Patrick M. Hare

Internal conversion to the electronic ground state occurs via two distinct pathways for pyrimidine bases in aqueous solution

The femtosecond transient absorption technique was used to study the relaxation of excited electronic states created by absorption of 267-nm light in all of the naturally occurring pyrimidine DNA and RNA bases in aqueous solution. The results reveal a surprising bifurcation of the initial excited-state population in <1 ps to two nonradiative decay channels within the manifold of singlet states. The first is the subpicosecond internal conversion channel first characterized in 2000. The second channel involves passage through a dark intermediate state assigned to a lowest-energy 1nπ* state. Approximately 10–50% of all photoexcited pyrimidine bases decay via the 1nπ* state, which has a lifetime of 10–150 ps. Three- to 6-fold-longer lifetimes are seen for pyrimidine nucleotides and nucleosides than for the corresponding free bases, revealing an unprecedented effect of ribosyl substitution on electronic energy relaxation. A small fraction of the 1nπ* population is proposed to undergo intersystem crossing to the lowest triplet state in competition with vibrational cooling, explaining the higher triplet yields observed for pyrimidine versus purine bases at room temperature. Some simple correlations exist between yields of the 1nπ* state and yields of some pyrimidine photoproducts, but more work is needed before the photochemical consequences of this state can be definitively determined. These findings lead to a dramatically different picture of electronic energy relaxation in single pyrimidine bases with important ramifications for understanding DNA photostability.

Hydrated electron extinction coefficient revisited.

The extinction coefficient of the hydrated electron (e(-))aq generated by pulse radiolysis is evaluated relative to the methyl viologen radical cation (*)MV(+), whose extinction coefficient at 605 nm has been carefully measured in the past. We find that the room temperature (e(-))aq extinction coefficients reported in the literature are underestimated by 10-20%. We obtain = 22,700 M(-1) cm(-1) for the 20 degrees C hydrated electron at 720 nm, assuming the (*)MV(+) extinction is 13,700 M(-1) cm(-1) at 605 nm. This has implications both for second-order reaction rate measurements of (e(-))aq and for the estimate of its integrated oscillator strength.

Solvated Electron Extinction Coefficient and Oscillator Strength in High Temperature Water

The decadic extinction coefficient of the hydrated electron is reported for the absorption maximum from room temperature to 380 degrees C. The extinction coefficient is established by relating the transient absorption of the hydrated electrons in the presence of a scavenger to the concentration of stable product produced in the same experiment. Scavengers used in this report are SF(6,) N(2)O, and methyl viologen. The room temperature value is established as 22,500 M(-1) cm(-1), higher by 10-20% than values used over the last several decades. We demonstrate how previous workers arrived at a low value by incorrect choice of a radiolysis yield value. With this revision, the integrated oscillator strength, corrected by refractive index, is definitely (ca. 10%) larger than unity. This result is fully consistent with EPR and resonance Raman results which indicate mixing of the hydrated electron wave function with solvent electronic orbitals. Oscillator strength appears to be conserved vs temperature.

Solvent Effects on the Steady State Photophysics of Estrone and 17β‐Estradiol

Absorption and emission yields for estrone and 17β‐estradiol were measured in a variety of room temperature solvents. Molar extinction coefficients were found to not vary as a function of solvent, while fluorescence yields were found to be significantly affected by the polarity and hydrogen‐bond accepting ability of the solvent, with the yield for 17β‐estradiol being highest in nonpolar, hydrogen‐bond donating solvents, and lowest in the nonpolar, hydrogen‐bond accepting solvent ethyl acetate. Estrone’s emission yield was found to be a factor of ten smaller than 17β‐estradiol’s. Strong solvent and excitation wavelength dependences were found for the relative amounts of emission between estrone’s two emission bands, with increased relative emission occurring in nonpolar aprotic solvents, and under higher excitation energies. These results are interpreted with the aid of vertical excitation energies from time‐dependent density functional calculations using both explicit and implicit solvation models.

Chapter 90 - Ultrasfast excited-state dynamics in DNA and RNA polymers

Recent progress in understanding the photophysics of single nucleobases has now focused attention on the challenging problem of excited-state dynamics in DNA and RNA polymers. It is often stated that ultrafast non-radiative decay by the monomeric nucleobases greatly reduces the likelihood of their photochemical damage. This is a tempting explanation for why the present-day bases may have been favored over alternative compounds as the carriers of genomic information in all living organisms. However, it is yet to be shown that the DNA and RNA polymers enjoy the same degree of photostability as the monomeric nucleobases. In fact, evidence is now accumulating that considerably longer-lived excitations are formed in natural and synthetic nucleic acid polymers. This chapter discusses how the noncovalent interactions responsible for nucleic acid secondary structure (that is, base stacking and base pairing) affect the photophysics of these multi-chromophoric systems. It describes initial experimental results that demonstrate dramatic differences in excited-state dynamics of nucleic acid polymers compared to their constituent monomers. Although ultrafast internal conversion is the dominant relaxation pathway for single bases, electronic energy relaxation in single-stranded polynucleotides occurs over a wide range of time scales extending from femtoseconds to nanoseconds. Since base stacking increases the electronic coupling between adjacent nucleobases, polymer conformation significantly affects the electronic structure of these important macromolecules. The experiments in the chapter demonstrate that the intrinsic UV chromophores of nucleic acids, the DNA and RNA bases, are useful ultrafast optical probes of secondary polymer structure.

Solvent‐Dependent Fluorescence Lifetimes of Estrone, 17β‐Estradiol and 17α‐Ethinylestradiol

The fluorescence lifetimes of the estrogens, estrone, 17β‐estradiol and 17α‐ethinylestradiol, were studied in various solvents. The fluorescence lifetimes of 17β‐estradiol and 17α‐ethinylestradiol decreased from 4.7 to 0.9 ns as the solvent hydrogen bond accepting ability increased, in good agreement with other phenolic molecules. Estrones two fluorescence bands had distinct lifetimes, with the 304 nm band having a lifetime shorter than 200 ps, reflecting efficient energy transfer to the carbonyl group, which had lifetimes ranging from 4.4 to 4.9 ns depending on the solvent. Solvent effects on the 1ππ*, 1πσ* and 1nπ* states that are relevant to estrogen photophysics can adequately explain these trends. The solvent dependence on the excited states of these potent endocrine disruptors has significant implications for their photochemistry.