Absorption and Fluorescence

The colors that we perceive from various materials comes from the light absorption properties of the molecules within the material. All materials absorb light somewhere in the electromagnetic spectrum, and the chemical structure of the individual compounds making up the material will exhibit unique wavelengths (or energies) of absorption. For organic materials, there will be specific infrared and ultraviolet absorptions, with some exhibiting visible light absorptions. Absorption can be measured by simply passing light through a sample, and measuring the wavelength and amount of light not transmitted to the other side of the sample (see below). That light that was lost to the sample must have been absorbed. The mechanism for light absorption is unique to the various regions of the electromagnetic spectrum. This is due to those wavelengths accessing different energies which correspond to "exciting" different types of motion resonant with those energies. For example, IR absorptions generally occur because nuclear motion, or vibrations, are excited within a molecule. This type of mechanism is the origin for the greenhouse effect for example. Visible and UV absorption is associated with higher energies that access transitions between electronic states of an atom or molecule. The wavelengths that these absorptions occur are very sensitive to the chemical and electronic structure of the material - thus one can use the measurement to learn about those properties. 

While absorption spectroscopy is very versatile and commonplace in the chemical analysis lab, it lacks species selectivity as well as relaying information that takes place after the light absorption event. Fluorescence, or light emission from a singlet state, is a process that takes place from the excited species some time after it has absorbed a photon (usually nanoseconds). This excited state deactivation pathway is a way to bring the molecule back to its lowest energy (ground) state, or relax, by emission of a photon with an energy matching the difference between ground and excited state (see above). By collecting the emitted light and dispersing its wavelengths across a detector, a fluorescence spectrum is measured which shows transitions from the excited state mapping back onto the ground state. Often these spectra resemble mirror images of their corresponding absorption spectrum; however, when they don't it ofttimes signals the existence of other excited state dynamics taking place. With that in mind, the total amount of fluorescence can also signal the propensity for the molecule to undergo other dynamics prior to fluorescence, such as internal conversion, intersystem crossing, photochemistry, etc. With absorption and fluorescence utilized in tandem, a myriad of properties relating to a molecule's light harvesting ability or light-activated function can be determined.