photochemistry

The study of chemical reactions of molecules in electronically excited states produced by the absorption of infrared (700–1000 nanometers), visible (400–700 nm), ultraviolet (200–400 nm), or vacuum ultraviolet (100–200 nm) light. Bond making and bond breaking as well as electron transfer and ionization are often observed in both organic and inorganic compounds as a consequence of such excitation.

Electronic absorption

An important generalization sometimes called the first law of photochemistry is that only light that is absorbed can induce chemical change. The absorption of a photon induces an electronic transition in which an electron originally present in a molecular orbital, usually a bonding or nonbonding molecular orbital of the ground state of the absorbing molecule, is promoted to a higher-lying orbital. The excited state produced by absorption of light has a different electronic structure than its ground-state precursor and can reasonably be regarded as an isomeric species with distinct and characteristic chemical and physical properties.

Most organic molecules exist as ground-state singlets in which all electrons are paired. Because photoexcitation causes the promotion of only a single electron, two singly occupied orbitals are produced upon excitation. If the electronic transition takes place without a spin inversion, these two electrons have opposite spins, and a singlet excited state is produced. The number of unpaired electrons in a molecule determines its multiplicity: a molecule with no unpaired spins is a singlet; one with one unpaired spin is a doublet; one with two unpaired spins is a triplet; one with three unpaired spins is a quartet, and so forth. If an electronic transition were to take place with a spin inversion, the two singly occupied orbitals would be populated by electrons with parallel spins, producing a triplet excited state. Spin restrictions forbid spin inversion during excitation, and only singlet-singlet electronic transitions are easily observed spectroscopically. After excitation, however, a change in state multiplicity can take place by a process called intersystem crossing. The facility of intersystem crossing is influenced by the magnitude of spin-orbital coupling, which can be enhanced by the presence of a heavy atom (an atom in the third row or below of the periodic table), either bound to the absorbing molecule or present externally as solvent. See also Periodic table; Triplet state.

Transitions

A chromophore is that part of the molecule that accounts for its absorption of light and its photochemical activity. The absorption corresponding to a particular chromophore depends on the type of transition involved in that particular excitation. The promotion of an electron from a π-bonding molecular orbital to a π-antibonding orbital is referred to as a π,π* (read pi to pi star) transition. Such transitions are frequently encountered in alkenes, alkynes, aromatic molecules, and other unsaturated compounds. Because the spatial overlap of π and π* orbitals is substantial, such a transition typically has high oscillator strength and a large extinction coefficient (absorptivity). Promotion of an electron from a nonbonding molecular orbital to a π-antibonding orbital, referred to as an n,π* transition, involves orbitals that are nearly orthogonal; and it takes place only inefficiently; that is, it has a low oscillator strength and a small extinction coefficient. Such transitions are often encountered in compounds containing carbon-heteroatom or heteroatom-heteroatom double bonds. Because nonbonding molecular orbitals lie at higher energy than bonding ones, n,π* transitions are of lower energy than the corresponding π,π* transitions. Both n,π* and π,π* transitions are usually found in the ultraviolet region of the electromagnetic spectrum. Transitions involving sigma (σ) bonds (for example nσ* transitions in amines, alcohols, ethers, and alkyl halides and σ,σ* transitions in alkanes) are usually encountered at the high-energy end of the ultraviolet spectrum or in the vacuum ultraviolet region. See also Chemical bonding.

Each allowed transition of a compound registers as a band in the absorption spectrum, with the intensity of the transition (measured by its extinction coefficient) being governed by the operative selection rules. The transition intensity of a given absorption is measured by integrating over the whole absorption band. The resulting integrated absorption coefficient is directly proportional to the oscillator strength of the transition. The oscillator strength, a measure of the allowedness of an electric dipole transition compared to that of a free electron oscillating in the three dimensions, is directly related to an experimentally measured value, the extinction coefficient (ε). Beer's law is given by Eq. (1),
1.
where A is the observed absorbance, ε is the extinction coefficient, b is the path length (in centimeters) of the cell used for the measurement, and c is the molar concentration of the absorbing species. This law is used to correlate the observed absorbance with the extinction coefficient and concentration of the absorbing species.

