Is 1-methylcytosine a faithful model compound for ultrafast deactivation dynamics of cytosine nucleosides in solution?

1-Methylcytosine (1mCyt) is the base for nucleoside N1-methylpseudodeoxycytidine of Hachimoji nucleic acids and a frequently used model compound for theoretical studies on excited states of cytosine nucleosides. However, there is little experimental characterization of spectra and photo-dynamic properties of 1mCyt. Herein, we report a comprehensive investigation into excited state dynamics and effects of solvents on fluorescence dynamics of 1mCyt in both water and acetonitrile. The study employed femtosecond broadband time-resolved fluorescence, transient absorption, and steady-state spectroscopy, along with density functional theory and time-dependent density functional theory calculations. The results obtained provide the first experimental evidence for identifying a dark-natured ∼5.7 ps lifetime nπ* state in the ultrafast non-radiative deactivation with 1mCyt in aqueous solution. This study also demonstrates a significant effect of the solvent on 1mCyt's fluorescence emission, which highlights the crucial role of solute-solvent hydrogen bonding in altering structures and reshaping the radiative as well as nonradiative dynamics of the 1mCyt's ππ* state in the aprotic solvent compared to the protic solvent. The solvent effect exhibited by 1mCyt is distinctive from that known for deoxycytidine, indicating the need for caution in using 1mCyt for modelling the ultrafast dynamics of Cyt nucleosides in solvents with varying properties. Overall, our study unveils a deactivation mechanism that confers a high degree of photo-stability for 1mCyt in solution, shedding light on the molecular basis for solvent-induced effects on the excited state dynamics of nucleobases and derivatives.

Long living excited state of protonated adenosine unveiled by ultrafast fluorescence spectroscopy and density functional theoretical study.

Adenosine (Ado) possesses ultrafast nonradiative dynamics accounting for its remarkably high photostability. The deactivation dynamics of Ado after protonation in an aqueous solution remains an elusive issue. Herein we report an investigation of the excited state dynamics of protonated Ado (AdoH+) performed using ultrafast time-resolved fluorescence spectroscopy combined with density functional theoretical calculation. The result obtained from comparison of conformers with protonation at different sites revealed that the syn-conformer with protonation occurring at the N3 position (syn-N3) is the predominant form of AdoH+ in the ground state, similar to that of Ado. In contrast, the fluorescence of AdoH+ with maximum intensity at 385 nm, significantly red-shifted from that of Ado, displaying decay dynamics composed of an ultrafast component with the lifetime of ∼0.5 ps and a slower one of ∼2.9 ns. The former is because of the decay of the syn-N3 conformer, similar to that reported for AdoH+ under the gas phase condition. The latter is due to the syn-N1 conformer formed via ultrafast proton transfer of the syn-N3. The excited state of syn-N1 has a peculiar nonplanar conformation over the purine molecule, which is responsible for the substantial Stokes shift showed in the fluorescence spectrum and correlates with a large energy barrier for nonradiative decay likely involving a reversed proton transfer. This study demonstrates the importance of protonation and solvent environment in altering dramatically the excited states of Ado, providing insight for better understanding nonradiative dynamics of both the monomeric bases and the oligomeric or polymeric DNAs.

Excitation-Wavelength-Dependent and Auxiliary-Ligand-Tuned Intersystem-Crossing Efficiency in Cyclometalated Platinum(II) Complexes: Spectroscopic and Theoretical Studies.

