Experiment 1: Excited-state properties of 2-naphthol (the acidity constants)

The electronic structure of a molecule determines such physical and chemical properties as its charge distribution, geometry (and therefore the dipole moment), ionization potential, electron affinity, and of course, chemical reactivity. If the electronic structure of a molecule were to be changed, one would expect its physical and chemical properties to be altered. Such a rearrangement can occur when a molecule is raised to an electronically excited state via the absorption of a quantum of light (i.e., a photon) whose energy matches the energy gap between the ground and excited states. For most organic molecules that contain an even number of electrons, the ground state is characterized by having all electron spins paired; the net spin angular momentum is zero, and such an arrangement is called a singlet state. When considered in terms of molecular orbitals (MOs), electronic excitation involves the promotion of an electron from a filled MO to a higher, unoccupied MO. This new electronic configuration, which characterizes the electronically excited state, may be one in which the two electrons reside in the singly occupied MO’s with opposite spins. Accordingly, this electronically excited state is also a singlet. The ground, and lowest electronically excited, singlet states are often denoted as S0 and S1, respectively. Higher excited singlet states are referred to as S2, S3, ..., Sn. Although measurements of the physical and chemical properties of a molecule in its ground state can be carried out, more or less, at leisure (assuming that the molecule is thermally stable), the examination of these properties in its excited states is severely hampered by the fact that these states are very short-lived. For most molecules, S1 states have lifetimes ranging from 10 – 10 s (i.e., μs to some ps). Excited states are metastable; they undergo decay processes that dissipate the energy they possess relative to more stable products. For example, the excited state of a molecule may, in general: spontaneously return (spontaneous emission; lifetime 10 to 10 s) to the ground state via photon emission (fluorescence), convert electronic excitation into vibrational energy (eventually generating heat), undergo bond dissociation or rearrangement (“fast”) or a change in electron spin multiplicity (“slow”). Because spontaneous emission from an excited state (i.e., fluorescence) often takes place very rapidly, fluorescence can be used as a probe, or measurement, of excited state concentration (i.e., fluorescence assay). In addition, fluorescence studies can provide information about the physical and chemical properties of these short-lived singlet states. This field of experimentation is called photochemistry or photophysics depending on what is being studied. In this experiment, the ground and excited state acidity constants of 2-naphthol (ArOH; see Fig. 1) will be determined. For more information on optical spectroscopy, see Refs. [1–3].