Transient species formed by photoexcitation (400 nm) of [Ru(dppz)(tap)2]2+ (1) (dppz = dipyrido[3,2-a:2',3'-c]phenazine; tap=1,4,5,8-tetraazaphenanthrene) in aqueous solution and when intercalated into a double-stranded synthetic polynucleotide, [poly(dG-dC)]2, have been observed on a picosecond timescale by both visible transient absorption (allowing monitoring of the metal complex intermediates) and transient infrared (IR) absorption spectroscopy (allowing direct study of the DNA nucleobases). By contrast with its behavior when free in aqueous solution, excitation of 1 when bound to [poly(dG-dC)]2 causes a strong increase in absorbance at 515 nm due to formation of the reduced complex [Ru(dppz)(tap)2]+ (rate constant=(2.0+/-0.2) x 10(9) s(-1)). The subsequent reformation of 1 proceeds with a rate constant of (1.1+/-0.2) x 10(8) s(-1). When the process is carried out in D2O, the rates of formation and removal of [Ru(dppz)(tap)2]+ are reduced (rate constants (1.5+/-0.3) x 10(9) and (0.7+/-0.2) x 10(8) s(-1) respectively) consistent with proton-coupled electron transfer processes. Picosecond transient IR measurements in the 1540-1720 cm(-1) region in D2O solution confirm that the reduction of 1 intercalated into [poly(dG-dC)]2 is accompanied by bleaching of IR ground-state bands of guanine (1690 cm(-1)) and cytosine (1656 cm(-1)), each with similar rate constants.

A strong infrared band at 1574 cm 21 is observed following 267 nm excitation of 29-deoxycytidine (t = 37 ¡ 4 ps) or 29-deoxycytidine 59-monophosphate (t = 33 ¡ 4 ps); this band is provisionally attributed to an 1 n N p* state and is absent for cytosine.The nature of the electronic excited states of DNA continues to be a focus of significant interest because of their role in UV-light induced modifications that could potentially be mutagenic. 1 The nucleobases fluoresce extremely weakly and this has led to the conclusion that their excited states must be very short-lived. 2 This is supported by femtosecond transient visible absorption 3 and fluorescence upconversion 4 measurements that reveal that the bases and mononucleotides possess sub-picosecond excited state lifetimes. However, the reasons for the short lifetimes are still a matter of debate, with theoretical studies emphasising the importance of ultrafast non-radiative processes. 5 IR methods, including 2D-IR, are proving an increasingly valuable tool for the study of DNA systems. 6,7 Time-resolved IR (TRIR) absorption spectroscopy 8 can provide structural as well as kinetic information on the ultrafast processes in DNA and our first paper in this area allowed the observation of the vibrationally excited ground states formed from nucleotides excited at 267 nm. 9 Subsequently the method has been used to identify the products of the photoionisation of GMP after excitation at 200 nm 10 and very recently the formation of thymine dimers. 11 In this paper we probe the transient species formed from 29-deoxycytidine 59-monophosphate (dCMP), 29-deoxycytidine (dCyd) and cytosine (Cyt) using vibrational spectroscopy. The presence of a new short-lived species is shown for both dCyd and dCMP but not for the nucleobase Cyt. Fig. 1 shows the transient IR difference spectra recorded between 2 ps and 1000 ps after 267 nm (150 fs) excitation of the mononucleotide dCyd in neutral (buffered pH 6.9) solution. (Identical spectra were also recorded at pH 8.5). The spectrum shows areas corresponding to ground state depletion (bands at 1505, 1524, 1617 and 1652 cm 21 as found in the FTIR) and ones where the transient absorbs more strongly. Similar behaviour is observed for dCMP (see ESI).The transient decay (and the ground state recovery) is essentially complete over the 1000 ps time scale of the experiment. However, in contrast to the results for other nucleotides such as dAMP and dGMP, 9 the kinetics are distinctly biphasic (see Fig. 2). At short times (2-8 ps -shown in red in Fig. 1) the transient absorption bands are observed to shift to higher wavenumber, behaviour which is consistent with relaxation of vibrationally excited ground states, as previously proposed. 9 The time constant for this is determined to be 2.6 ¡ 0.3 ps for dCyd.After this cooling process is complete a strong transient absorption at 1574 cm 21 persists. This band is the only one observable in the IR absorption spectrum in the region 1450-1750 cm 21 (assuming that there is no accidental overlap of other band...

The role of N1-substitution in controlling the deactivation processes in photoexcited cytosine derivatives has been explored using picosecond time-resolved IR spectroscopy. The simplest N1-substituted derivative, 1-methylcytosine, exhibits relaxation dynamics similar to the cytosine nucleobase and distinct from the biologically relevant nucleotide and nucleoside analogues, which have longerlived excited-state intermediates. It is suggested that this is the case because the sugar group either facilitates access to the long-lived 1 n O π* state or retards its crossover to the ground state.

The transient IR absorption spectrum for UV-excited i-motif DNA is reported for the first time and found to possess complex dynamics pointing to multiple decay processes, including possible charge transfer between packed hemi-protonated C bases.

Recent developments of the picosecond transient absorption infrared technique and its ability to elucidate the nature and kinetic behaviour of transient species formed upon pulsed laser excitation of nucleic acids are described.