Femtosecond broad-band pump−probe spectroscopy has been used to study intramolecular bichromophoric coupling and structural relaxation in pyrene and aryl pyrene derivatives in solution. The influence of aryl substituents on the S2 → S1 internal conversion process, which occurs with a time constant of ∼75 fs in pyrene, has been investigated. While in 1-phenylpyrene the internal conversion is faster than 50 fs, it is slower in 1-biphenyl-4-yl-pyrene (105 fs). The temporal evolution of the transient absorption spectrum indicates strong mixing of several “zero-order” electronic configurationswhich evolve separately with timein the S1 and the S2 states. The time-resolved spectra are interpreted within the framework of an adiabatic state model which includes interchromophoric electronic coupling. In this paper we give a full description of the experimental setup, the data acquisition procedure, and several experimental details about the characterization of the broad-band femtosecond white light source.

Düfte mit Maiglöckchennote leiten vermutlich menschliche Spermien auf dem Weg zur Eizelle. para‐Substituierte 3‐Phenylpropanale wie Cyclamal und Bourgeonal werden von einem Geruchsrezeptor des Spermiums erkannt und lösen über cyclisches Adenosin‐3′‐monophosphat als Second Messenger einen Einstrom von Ca2+‐Ionen aus. Das elektrische Signal moduliert das Schwimmverhalten und die Geschwindigkeit der Spermien. Undecanal wirkt inhibierend und macht die Spermien langsam und orientierungslos.

Charge-transfer processes through DNA have been studied intensively in the past 15 years. [1] It is important to emphasize, that in most of these experiments oxidative hole transfer has been observed. On the other hand, reductive electron transfer (ET) is currently used extensively in DNA chip technology [2] and DNA nanotechnology [3] without an understanding of the mechanism of this type of charge-transfer. Recently, Carell and co-workers described the repair of a thymine±thymine dimer by a distant flavine derivative, which was incorporated into the DNA as an artificial base. [4] Despite the fact that spectroscopic measurements with this system have not been published, the cleavage of the thymine±thymine dimer was interpreted as the chemical result of a reductive ET through the DNA base stack. To date, suitable DNA assays for the time-resolved spectroscopic investigation of reductive ET through DNA are elusive.We present herein the synthesis and pH-dependent spectroscopic investigation of the ET in the model nucleoside 5pyrenyl-2'-deoxyuridine (Py-dU, 1). Pyrene derivatives have been used previously as artificial DNA bases by Kool et al. [5] We chose a different approach and attached the pyrenyl group covalently to the uracil or thymine nucleobases. Excitation of the pyrene moiety at 340 nm leads to an intramolecular ET, which yields the corresponding uracil radical anion and the pyrenyl radical cation (PyC þ -dUC À ). This charge-transfer assignment has been proven previously by Netzel et al. [6] Based on the reduction potential for the PyC þ /Py redox couple of 1.52 V (vs. normal hydrogen electrode) [7] and E 00 ¼ 3.25 eV, [6] the driving force DG of this ET process has a maximum value of À0.5 eV based on the potential of À1.2 V for the dU/dUC À redox couple. [8] However, this value of j DG j seems to be too large with respect to a recent femtosecond time-resolved study on the reduction of thymine, which suggests a potential of approximately À1.8 V for the dT/dTC À redox couple. [9] Based on steady-state fluorescence spectroscopy and nanosecond fluorescence lifetime measurements, it was proposed that ET from the photoexcited pyrene to the uracil moiety should be more favorable in MeOH than in MeCN because of a proton-coupled ET process. [6] However, this hypothesis is in disagreement with the proposed pK a value of 6.9 for the protonated uracil radical anion dU(H)C reported by Steenken. [10] Therefore, the prononated radical dU(H)C represents a stronger acid than MeOH and cannot be protonated by this solvent. To elucidate the possibility of a proton-coupled ET in more detail, we chose water at different pH values and measured the steady-state fluorescence and time-resolved transient absorption spectra. An understanding of the protonation dynamics of radical anions of DNA bases is crucial to understand both ET and hole transfer through DNA. Moreover, to investigate the competition between ET to adjacent DNA bases and protonation by surrounding water molecules and/or hydrogen-bonded bases, it is particularly ...