Principles Of Molecular Photochemistry: An Introduction.mobi
Shanae Maerz
Principles of Molecular Photochemistry: An Introduction
Molecular photochemistry is the study of the interactions between molecules and light, and the resulting chemical and physical changes. Photochemical reactions are important in many fields, such as organic synthesis, solar energy conversion, environmental chemistry, biological processes, and nanotechnology. In this article, we will introduce some basic principles of molecular photochemistry, such as electronic states, photophysical processes, radiative and radiationless transitions, energy transfer and electron transfer.
Electronic States
A molecule can exist in different electronic states, which are characterized by the distribution of electrons among the molecular orbitals. The lowest energy state is called the ground state, and the higher energy states are called excited states. The energy difference between two electronic states is equal to the energy of a photon that can induce a transition between them. The transition can be either absorption or emission of a photon, depending on the direction of the energy change.
The electronic states can be further classified by their spin multiplicity, which is determined by the number of unpaired electrons in the molecule. A singlet state has no unpaired electrons and a spin multiplicity of one, while a triplet state has two unpaired electrons and a spin multiplicity of three. The spin multiplicity affects the symmetry and stability of the electronic states, as well as the selection rules for the transitions between them.
The electronic states can also be described by their vibrational and rotational levels, which are determined by the nuclear motions in the molecule. Each electronic state has a set of vibrational and rotational levels, which are quantized according to the quantum mechanical model. The vibrational levels are separated by larger energy gaps than the rotational levels, and they can be represented by a potential energy curve or surface. The shape of the potential energy curve or surface depends on the geometry and bonding of the molecule in each electronic state.
Photophysical Processes
When a molecule absorbs a photon, it undergoes a transition from the ground state to an excited state. The excited state can then undergo various photophysical processes, which are changes in the energy or spin of the molecule without breaking or forming any bonds. Some common photophysical processes are fluorescence, phosphorescence, internal conversion, intersystem crossing, and external conversion.
Fluorescence is the emission of a photon from an excited singlet state to a lower singlet state, usually the ground state. Fluorescence occurs within nanoseconds after absorption, and it is usually accompanied by vibrational relaxation, which is the loss of excess vibrational energy to heat. Fluorescence can be detected by measuring the intensity and spectrum of the emitted light.
Phosphorescence is the emission of a photon from an excited triplet state to a lower singlet state, usually the ground state. Phosphorescence occurs after intersystem crossing, which is a spin-flip transition from a singlet state to a triplet state. Intersystem crossing is usually induced by spin-orbit coupling, which is an interaction between the orbital angular momentum and the spin angular momentum of the electrons. Phosphorescence occurs on a longer timescale than fluorescence, ranging from microseconds to hours, depending on the rate of intersystem crossing and the lifetime of the triplet state. Phosphorescence can be detected by measuring the intensity and spectrum of the emitted light after removing or delaying the fluorescence signal.
Internal conversion is a non-radiative transition from an excited state to a lower state of the same spin multiplicity. Internal conversion is usually induced by vibronic coupling, which is an interaction between the vibrational and electronic motions of the molecule. Internal conversion occurs on a very fast timescale, ranging from femtoseconds to picoseconds, and it is usually accompanied by vibrational relaxation. Internal conversion can be detected by measuring the heat or sound generated by the molecule.
External conversion is a non-radiative transition from an excited state to a lower state of the same spin multiplicity, mediated by the interaction with the surrounding molecules or solvent. External conversion occurs on a similar timescale as internal conversion, and it is also accompanied by vibrational relaxation. External conversion can be detected by measuring the heat or sound generated by the molecule and the solvent.
Radiative and Radiationless Transitions
The photophysical processes can be classified into two types: radiative and radiationless transitions. Radiative transitions are those that involve the emission or absorption of a photon, such as fluorescence and phosphorescence. Radiationless transitions are those that do not involve any photons, such as internal conversion and external conversion. The relative rates of radiative and radiationless transitions determine the quantum yield and the lifetime of the excited state.
The quantum yield is the ratio of the number of molecules that undergo a certain photophysical process to the number of molecules that absorb photons. For example, the fluorescence quantum yield is the ratio of the number of molecules that emit fluorescence photons to the number of molecules that absorb photons. The quantum yield can range from zero to one, depending on the efficiency of the photophysical process.
The lifetime is the average time that a molecule spends in an excited state before undergoing a photophysical process. For example, the fluorescence lifetime is the average time that a molecule spends in an excited singlet state before emitting a fluorescence photon or undergoing a non-radiative transition. The lifetime can range from femtoseconds to hours, depending on the rate constants of the photophysical processes.
Energy Transfer and Electron Transfer
Besides undergoing photophysical processes, an excited molecule can also interact with another molecule and transfer its energy or electron. These processes are called energy transfer and electron transfer, respectively. Energy transfer and electron transfer are important in many photochemical reactions, such as photosynthesis, photovoltaic devices, photocatalysis, and photoinduced charge separation.
Energy transfer is the transfer of excitation energy from a donor molecule to an acceptor molecule without exchanging any electrons. Energy transfer can occur by two mechanisms: resonance energy transfer and collisional energy transfer. Resonance energy transfer is a long-range process that involves dipole-dipole coupling between the donor and acceptor molecules. Resonance energy transfer requires spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, as well as proper orientation of their transition dipole moments. Collisional energy transfer is a short-range process that involves direct contact between the donor and acceptor molecules. Collisional energy transfer requires thermal activation and collision frequency between the donor and acceptor molecules.
Electron transfer is the transfer of an electron from a donor molecule to an acceptor molecule, resulting in the formation of a radical pair. Electron transfer can occur by two mechanisms: photoinduced electron transfer and thermally activated electron transfer. Photoinduced electron transfer is a process that involves direct excitation of either the donor or acceptor molecule by a photon, followed by electron transfer to or from another molecule. Photoinduced electron transfer requires favorable energetics and kinetics between the donor and acceptor molecules, as well as proper alignment of their orbitals. Thermally activated electron transfer is a process that involves thermal excitation of either the donor or acceptor molecule by heat, followed by electron transfer to or from another molecule. Thermally activated electron transfer requires high temperature and high concentration of the donor and acceptor molecules, as well as proper alignment of their orbitals.
The outcome of energy transfer and electron transfer depends on the relative rates of various processes, such as charge recombination, charge separation, quenching, sensitization, and catalysis. These processes can be influenced by various factors, such as solvent polarity, pH, temperature, concentration, and additives. The efficiency and mechanism of energy transfer and electron transfer can be studied by various spectroscopic techniques, such as fluorescence, phosphorescence, absorption, emission, transient absorption, time-resolved fluorescence, and electron paramagnetic resonance.
Conclusion
In this article, we have introduced some basic principles of molecular photochemistry, such as electronic states, photophysical processes, radiative and radiationless transitions, energy transfer and electron transfer. We have also discussed some examples and applications of photochemical reactions in various fields. Molecular photochemistry is a fascinating and interdisciplinary subject that combines chemistry, physics, biology, and engineering. We hope this article has sparked your interest and curiosity in molecular photochemistry.
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