Understanding the factors affecting the intersystem-crossing (ISC) rate constant (kISC) of transition-metal complexes is crucial to material design with tailored photophysical properties. Most of the works on ISC to date focused on the influence by the chromophoric ligand and the understanding of the ISC efficiency were mainly drawn from the steady-state fluorescence to phosphorescence intensity ratio and ground-state calculations, with only a few high-level calculations on kISC that take excited-state structural change and solvent reorganization into account for quantitative comparisons with the experimental data. In this work, a series of [Pt(thpy)X)]+ complexes were prepared [Hthpy = 2-(2'-thienyl)pyridine, where X = auxiliary ligands] and characterized by both steady-state and time-resolved luminescence spectroscopies. A panel of auxiliary ligands with varying σ-donating/π-accepting character have been used. For comparison, analogues of [Pt(ppy)(P^P)]+ (Hppy = 2-phenylpyridine and P^P = diphosphino ligand) were also examined. The [Pt(thpy)(P^P)]+ complexes exhibit dual fluorescence-phosphorescence emission, with their ISC rate constants varied with the electronic characteristics of the auxiliary ligand: the more electron-donating ligand induces faster ISC from the S1 excited state to the triplet manifold. Density functional theory (DFT)/time-dependent DFT calculations of kISC(S1→T2) at the optimized excited-state geometries give excellent quantitative agreement with the femtosecond time-resolved fluorescence measurements; it was revealed that the more electron-donating auxiliary ligand increases metal contributions to both occupied and virtual orbitals and decreases the energy gap of the coupling excited states, leading to a decrease in the activation energy and an increase in spin-orbit coupling. Furthermore, the ISC rate constants of [Pt(thpy)(P^P)]+ complexes are found to depend on the excitation wavelengths. The deviation from Kasha-Vavilov's rule upon photoexcitation at λexc < 350 nm is due to the ultrafast S2 → T2 and S2 → T3 ISCs, as demonstrated by the calculated τISC < 100 fs, giving hints as to why S2 → S1 internal conversion (τIC ∼ ps) is not competitive with this hyper-ISC.

A long lasting sunscreen controversy of 4-aminobenzoic acid and 4-dimethylaminobenzaldehyde derivatives resolved by ultrafast spectroscopy combined with density functional theoretical study.

4-Aminobenzoic acid (PABA) is one of the earliest patented and most commonly used sunscreen components. There is however a long-lasting controversy on its photo-protective efficacy owing to the lack of information on its protolytic equilibrium and photo-dynamics after absorption of ultraviolet radiation in physiologically relevant aqueous solution. The excitation dynamics in water also remains largely unknown for analogs of PABA such as 4-dimethylaminoacetophenone (DMAAP) and 4-dimethylaminobenzaldehyde (DMABA) which are recognized as prototypes for photo-induced twisted intramolecular charge transfer (TICT). Herein we report a combined application of femtosecond broadband time-resolved fluorescence and transient absorption coupled with density functional theoretical study for PABA, DMAAP, and DMABA under several solvent conditions with representative properties in terms of the pH, polarity and hydrogen bonding capacity. The results we gained demonstrate that, in a neutral aqueous solution, PABA taking the deprotonated anion form in the ground state undergoes rapid protonation after excitation, producing excited state species in the neutral form that may shift effectively by intersystem crossing (ISC) to the long-lasting triplet state capable of damaging nucleic acids. This provides evidence at the molecular level for the detrimental effect of PABA if used as a sunscreen ingredient. In contrast, our investigation on DMAAP and DMABA unveils an unusual solvent controlled deactivation dynamics rendered by the participation of the carbonyl oxygen associated nOπ* state featuring energy and structure strongly responsive to solvent properties. In particular, these molecules in water exhibit solute-solvent hydrogen bonding at the sites of the carbonyl oxygen and the amino nitrogen which is, respectively, weakened and strengthened after the excitation, leading to state reversal and formation of a nOπ* state with a peculiar non-planar structure. This quenches strongly the excitation, eliminates the TICT, suppresses the ISC and opens up the otherwise inaccessible internal conversion (IC) to account for ∼80% of the entire deactivation. The IC, observed to proceed at a rate of ∼2.5 ps, allows the effective recovery of the ground state, providing substantial protection against ultraviolet irradiation. Moreover, the revelation of highly solvent sensitive fluorescence emission from DMABA and DMAAP implies the potential application of these molecules as the functional element in the design of sensory materials for probing the polarity and hydrogen bonding character of the surrounding environment.

Photoprotection or photodamage: a direct observation of nonradiative dynamics from 2-ethylhexyl 4-dimethylaminobenzoate sunscreen agent.

