In resonant radiation, a photon of a particular wavelength is absorbed and an equivalent photon is immediately emitted, through which no significant internal energy transitions of the chemical substrate between absorption and emission are involved and the process is usually of an order of 10 nanoseconds.

When the chemical substrate undergoes internal energy transitions before relaxing to its ground state by emitting photons, some of the absorbed energy is dissipated so that the emitted light photons are of lower energy than those absorbed. One of such most familiar phenomenon is fluorescence, which has a short lifetime (10^-8 to 10^-4 s).

Phosphorescence is a radiational transition, in which the absorbed energy undergoes intersystem crossing into a state with a different spin multiplicity. The lifetime of phosphorescence is usually from 10^-4 - 10^-2 s, much longer than that of Fluorescence. Therefore, phosphorescence is even rarer than fluorescence, since a molecule in the triplet state has a good chance of undergoing intersystem crossing to ground state before phosphorescence can occur.

From the photoluminescence spectra of hexaphenylsilole (HPS, Figure 4) shown in Figure 5, it can be seen that as the water (bad solvent) fraction increases, the emission intensity of HPS increases. For BODIPY derivative (Figure 6) in Figure 7, it shows that the PL intensity peaks at 0 water content resulted from intramolecular rotation or twisting, known as twisted intramolecular charge transfer (TICT).

Figure 4: The structure of hexaphenylsilole (HPS).

Figure 5: PL spectra of HPS solutions in acetonitrile/water mixtures. Adapted from Y. Hong, J. W. Y. Lam, and B. Z. Tang, Chem. Commun., 2009, 4332. Copyright: The Royal Society of Chemistry (2009).

Figure 6: The structure of a triphenylamine–boradiazaindacene (BODIPY) derivative.

Figure 7: PL spectra of BODIPY derivative in THF/water mixtures. Adapted from Y. Hong, J. W. Y. Lam, and B. Z. Tang, Chem. Commun., 2009, 4332. Copyright: The Royal Society of Chemistry (2009).

The emission color of an AIE luminogen is scarcely affected by solvent polarity, whereas that of a TICT luminogen typically bathochromically shifts with increasing solvent polarity. In Figure 8, however, it shows different patterns of emission under different excitation wavelengths. At the excitation wavelength of 372 nm, which is corresponding to the BODIPY group, the emission intensity increases as water fraction increases. However, it decreases at the excitation wavelength of 530 nm, which is corresponding to the TPE group. The presence of two emissions in this compound is due to the presence of two independent groups in the compound with AIE and ACQ properties, respectively.

Figure 8: PL spectra of compound containing AIE and ACQ groups in THF/water mixtures at the excitation wavelength of 329 nm. Adapted from Y. Hong, J. W. Y. Lam, and B. Z. Tang, Chem. Commun., 2009, 4332. Copyright: The Royal Society of Chemistry (2009).

Figure 9 shows the photoluminescence spectroscopy of a BODIPY-TPE derivative of different concentrations. At the excitation wavelength of 329 nm, as the molarity increases, the emission intensity decreases. Such compounds whose PL emission intensity enhances at low concentration can be a good chemo-sensor for the detection of the presence of compounds with low quantity.

Figure 9: PL spectra of a BODIPY derivative solution in different concentrations in THF at excitation wavelength of 329 nm.

Band gap is the energy difference between states in the conduction and valence bands, of the radiative transition in semiconductors. The spectral distribution of PL from a semiconductor can be analyzed to nondestructively determine the electronic band gap. This provides a means to quantify the elemental composition of compound semiconductor and is a vitally important material parameter influencing solar cell device efficiency.

Radiative transitions in semiconductors involve localized defect levels. The photoluminescence energy associated with these levels can be used to identify specific defects, and the amount of photoluminescence can be used to determine their concentration. The PL spectrum at low sample temperatures often reveals spectral peaks associated with impurities contained within the host material. Fourier transform photoluminescence microspectroscopy, which is of high sensitivity, provides the potential to identify extremely low concentrations of intentional and unintentional impurities that can strongly affect material quality and device performance.

The return to equilibrium, known as “recombination”, can involve both radiative and nonradiative processes. The quantity of PL emitted from a material is directly related to the relative amount of radiative and nonradiative recombination rates. Nonradiative rates are typically associated with impurities and the amount of photoluminescence and its dependence on the level of photo-excitation and temperature are directly related to the dominant recombination process. Thus, analysis of photoluminescence can qualitatively monitor changes in material quality as a function of growth and processing conditions and help understand the underlying physics of the recombination mechanism.

The widely used conventional methods such as XRD, IR and Raman spectroscopy, are very often not sensitive enough for supported oxide catalysts with low metal oxide concentrations. Photoluminescence, however, is very sensitive to surface effects or adsorbed species of semiconductor particles and thus can be used as a probe of electron-hole surface processes.