We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 +/- 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1pipi* state and the electronic ground state with a 1pipi* charge-transfer state that is strongly stabilized by the transfer of a proton.

We report the relaxation dynamics of keto and enol or keto-imino cytosine, photoexcited in the wavelength range of 260-290 nm. Three transients with femtosecond to hundreds of picoseconds lifetimes are observed for the biologically relevant keto tautomer and are assigned to internal conversion and excited-state tautomerization. Only two transients with femtosecond and picosecond lifetimes are identified for the enol or keto-imino tautomer and are assigned to internal conversion processes. The results are discussed in the context of published ab initio theory.

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.

Ab initio calculations and time-resolved photoionization spectroscopy were carried out to characterize the role of the lowest two pi sigma* excited states for the photoinduced processes in the adenine monomer, adenine dimer, and adenine-water clusters. The calculations show--with respect to the monomer--a stabilization of 0.11-0.14 eV for the pi sigma* states in different isomers of adenine dimer and an even bigger stabilization of 0.14-0.36 eV for isomers of adenine-(H2O)1 and adenine-(H2O)3. Hence, the stabilized pi sigma* states should play an important role in the excited-state relaxation of partially or fully solvated adenine. This conclusion is supported by experimental results: In the adenine monomer, strong n pi* state signals are observed. Those signals are reduced in adenine dimer and vanish in water clusters due to the competing relaxation via the pi sigma* states.

Ab initio calculations and femtosecond pump-probe ionization experiments were carried out to identify excited state relaxation processes in isolated thymine monomer and small thymine-water clusters. Three transient species with life times of < or =100 fs, 7 ps and >1 ns were observed in the experiments on gas phase thymine. The longer-lived transients were weak or absent in thymine-water clusters. Available theoretical results on thymine agree with the assignment of low-lying pi-pi* and n-pi* excited states to the femtosecond and picosecond transients but the assignment of the third transient remains opaque. Our theoretical results seem to exclude the possibility of ground or excited state tautomerization as well as the involvement of states with pi-sigma* character. Remaining explanations for the observed transients are: very fast intersystem crossing to the triplet manifold or the observation of transient signals from local minima on the potential energy surfaces.

Hemithioindigo (HTI) photoswitches have a tremendous potential for biological and supramolecular applications due to their absorptions in the visible-light region in conjunction with ultrafast photoisomerization and high thermal bistability. Rational tailoring of the photophysical properties for a specific application is the key to exploit the full potential of HTIs as photoswitching tools. Herein we use time-resolved absorption spectroscopy and Hammett analysis to discover an unexpected principal limit to the photoisomerization rate for donor-substituted HTIs. By using stationary absorption and fluorescence measurements in combination with theoretical investigations, we offer a detailed mechanistic explanation for the observed rate limit. An alternative way of approaching and possibly even exceeding the maximum rate by multiple donor substitution is demonstrated, which give access to the fastest HTI photoswitch reported to date.

Controlling the internal motions of molecules by outside stimuli is a decisive task for the generation of responsive and complex molecular behavior and functionality. Light-induced structural changes of photoswitches are of special high interest due to the ease of signal application and high repeatability. Typically photoswitches use one reaction coordinate in their switching process and change between two more or less-defined states. Here we report on new twisted hemithioindigo photoswitches enabling two different reaction coordinates to be used for the switching process. Depending on the polarity of the solvent, either complete single bond (in DMSO) or double bond (in cyclohexane) rotation can be induced by visible light. This mutually independent switching establishes an unprecedented two-dimensional control of intramolecular rotations in this class of photoswitches. The mechanistic explanation involves formation of highly polar twisted intramolecular charge-transfer species in the excited state and is based on a large body of experimental quantifications, most notably ultrafast spectroscopy and quantum yield measurements in solvents of different polarity. The concept of pre-twisting in the ground state to open new, independent reaction coordinates in the excited state should be transferable to other photoswitching systems.

No abstract