Cytosine (Cyt) among all the nucleic acid bases features the most complex and least understood nonradiative deactivation, a process that is crucially important for its photostability. Herein, the excited state dynamics of Cyt and a series of its N1- and C5-derivatives, including the full set of Cyt nucleosides and nucleotides in DNA and RNA and the nucleosides of 5-methyl cytosine, 5-methylcytidine and 2'-deoxy-5-methylcytidine, have been investigated in water and in methanol employing femtosecond broadband time-resolved fluorescence coupled with fs transient absorption spectroscopy. The results reveal remarkable state-specific effects of the substitution and solvent in tuning distinctively the timescales and pathways of the nonradiative decays. For Cyt and the N1-derivatives, the nonradiative deactivations occur in a common two-state process through three channels, two from the light-absorbing ππ* state with respectively the sub-picosecond (∼0.2 ps) and the picosecond (∼1.5 ps) time constant, and the third is due to an optically dark nπ* state with the lifetime ranging from several to hundreds of picoseconds depending on solvents and substitutions. Compared to Cyt, the presence of the ribose or deoxyribose moiety at the N1 position of N1-derivatives facilitates the formation of the nπ* at the sub-picosecond timescale and at the same time increases its lifetime by ∼4-6 times in both water and methanol. In sharp contrast, the existence of the methyl group at the C5 position of the C5-derivatives eliminates completely the sub-picosecond ππ* channel and the channel due to the nπ*, but on the other hand slows down the decay of the ππ* state which after relaxation exhibits a single time constant of ∼4.1 to ∼7.6 ps depending on solvents. Varying the solvent from water to methanol accelerates only slightly the decay of the ππ* state in all the compounds; while for Cyt and its N1-derivatives, this change of solvent also retards strongly the nπ* channel, prolongs its lifetime from such as ∼7.7 ps in water to ∼52 ps in methanol for Cyt and from ∼30 ps in water to ∼186 ps in methanol for deoxycytidine. The spectral signatures we obtained for the ππ* and the nπ* states allow unambiguous evidence for clarifying uncertainties in the excited states of Cyt and the derivatives. The results provide a unifying experimental characterization at an improved level of detail about the photophysics of Cyt and its analogues under biologically relevant conditions and may help in understanding the photostability as well as photo-damages of the bases and related DNAs.

An organometallic cyclometalated platinum(II) complex, [Pt(L(3))Cl][PF(6)], has been synthesised from a specially designed cyclometalating ligand, HL(3) (triphenyl{5-[3-(6-phenylpyridin-2-yl)-1H-pyrazol-1-yl]pentyl}phosphonium chloride), that contains a pendant carbon chain carrying a terminal cationic triphenylphosphonium moiety. Aside from its room temperature single-photon luminescent properties in solution, [Pt(L(3))Cl](+) can also produce two-photon-induced luminescence at room temperature upon excitation at 700 nm from a mode-locked Ti:sapphire laser. Its two-photon absorption cross-section in DMF at room temperature was measured to be 28.0x10(-50) cm(4) s photon(-1). [Pt(L(3))Cl](+) is able to selectively stain the cell nucleolus. This has been demonstrated by two-photon confocal imaging of live and methanol-fixed HeLa (human cervical carcinoma) and 3T3 (mouse skin fibroblasts) cells. This organelle specificity is likely to be related to its special affinity for proteins within cell nucleoli. As a result of such protein affinity, [Pt(L(3))Cl](+) is an efficient RNA transcription inhibitor and shows rather profound cytotoxicity. On the other hand, the organelle-specific labelling and two-photon-induced luminescent properties of [Pt(L(3))Cl](+) renders it a useful nuclear dye for the 3-dimensional reconstruction of optical sections of thick tissues, for example, mouse ileum tissues, by multiphoton confocal microscopy.