A combined method of femtosecond broadband time-resolved fluorescence (fs-TRF) and transient absorption (fs-TA) was employed to investigate the excited state dynamics of 2'-deoxyguanosine (dG) and 2'-deoxyguanosine 5'-monophosphate (dGMP). Comparative fs-TRF and fs-TA measurements were conducted on dG and dGMP in neutral water, deuterated water, and methanol with excitation wavelengths of 245, 267 and 285 nm. Very similar results were observed with dG and dGMP. The data provide compelling evidence for the co-existence of two nonradiative pathways. One is the generally recognized Laππ* mediated channel, the other involves an unprecedented weakly emissive state which plays a significant role in the overall deactivation processes. The Laππ* channel features biphasic dynamics with time constants (τ1/τ2) of ~0.2/0.8 ps in water and ~0.25/1.0 ps in methanol. The biphasic decay arises due to a partial transfer with τ1 of the Laππ* population to the newly identified state followed by conversion in τ2 of the remaining Laππ* molecules into the electronic ground state. The channel mediated by the weakly emissive species shows solvent-dependent dynamics with time constants (τ3) of ~2.0 ps in water, ~2.3 ps in deuterated water, and ~4.1 ps in methanol. The species features absorption at UV wavelengths (~300-400 nm) and exhibits deeply red-shifted fluorescence (λmax ~ 520 nm) with polarization direction varied markedly from that of the Laππ* but close to the Lbππ*. This species acts as an effective quenching state to the radiative decay of the brightly emissive Laππ* and Lbππ*. It sets in promptly (<~50 fs) after the photoexcitation and is further populated through nonadiabatic coupling with the Laππ*. The overall involvement of this state is facilitated with excitation at high energy and is favoured in methanol over water. According to the spectral character and the solvent effect in particular the kinetic isotope effect, the species is tentatively associated to the πσ* state with charge transfer (CT) character which is considered to be preferentially stabilized by hydrogen-bonding between the guanine amino and surrounding solvent molecules. The result of this study leads to a dramatically different picture of guanine deactivation. It demonstrates a crucial role of the solvent in shaping the nonradiative dynamics of guanine nucleosides and nucleotides. The data presented are important for understanding the detailed photophysics of not only the monomeric guanine but also DNA assemblies that contain guanine in base pairs or have a guanine tetrad as the structural motif.

A water-soluble porphyrinato ytterbium complex linked with rhodamine B (Yb-2) showed mitochondria-specific subcellular localization and strong two-photon-induced NIR emissions (λ(em) = 650 nm, porphyrinate ligand π → π* transition; λ(em) = 1060 nm, Yb(III) (5)F(5/2) → (5)F(7/2) transitions; σ(2) = 375 GM in DMSO) with an impressive Yb(III) NIR emission quantum yield (1% at λ(ex) = 340 nm; 2.5% at λ(ex) = 430 nm) in aqueous solution.

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.

A combined application of femtosecond broadband time-resolved fluorescence (fs-TRF), fluorescence anisotropy (fs-TRFA) and fs to microsecond (μs) transient absorption (TA) have been used to probe directly the dynamics, nature, formation and decay paths of the singlet intramolecular charge transfer ((1)ICT) state of methyl 4-dimethylaminobenzoate (1a) in acetonitrile. The result reveals explicit evidence for a common electronic origin (the L(a) nature) of the (1)ICT state and its precursor the locally excited ((1)LE) state to account jointly for the dual florescence known to this system. It also shows that the ICT reaction from the (1)LE to (1)ICT state occurs with time constant of ~0.8 ps and the (1)ICT state formed decays with a ~1.9 ns time constant leading mainly to a ππ* natured triplet state ((3)T(1)). The (3)T(1) then relaxes with a ~4 μs lifetime under deoxygenated condition resulting in full recovery of the ground state (S(0)). As a case study, this work contributes novel experimental data for improved understanding of the mechanism of ICT reaction; it also reveals a distinct deactivation pattern for this prototype para-amino substituted aromatic carbonyl compound in acetonitrile.