Apart from being an analogue of the prototype for photoinduced intramolecular charge transfer (ICT), 2-ethylhexyl 4-dimethylaminobenzoate (EHDMABA) is also one of the earliest patented and most commonly used sunscreen components. There is, however, little documented information about the photophysics and factors affecting the photophysics of this molecule. Such information is of importance for both the understanding of the ICT reaction and assessing the underlying process of photoprotection, especially in view of the "sunscreen controversy" that has arisen from the contrasting in vivo vs. in vitro photobiological results on this and related UV filters. We report herein a femtosecond broadband time-resolved fluorescence (fs-TRF), complemented by transient absorption (fs-TA) to allow a full probe of the excited state cascades for EHDMABA and two of its derivatives in solvents of varied properties. The results provide direct evidence for a nearly solvent independent inner sphere ICT reaction occurring on the sub-picosecond time scale, and an ensuing solvent dictated deactivation of the ICT state. The ICT state in the aprotic solvent acetonitrile decayed solely through the intrinsic intersystem crossing (ISC) to produce a potentially harmful triplet excited state. In the protic solvent, the solvation and formation of ICT-induced solute-solvent hydrogen (H)-bonding opened the originally inaccessible internal conversion (IC) channel of the ICT state, leading to the rapid reformation of the ground state molecule with a unitary efficiency in the aqueous solution. This H-bonding-mediated IC restrained or eliminated the intrinsic ISC, providing a mechanism at the molecular level for the benign dissipation of the electronic excitation. The precise rate of IC was observed to vary with the alkoxy substituent and its efficiency was affected by the H-bonding capacity of the solvent. The findings of this work demonstrate the pivotal role of the microenvironment and the direct participation of solvent molecules through H-bonding in drastically altering the nonradiative dynamics and promoting or inhibiting photostability and photoprotection. This may assist in developing next-generation UV filters and help in improving formulation design for the optimal efficacy of sunscreen products. The pronounced H-bonding-induced fluorescence quenching and variation in the fluorescence wavelength imply that these molecules may also serve as a sensitive fluorescence probe for the H-bonding properties of the microenvironment.

Nonradiative dynamics determined by charge transfer induced hydrogen bonding: a combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water.

As a case study of the interplay and the consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a combined femtosecond time-resolved fluorescence (fs-TRF) and density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) largely in a water solvent. Direct observation of the broadband spectra, anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with a time constant of ∼0.7 ps from an initial locally excited (LE) state identified to have the Laππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting a decay time of ∼49.7 ps, unusually faster than the nanosecond range lifetime in a polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to the study of the solvent kinetic isotope effect, is identified to originate from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows elimination of the ISC, and enables the nonradiative decay to proceed almost entirely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for the extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain an important insight for the in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the "sunscreen controversy" of the DMABA type of UVB sunscreen molecule.

Deciphering Photoluminescence Dynamics and Reactivity of the Luminescent Metal-Metal-Bonded Excited State of a Binuclear Gold(I) Phosphine Complex Containing Open Coordination Sites.

Luminescent metal complexes having open coordination sites hold promise in the design of sensory materials and photocatalysts. As a prototype example, [Au2 (dcpm)2)](2+) (dcpm = bis(dicyclohexylphosphanyl) is known for its intriguing environmental sensitive photoluminescence. By integrating a range of complementary ultrafast time-resolved spectroscopy to interrogate the excited state dynamics, this study uncovers that the events occurring in extremely rapid timescales and which are modulated strongly by environmental conditions play a pivotal role in the luminescence behavior and photochemical outcomes. Formed independent of the phase and solvent property within ∼0.15 ps, the metal-metal bonded (3)5dσ*6pσ state is highly reactive possessing strong propensity toward increasing coordination number at Au(I) center, and with ∼510 ps lifetime in dichloromethane is able to mediate light induced C-X bond cleavage.

Remarkable effects of solvent and substitution on the photo-dynamics of cytosine: a femtosecond broadband time-resolved fluorescence and transient absorption study.