Photophysics

The excited state produced by absorption of a photon is not generally a stable species. After a characteristic lifetime that can vary from femtoseconds (10⁻¹⁵ s) to hours, the excited molecule will either relax to its ground-state precursor or undergo a chemical transformation. The term photophysics is used to describe nonreactive relaxation processes, which include radiative (taking place with the emission of light) and nonradiative (taking place without the emission of light) pathways.

The energies of the lowest singlet and triplet excited states (relative to the ground state) can be obtained from the longest wavelength band of the fluorescence and phosphorescence spectra, respectively. This band is called a 0,0 band to indicate a transition between the lowest vibrational levels of the lowest-lying states. Singlet and triplet energies can also be determined indirectly by measuring quenching efficiencies. The shift between the 0,0 bands for absorption and emission in a single molecule is called its Stokes shift. A small Stokes shift is usually observed when the excited state has a geometry similar to the ground state. A Jablonski diagram (see illustration) is often used to graphically depict the relationship between competing photophysical processes.

Jablonski diagram. Solid arrows represent radiative processes; and wavy arrows nonradiative processes. S terms = singlet states; T terms = triplet states.

Quantum yield, or quantum efficiency, is defined as the number of molecules participating in a given photophysical process or reaction divided by the number of photons absorbed. The quantum yield ranges between zero and one for photoreactions induced by a single photon; values larger than one are indicative of a chain process in which product is formed in a repeating, dark cycle initiated by the photoexcitation. For a photochemical reaction, the number of molecules participating in the reaction is determined spectroscopically or chromatographically as a chemical yield per volume unit per time. The number of photons absorbed is obtained by measuring with a radiometer the light flux per volume unit per time or by employing a chemical actinometer, a known chemical reaction for which the quantum yield is known and accepted as a standard. See also Quantum chemistry.

Energy transfer

The process by which an excited state molecule, M*, in an excited singlet or triplet state transfers all or part of its excitation energy to a reaction partner or quencher, Q, is called energy transfer or quenching when the molecule of interest is M [reaction (2)]. 2 This same process is called sensitization when the molecule of interest is Q. In the latter case, M is called the sensitizer. Energy transfer permits an exception to the first law of photochemistry in that Q* is produced without having absorbed the incident light.

For energy transfer to take place, an incident wavelength must be chosen so that M is primarily excited, producing an excited state M* whose energy lies above that of Q*. Symmetry selection rules require that all energy transfer events preserve spin multiplicity. Thus, if M* is an excited singlet and Q is a ground-state singlet, M will be produced as a ground-state singlet and Q* as an excited singlet. If M* is an excited triplet and Q is a ground-state singlet, M will be produced as a ground-state singlet and Q* as an excited triplet.

Photochemical mechanisms

As in all studies of mechanisms of chemical reactions, determining the structure of all products is the first step in the specification of a photochemical reaction. Spectroscopic (nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, infrared spectroscopy, mass spectroscopy, x-ray analysis, absorption spectroscopy) and chromatographic (gas, liquid, or thin-layer chromatography) techniques are used to establish product structure and to determine product yields. Monitoring the effect of solvent polarity on reaction rate, the retention or loss of optical activity during the reaction, the positions of isotopic labels, and the success of intermediate trapping experiments can distinguish step-wise chemical reactions (those that proceed through one or more intermediates) from concerted reactions (those that proceed without intermediates). In addition to these mechanistic approaches, the identity and lifetime of the reactive excited state (singlet, triplet, and so forth) and the quantum yields for both product formation and for other competing photophysical processes are required for a full photochemical mechanistic characterization. Time-resolved flash photolysis and pulse radiolysis measurements can, in addition, be used sometimes for direct spectroscopic detection of absorptive or emissive intermediates encountered in a photochemical mechanism, as well as for their kinetic characterization. The addition of specific reactive quenchers or traps, conducting a photoreaction in a low-temperature matrix in which diffusion processes are stopped, and sensitization experiments are effective means for assigning the observed transient absorptions or emissions. The energetics of a well-defined photochemical reaction can be obtained by photoacoustic calorimetry measurements. See also Chromatography; Matrix isolation; Photolysis; Spectroscopy.

The branch of chemistry that deals with the chemical properties or effects of light rays or other radiation.

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