Question

Effects of green and blue fluroescent dyes were explored in the synthesis of benzopinacol. Dye A quenched the synthesis of benzpinacol and had a blue fluoresent light in the chemilluninescent reaction. Dye B also quenced the synthesis of benzpinacol and gave a green fluorsecence in the chemilluninescent reaction. What can you say about the energy levels of the S1 and T1 states of the 2 dyes? For T1, compare the dyes and benzpinacol and for S1, compare the green and blue dyes.

I dont need to see any mechanisms.

Answer 1

Electromagnetic radiation in the ultraviolet and visible region spans a wavelength range of about 800-100 nm corresponding to energies of 36-286 kcal/mole. Absorption of such radiation by molecules is not to be regarded as equivalent to simple excitation by thermal energy of 36-286 kcal/mole. All the energy of the light quantum is in exited state of a high energy of anti bonding orbital to ground state of low energy of bonding orbital. The absorption of a quantum of radio-frequency energy in NMR spectroscopy, where absorption process may be slow compared to chemical reactions. The schematic potential-energy curves for a molecule A-B in the ground state (A-B) and in excited electronic states (A-B)". In the ground states of most molecules all electrons are paired, excited states also can have all electrons paired. Such states with paired electrons are called singlet states. But, because the bonding is weaker in excited states, the average bond length between the nuclei is greater in the excited state than in the ground state. For this reason the upper curve (S,) displaced toward a larger average bond length relative to the lower or ground-state curve (So). Excited states also can have unpaired electrons. States with two unpaired electrons are called triplet states (T) and normally are more stable than the corresponding singlet states because, by Hund's rule, less inter electronic repulsion is expected with unpaired than paired electrons. The potential-energy curve for the excited triplet state (TI) of A-B is given an unrealistically long equilibrium bond distance, which puts it to the right of the curve for the S, state. The electronic configurations for ground singlet (S,), excited singlet (S,), and triplet (TI) states of the sigma electrons of a diatomic molecule.

When a molecule absorbs sufficient radiant energy to cause electronic excitation, the spin of the excited electron remains unchanged in the transition. The ground-state molecules with paired electrons (So) give excited states with paired electrons (S,), not triplet states (T,). The transition corresponds to a singlet-singlet (So S,) transition from a relatively high vibrational level of A-B. The energy change occurs with no change in r (Franck-Condon principle), and the electronic energy of the A-B* molecule so produced is seen to be above the level required for dissociation of A-B*. The vibration of the excited molecule therefore has no restoring force and leads to dissociation to A and B atoms. In contrast, the transition marked 2 leads to an excited vibrational state of A-B*, which is not expected to dissociate but can lose vibrational energy to the surroundings and come down to a lower vibrational state. This is called vibrational relaxation and usually requires about 10-l2 sec. The vibrationally "relaxed" excited state can return to ground state with emission of radiation (transition F, S, --+ So); this is known as fluorescenee, the wavelength of fluorescence being different from that of the original light absorbed. Normally, fluorescence, if it occurs at all, occurs in to low7 sec after absorption of the original radiation. In many cases, the excited state (S,) can return to the ground state (So) by non radioactive processes. The most important processes are: 1. By chemical reaction, often with surrounding molecules. By transfer of its excess electronic energy to other molecules. This kind of energy transfer also is a very important aspect of photochemistry. By decay through a lower energy state. They may actually cross at some point, thus providing a pathway for S, to relax to So without fluorescing. Conversion of a singlet excited state to a triplet state (S, --+ T,) is energetically favorable but usually occurs rather slowly, in accord with the spectroscopic selection rules, which predict that spontaneous changes of electron spin should have very low probabilities. The singlet state is sufficiently long-lived, the singlet-triplet change, S, ---+ T,, (often called intersystem crossing) may occur for a very considerable proportion of the excited singlet molecules. The triplet state, like the singlet state, can return to the ground state by nonradiative processes, but in many cases a radiative transition (TI ---+ So) occurs, even though it has low probability. Such transitions result in emission of light of considerably longer wavelength than either that absorbed originally or resulting from fluorescence. This type of radiative transition is called phosphorescence. Because phosphorescence is a process with a low probability, the T, state may persist from fractions of a second to many seconds. For benzene at -200°, the absorption of light at 254 nm leads to fluorescence centered on 290 nm and phosphorescence at 340 nm. The half-life of the triplet state of benzene at -200" is 7 sec.