A molecule exists in this electronic state when its total spin angular momentum quantum number S is equal to one. The triplet state is an important intermediate of organic chemistry. In addition to the wide range of triplet molecules available through photochemical excitation techniques, numerous molecules exist in stable triplet ground states, for example, oxygen molecules. Theoretical calculations, furthermore, make predictions concerning the spin multiplicities of the ground states of many prototype organic molecules such as cyclobutadiene, trimethylene methane, and methylene, and indicate that they will be triplets. See also: Atomic structure and spectra; Reactive intermediates; Spin (quantum mechanics)
A good working definition of a triplet state for the chemist is the following: A triplet is a paramagnetic even-electron species which possesses three distinct but energetically similar electronic states as a result of the magnetic interaction of two unpaired electron spins. The several important terms of this definition allow some insight as to the essential features of a triplet. First of all, a triplet is paramagnetic, and should thus display this property in a magnetic field. This paramagnetism serves as the basis for experimental magnetic susceptibility and electron spin resonance studies of the triplet state. However, one can imagine many paramagnetic odd-electron species which are not triplets, for example, nitric oxide. Thus, the criterion that a triplet must also be an even-electron species is apparent.
However, one can imagine paramagnetic, even-electron species which possess (1) only two distinct electronic states or (2) five or more electronic states. The former occurs when the paramagnetism results from two electrons which act as two independent odd electrons. For example, two carbon radicals separated by a long saturated chain will behave as two doublet states if there is sufficient separation to prevent spin interactions. Five or more electronic states result when four or six parallel electronic spins interact (to yield quintet and septet states, respectively). See also: Electron spin; Paramagnetism
One can now see that conceptual difficulties may arise in differentiating a biradical state (that is, a species possessing two independent odd-electron sites) from a triplet. Suppose two carbon radicals are separated by a long methylene chain as in Fig. 1a. If the methylene chain is sufficiently long and the odd-electron centers are so far removed from one another that they do not interact (magnetically and electronically) with one another, then the system is a doublet of doublets, that is, two independent odd electrons or a true biradical. If the methylene chain should be folded (Fig. 1b) so that the odd electrons begin to interact (magnetically and electronically) with one another, then at some distance R between the CH2 groups the doublet or doublets will become a triplet state. This state will result from the fact that the spin of the electron on carbon A is no longer independent of the spin on carbon B. Since the spins are quantized, selection
This leads to a difficulty in terminology: The “triplet state” is not one state but three states even in the absence of an external magnetic field. Indeed, under favorable conditions transitions may be observed between triplet levels at zero external magnetic field. The effect of an external magnetic field is to further split the triplet levels and allow transitions between them to be more easily detected.
A triplet may result whenever a molecule possesses two electrons which are both orbitally unpaired and spin unpaired. As shown in Fig. 2, orbital unpairing of electrons results when a molecule absorbs a photon of visible or ultraviolet light. Direct formation of a triplet as a result of this photon absorption is a very improbable process since both the orbit and spin of the electron would have to change simultaneously. Thus, a singlet state is generally formed by absorption of light. However, quite often the lifetime of this singlet state is sufficiently long to allow the spin of one of the two electrons to invert, thereby producing a triplet. The following discussion considers the ways in which such a species is unambiguously characteristic. See also: Molecular orbital theory
The question to be answered is: What are the general properties to be expected of a molecule in the triplet state? Some of the more important physical properties are (1) paramagnetism; (2) absorption between triplet sublevels; (3) electronic absorption from the lowest triplet to upper triplets; (4) electronic emission from the lowest triplet to a lower singlet ground state (if the triplet level is not the ground state). The paramagnetism of the triplet results from the interaction of unpaired spins and the fact that an unpaired spin shows a paramagnetic effect (is attracted) in a magnetic field.
Absorption between triplet sublevels may be observed directly by the use of an electron spin resonance spectrometer. See also: Electron paramagnetic resonance (EPR) spectroscopy
The triplet, like any other electronic state, may be excited to upper electronic states of the same spin as the result of light absorption. In favorable cases this may be observed by the method of flash spectroscopy. See also: Photochemistry
For most organic molecules the lowest triplet state is an excited electronic state and may emit light and pass to the ground singlet state. Since light absorption to form a triplet from a singlet is improbable, the symmetrically related emission of light from a triplet returning to a ground state is likewise improbable. Indeed, it takes the triplet states of some aromatic molecules an average of about 30 s to emit light. This phenomenon is known as phosphorescence and is to be contrasted with fluorescence, the emission of light from an excited singlet state returning to a singlet ground state, a process which often occurs in nanoseconds. See also: Fluorescence; Phosphorescence
Although phosphorescence (long-lived emission) was the first method employed to study triplets, it is not a specific device for establishing whether a long-lived emission occurs from a triplet. For instance, examples are known for which the slow combination of positive and negative sites will generate excited molecules which emit light. In this case the combination reaction may be rate-determining for light emission.
Similarly, absorption from one triplet to another is not a specific method since the precise triplet-triplet absorption characteristics cannot be predicted accurately. It would thus remain to be proven that the absorbing species is indeed a triplet and not some other transient species.
Even paramagnetism is not an infallible probe for a triplet state since free radicals which are also paramagnetic are often produced by the absorption of light.
It appears that electron spin resonance (ESR) is probably the most powerful single method for establishing that a molecule is in its triplet state. The nature of the ESR signals may be predicted and fitted to theoretical relation (2), which