical spectroscopy is a useful approach for both qualitative and quantitative studies of physical and chemical processes involving matter in most of its states by measurement of absorption, emission, or scattering of electromagnetic radiation moreover, optical spectroscopic measurements can be very sensitive, nondestructive, and typically require only small amounts of material for analysis.Ībsorption spectra are usually acquired for analytes dissolved in nonabsorbing solvents.
The earliest prospect of making spectroscopic measurements came with the observation that visible light can be dispersed by an optical prism, and the concomitant recognition that matter could be intimately investigated through its response to optical radiative energy as a function of frequency, defining what is referred to as optical spectroscopy. Spectroscopy is the branch of science dealing with the interaction of electromagnetic and other forms of radiated energy with matter. Introduction: overview of molecular spectroscopy and quantum calculations Furthermore, the calculations show that the geometric structure of the porphyrin is strongly dependent on protonation and the nature of the meso-substituted functional groups.ġ.
This finding suggests that the meso-sulfonatophenyl substitution groups are able to rotate around Cm─Cϕ bond at room temperature because the thermal energy (kBT) at 298 K is 207.2 cm−1. The ground state RPES curve indicates that when the molecule transitions from the lowest ground state to a local state, the calculated highest potential energy barrier at the dihedral angle of 90° is only 177 cm−1. A relaxed potential energy surface (RPES) scan has been utilized to calculate ground and excited state potential energy surface (PES) curves as functions of the rotation of one of the meso-substituted sulfonatophenyl groups about dihedral angles θ (corresponding to Cα─Cm─Cϕ─C) ranging from 40 to 130°, using 10° increments. The calculations show that protonation of core nitrogen atoms of porphyrin and meso-substituted porphyrins produces a substantial shift in Soret and Q-absorption bands, relative to their positions in corresponding nonprotonated and nonsubstituted chromophores. The results of the calculations are compared with experimental data. In this chapter, we discuss protonation and substitution effects on the absorption spectra of porphyrin molecules based on density functional theory (DFT) and time-dependent DFT calculations.
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