Excited states of six-membered AZA-aromatic rings. X. Acid-base equilibria of electronically excited quinoxaline as an example of anomalous diazines

Photophysical characterization of 2,3-difunctionalized thieno[3,4-b]pyrazines

“…As protonation should result in stabilization of the nonbonding MOs, the energy of the n-p à transition should increase, causing a blue shift of the transition. In contrast, the p-p à transitions of the analogous quinoxaline exhibit a red shift upon protonation, which has been attributed to the destabilization of the p MOs [31]. However, as protonation results in a reduction of nitrogen electron density, one would expect stabilization of both the nitrogen contributing p MOs and p à MOs.…”

“…For example, the n-p à transition in the parent pyrazine typically undergoes a 10-20 nm blue-shift from hydrocarbon to alcohol [28][29][30]. In contrast, the p-p à transitions of pyrazine show no solvent dependence with the single exception of a strong red-shift under acidic conditions [30,31]. The lack of solvent dependence for this low-energy transition of the thieno [3,4-b]pyrazine strengthens the argument for a p-p à assignment.…”

The Förster Cycle Reconsidered

“…Normalisation of the luminescence spectrum to the corresponding absorption maximum is often arbitrary. c) Change of orbital origin between substrate and product In numerous systems A* and B* are of different orbital origin, e.g., the lowest excited singlet state of quinoxaline is an 1( , *) state, while the conjugate acid, quinoxalinium ion, is a 1( , *) state [12]. 4).…”

“…Hn pH Fig. Examples of acid-base equilibria (12) and (13) in several diazines in three electronic states. Examples of acid-base equilibria (12) and (13) in several diazines in three electronic states.…”

“…5. Interrogation-the value could not bo determined experimentally Polynuclear diazines containing the pyrazine ring exhibit particular properties : unusually high basicity in the triplet state, and the lack of room temperature fluorescence of the protonated forms [12,27,28]. Vertical segments mark pKG values.…”