When a starting material has two or more functional groups, one may influence how another functional group in the molecule reacts if the groups are close enough. This chapter discusses difunctional molecules and their chemical reactions. It discusses the fundamental principles of photochemistry. Photochemistry refers to reactions brought about by the absorption of light. Both UV and visible light can excite molecules to higher electronic states. Different functional groups absorb light according to their individual structure. Benzophenone is a common triplet sensitizer. The use of a sensitizer will influence the choice of the irradiation wavelength. Tuning of the wavelength plays a key role in the outcome of a photochemical path. The instrument used to measure the effect of UV light on a molecule is called an ultraviolet spectrophotometer.