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.

Design and synthesis of bipyridine platinum(II) bisalkynyl fullerene donor-chromophore-acceptor triads with ultrafast charge separation.

Donor-chromophore-acceptor triads, (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet-triplet energy transfer from the (3)MLCT/LLCT excited state within 4 ps to give the (3)C60* state, while in (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the (3)MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of (3)C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.

A theoretical investigation into the luminescent properties of d8-transition-metal complexes with tetradentate Schiff base ligands.

A theoretical investigation on the luminescence efficiency of a series of d(8) transition-metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M-salen](n) complexes (salen = N,N'-bis(salicylidene)ethylenediamine; M = Pt, Pd (n = 0); Au (n = +1)) in acetonitrile solutions at room temperature: [Pt-salen] is phosphorescent and [Au-salen](+) is fluorescent, but [Pd-salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron-withdrawing groups at the 4-position of the Schiff base ligand should widen the (3)MLCT-(3)MC gap (MLCT = metal-to-ligand charge transfer and MC = metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding Pd(II) Schiff base complex. Although it is experimentally proven that [Pd-salph-4E] (salph = N,N'-bis(salicylidene)-1,2-phenylenediamine; 4E means an electron-withdrawing substituent at the 4-position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding Pt(II) Schiff base complex, [Pt-salph-4E], is also much less emissive than the unsubstituted analogue, [Pt-salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M-salph-X] (X is a substituent on the salph ligand, M = Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low-frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin-orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well.

Luminescent cyclometalated platinum(II) complex forms emissive intercalating adducts with double-stranded DNA and RNA: differential emissions and anticancer activities.

Luminescent metallo-intercalators are potent biosensors of nucleic acid structure and anticancer agents targeting DNAs. There are few examples of luminescent metallo-intercalators which can simultaneously act as emission probes of nucleic acid structure and display promising anticancer activities. Herein, we describe a luminescent platinum(II) complex, [Pt(C^N^N)(C≡NtBu)]ClO4 (1 a, HC^N^N= 6-phenyl-2,2'-bipyridyl), that intercalates between the nucleobases of nucleic acids, accompanied by an increase in emission intensity and/or a significant change in the maximum emission wavelength. The changes in emission properties measured with double-stranded RNA (dsRNA) are different from those with dsDNA used in the binding reactions. Complex 1 a exhibited potent anticancer activity towards cancer cells in vitro and inhibited tumor growth in a mouse model. The stabilization of the topoisomerase I-DNA complex with resulting DNA damage by 1 a is suggested to contribute to its anticancer activity.

Ultrafast excited state dynamics of isocytosine unveiled by femtosecond broadband time-resolved spectroscopy combined with density functional theoretical study.

Isocytosine, having important applications in antivirus and drug development, is among the building blocks of Hachimoji nucleic acids. In this report, we present an investigation of the excited state dynamics of isocytosine in both protic and aprotic solvents, which was conducted by a combination of methods including steady-state spectroscopy, femtosecond broadband time-resolved fluorescence, and transient absorption. These methods were coupled with density functional and time-dependent density functional theory calculations. The results of our study provide the first direct evidence for a highly efficient nonradiative mechanism achieved through internal conversion from the ππ* state of the isocytosine keto-N(3)H form occurring within subpicoseconds and picoseconds following photo-excitation. Our study also unveils a crucial role of solvent, particularly solute-solvent hydrogen bonding, in determining the tautomeric composition and regulating the pathways and dynamics of the deactivation processes. The deactivation processes of isocytosine in the solvents examined are found to be distinct from those of cytosine and the case known for isocytosine in the gas phase mainly due to different tautomeric forms involved. Overall, our findings demonstrate the high photo-stability of isocytosine in the solution and showcase the remarkable effect of covalent modification in altering the spectral character and excited state dynamics of nucleobases.

The solvent effect and identification of a weakly emissive state in nonradiative dynamics of guanine nucleosides and nucleotides--a combined femtosecond broadband time-resolved fluorescence and transient absorption study.

