The luminescence of the scorpion's outer shell has been shown to be due to fluorescence of very short lifetime (nanoseconds). The emission and excitation spectra have been determined, and the potential biological significance of this photoluminescence is discussed.

The rate constants of quenching of the triplet state T₁ of four aromatic ketones by triethylamine in acetonitrile show no correlation with the Gibbs energy of the reaction, but depend on the nature of T₁, i.e. n–π*, π–π* or charge transfer (CT) type. For example, the ratio of rate constants between two systems of n–π* and CT type, with the same driving force, is over 10⁸. It is concluded that these reactions are kinetically controlled, the decisive factor for the activation energy being the electrostatic charge distribution of the carbonyl group in the triplet state. 

The free ion yield (R) resulting from the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by various electron donors in acetonitrile has been studied using ns laser photoconductivity. The influence of the chemical nature of the doors is established in a general manner. For a given oxidation potential Eox(D/D+), the rate constant of geminate ion recombination, kbac, decreases significantly as the electronic delocalization of the donor increases. As a consequence multiple Marcus plots are observed in the inverted region. These plots show decreasing curvature when going from stilbenes to amines as donors. This fan effect is tentatively explained by considering the detailed roles of the parameters V, and h in the Marcus model.

A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.

The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives

In conditions of laser flash photolysis, the kinetics of decay of the absorption of the benzophenone radical anion show that free, solvated ions are formed after electron transfer between the title compounds in neat, dry acetonitrile. Furthermore, it is shown that the opposite conclusion claimed by Devadoss and Fessenden (J. Phys. Chem., 1990, 94,4540), Le., no ion pair dissociation, results from a misinterpretation of the transient decay rate.

The energy balance of a photoinduced electron transfer reaction is given by the Rehm-Weller equation which combines the oxidation potential Eox(D) of the electron donor, the reduction potential Ered(A) of the electron acceptor, an electrostatic correction term C and the excited state energy of the light-absorbing species: It is shown that if light carries a thermodynamic entropy the excitation energy term must be given by ηE*, η being the efficiency of the conversion of the energy of light into chemical free energy. Measurements of fluorescence quenching through electron transfer at very low light intensities show that the Rehm-Weller equation remains valid in spite of its implied assumption that η = 1; it is concluded that contrary to much current thinking light is a form of high grade energy which can be converted in principle entirely into chemical free energy and electrical energy.

A study of the temperature dependence (from 233 to 353 K) of the rate of back electron-transfer reactions within geminate radical pairs by measurement of the free radical yield is reported. The radical pair is generated by photoinduced electron transfer with rhodamine 6G and oxazine 118 cations as electron acceptors and aromatic amines and methoxy-benzene derivatives as electron donors in acetonitrile, methanol and ethanol. In acetonitrile, the back electron transfer is non-adiabatic and apparent negative activation energies are observed for barrierless reactions. In alcohol solvents, an anomalously large temperature dependence is observed, which is attributed to a solvent-controlled adiabatic behaviour.

A study of photo-induced electron-transfer reactions in MeCN with 9,10-dicyanoanthracene as acceptor and 21 electron donors with transient photoconductivity measurements is reported. The free-ion yield and the rate constant of back electron transfer are determined. For exergonic reactions, the Marcus-inverted region is observed. The fit with the theory is best, when a nearly solvent-independent Coulomb term is used in the calculation of the energy balance.

Photoinduced electron transfer reactions in acetonitrile with bensopheneone, anthraquinone, 9-cyanoanthracene and 9,10-dicyanoanthracene as electron acceptors, and with 1,4-diasabicyclo[2,2,2]octane and N,N-dimethylaniline as electron donors have been studied with ns-laser flash photolysis and fluorescence quenching measurements. For these systems the resulting free ion yield depends on the spin state of the geminate ion pair: its separation is very efficient if formed in a triplet state (carbonyl compounds/donors), while it is very inefficient if formed in a singlet state (cyanoanthracenes/donors). In the triplet systems, geminate back electron transfer is limited by the rate of spin flip.

Naphthalene and anthracene undergo a monophotonic ionization process in MeCN to produce the radical cations in low quantum yields (around 0.06 for anthracene). This reaction originates from the relaxed singlet excited state S1, and it is not due to traces of H2O in the solvent.

Laser-flash-photolysis experiments show that, in MeCN at 20°, perylene (P) undergoes three distinct electron-transfer reactions: These processes originate probably from the thermally relaxed excited states of P.