Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and 1700 cm⁻¹ in room-temperature acetonitrile-d₃ solution. These bands and additional ones observed between 1300 and 1450 cm⁻¹ are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4O carbonyl exhibits substantial single-bond character, explaining the large (70 cm⁻¹) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed ¹nπ^* state as the triplet precursor.     

A new type of Littlewood-Paley partition

We define a partition of Z into intervals {I _j} and prove the Littlewood-Paley inequality ‖f‖ _p ≦C _p‖Sf‖ _p , 2≦p<∞. Heref is a function on [o, 2π) and . This is a new example of a partition having the Littlewood-Paley property since the {I _j} are not of the type obtained by iterating lacunary partitions finitely many times.     

A new type of Littlewood-Paley partition

We define a partition of Z into intervals {I _j} and prove the Littlewood-Paley inequality ‖f‖ _p ≦C _p‖Sf‖ _p , 2≦p<∞. Heref is a function on [o, 2π) and \(Sf = (\sum |\Delta _j |^2 )^{1/2} , \hat \Delta j = \hat f\chi _{Ij} \) . This is a new example of a partition having the Littlewood-Paley property since the {I _j} are not of the type obtained by iterating lacunary partitions finitely many times.     

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.     

Solvent-dependent photophysics of 1-cyclohexyluracil: ultrafast branching in the initial bright state leads nonradiatively to the electronic ground state and a long-lived 1npi* state

The modified nucleic acid base, 1-cyclohexyluracil, was studied by femtosecond transient absorption spectroscopy in protic and aprotic solvents of varying polarity. UV excitation at 267 nm populates the lowest-energy bright state, a (1)pipi* state, which has a lifetime of 120-270 fs, depending on the solvent. In all solvents, this initial bright state population bifurcates with approximately 60% undergoing subpicosecond nonradiative decay to the electronic ground state and the remaining population branching to a singlet dark state. The latter absorbs between 340 and 450 nm. The latter state is assigned to the lowest-energy (1)npi* state. It decays to the electronic ground state with a lifetime that varies from 26 ps in water to at least several nanoseconds in aprotic solvents. The results suggest that the two nonradiative decay pathways identified for photoexcited uracil in recent quantum chemical calculations (Matsika, S. J. Phys. Chem. A. 2004, 108, 7584) are simultaneously operative in a wide variety of solvent environments. The lowest-energy triplet state was also detected by transient absorption. The triplet population appears in a few picoseconds and is not formed from the thermalized (1)npi* state. It is suggested that high spin-orbit coupling is found only along initial segments of the nonradiative decay pathways. Efficient intersystem crossing prior to vibrational cooling offers a possible explanation for the wavelength-dependent triplet yields seen in single DNA bases.