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.

Excited-State Charge Transfer, Proton Transfer, and Exciplex Formation Revealed by Ultrafast Time-Resolved Spectroscopy in Human Telomeric Ribonucleic Acid Quadruplex.

Guanine quadruplexes (GQs), important for genome stability and biotechnology application, can form from both DNA and RNA. However, unlike the study of DNA GQs, little research has been conducted on excited states of GQs from RNA, which due to the ribose 2'-hydroxy group have structures distinct from their DNA counterparts. Combining ultrafast broadband time-resolved fluorescence and transient absorption measurements, we report the first direct probe of excitation dynamics for a bimolecular GQ from human telomeric repeat-containing RNA taking the typical highly compacted parallel folding with a propeller-like loop structure. The result revealed a multichannel decay containing an unusual high-energy excimer featuring charge transfer deactivated by rapid proton transfer in the tetrad core region. It also identified an unprecedented exciplex displaying massively red-shifted fluorescence produced from charge transfer in the loop region. The findings underscore the role of structural conformation and base content in determining the energy, electronic attribution, and decay dynamics of GQ excited states.

Charge dynamics in solar cells with a blend of π-conjugated polymer-fullerene studied by transient photo-generated voltage.

The biphasic feature of transient photo-generated voltage (TPV) is investigated in organic solar cells (OSCs) with a blend active layer of poly(3-hexylthiophene) (P3HT) and phenyl C61 butyric acid methyl ester (PCBM). The positive and negative components in biphasic TPV are explained through PCBM only and P3HT only devices. The negative and positive components are ascribed to the dipole formation at the buried interface of P3HT/indium tin oxide (ITO) and PCBM/ITO respectively. Based on these findings, two fundamental phenomena are revealed as follows: (1) interfacial modification on the buried interface inverts the negative component in biphasic TPV to a positive component, which prevents the leakage current channel in the conventional OSC structure; and (2) the solvent chosen transforms the positive component in biphasic TPV into a negative signal, which blocks the leakage current channel in the inverted OSC structure. Consequently, the study of TPV polarity provides the justification of the interaction at the buried interface. Besides, the decay of TPV is found to be bi-exponential, which can be used as a tool to estimate the degree of charge balance in OSCs.

Dual Time-Scale Proton Transfer and High-Energy, Long-Lived Excitons Unveiled by Broadband Ultrafast Time-Resolved Fluorescence in Adenine-Uracil RNA Duplexes.

In contrast to the immense amount of research on electronically excited DNA, surprisingly little has been done about the excited states of RNA. Herein, we demonstrate an ultrafast broadband time-resolved fluorescence and fluorescence anisotropy study to probe directly the intrinsic fluorescence and overall dynamics of the fluorescence from a homopolymeric adenine·uracil RNA duplex adopting the A-form structure. The results unveiled complex deactivation through distinctive multichannels mediated by states of varied energy, a character of charge transfer, and a lifetime from sub-picosecond to nanoseconds. In particular, we observed an unprecedented kinetic isotopic effect and participation of unusual proton transfer from states in two discrete energies and time domains. We also identified a high-energy nanosecond emission that we attributed to its fluorescence anisotropy to long-lived weakly emissive excitons not reported in DNA. These distinguishing features originate from the stacking, pairing, and local hydration environment specific to the A-form conformation of the adenine·uracil double helix.

Organic triplet excited states of gold(I) complexes with oligo(o- or m-phenyleneethynylene) ligands: conjunction of steady-state and time-resolved spectroscopic studies on exciton delocalization and emission pathways.

A series of mononuclear and binuclear gold(I) complexes containing oligo(o- or m-phenyleneethynylene) (PE) ligands, namely [PhC≡C(C(6)H(4)-1,2-C≡C)(n-1)Au(PCy(3))] (n = 2-4, 4a-c), [µ-{C≡C-(1,2-C(6)H(4)C≡C)(n)}{Au(PCy(3))}(2)] (n = 1-6, 8, 5a-g), [PhC≡C(C(6)H(4)-1,3-C≡C)(n-1)Au(PCy(3))] (n = 2-4, 6a-c), and [µ-{C≡C-(1,3-C(6)H(4)C≡C)(n)}{Au(PCy(3))}(2)] (n = 1, 2, 7a,b), were synthesized and structurally characterized. Extensive spectroscopic measurements have been performed by applying combined methods of femtosecond transient absorption (fs-TA), fs time-resolved fluorescence (fs-TRF), and nanosecond time-resolved emission (ns-TRE) coupled with steady-state absorption and emission spectroscopy at both ambient and low (77 K) temperatures to directly probe the temporal evolution of the excited states and to determine the dynamics and spectral signatures for the involved singlet (S(1)) and triplet (T(1)) excited states. The results reveal that S(1) and T(1) both feature ligand-centered electronic transitions with ππ* character associated with the phenyl and acetylene moieties. The (3)ππ* emission of the PE ligands is switched on by the attachment of [Au(PCy(3))](+) fragment(s) due to the heavy-atom effect. T(1)((3)ππ*) was found to form with nearly unity efficiency through intersystem crossing (ISC) from S(1)((1)ππ*). The ISC time constants were determined to be ∼50, 35, and 40 ps for 4b and 6a,b, respectively. Dual emission composed of fluorescence from S(1) and phosphorescence from T(1) were observed for most of the complexes except 5a and 7a, where only phosphorescence was found. The fluorescence at ambient temperature is accounted for by both the short-lived prompt fluorescence (PF) and long-lived delayed fluorescence (DF, lifetime on microsecond time scale). Explicit evidence was presented for a triplet-triplet annihilation mechanism for the generation of DF. Ligand length and substitution-dependent dynamics of T(1) are the key factors governing the dual emission character of the complexes. By extrapolation from the plot of emission energy against the PE chain length of the [Au(PCy(3))](+) complexes with oligo(o-PE) or oligo(m-PE) ligands, the triplet emission energies were estimated to be ∼530 and ∼470 nm for poly(o-PE) and poly(m-PE), respectively. Additionally, we assign the unusual red shifts of 983 cm(-1) from [PhC≡CAu(PCy(3))] (1) to [µ-{1,3-(C≡C)(2)C(6)H(4)}{Au(PCy(3))}(2)] (7a) and 462 cm(-1) from 7a to [µ(3)-{1,3,5-(C≡C)(3)C(6)H(3)}{Au(PCy(3))}(3)] (8) in the phosphorescence energies to excitonic coupling interactions between the C≡CAu(PCy(3)) arms in the triplet excited states. These complexes, together with those previously reported [Au(PCy(3))](+) complexes containing oligo(p-PE) ligands ( J. Am. Chem. Soc. 2002 , 124 , 14696 - 14706 ), form a collection of oligo(phenyleneethynylene) complexes exhibiting organic triplet emission in solution under ambient conditions. The remarkable feature of these complexes in exhibiting TTA prompted DF in conjunction with high formation efficiency of T(1)((3)ππ*) affords an opportunity for emission spectra to cover a wide range of wavelengths. This may have implication in the development of PE-based molecular materials for future optical applications.

Water-soluble mitochondria-specific ytterbium complex with impressive NIR emission.

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.

Impressive europium red emission induced by two-photon excitation for biological applications.

Three triazine-based europium(III) complexes were synthesized that demonstrated strong two-photon induced europium emission with a high two-photon absorption cross-section. The modified triazine ligand of complex 3 initiated over 100% enhancement of the two-photon absorption cross-section (σ(2): 320 GM) when compared with complex 1 (σ(2): 128 GM) in a solution of DMSO. Europium complex 3 is also stable in vitro, and power-dependence curves were obtained in vitro to confirm the two-photon-induced f-f emission in HeLa cells.

Femtosecond broadband time-resolved fluorescence and transient absorption study of the intramolecular charge transfer state of methyl 4-dimethylaminobenzoate.

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.