Excited-state Deactivation Dynamics Research Articles

Solvent Mediated Excited State Proton Transfer in Indigo Carmine.

Excited state proton transfer (ESPT) is thought to be responsible for the photostability of biological molecules, including DNA and proteins, and natural dyes such as indigo. However, the mechanistic role of the solvent interaction in driving ESPT is not well understood. Here, the electronic excited state deactivation dynamics of indigo carmine (InC) is mapped by visible pump-infrared probe and two-dimensional electronic-vibrational (2DEV) spectroscopy and complemented by electronic structure calculations. The observed dynamics reveal notable differences between InC in a protic solvent, D2O, and an aprotic solvent, deuterated dimethyl sulfoxide (dDMSO). Notably, an acceleration in the excited state decay is observed in D2O (<10 ps) compared to dDMSO (130 ps). Our data reveals clear evidence for ESPT in D2O accompanied by a significant change in dipole moment, which is found not to occur in dDMSO. We conclude that the ability of protic solvents to form intermolecular H-bonds with InC enables ESPT, which facilitates a rapid nonradiative S1 → S0 transition via the monoenol intermediate.

Resonance Raman spectroscopic and density functional theoretical study on microsolvated 2-Thiocytosine clusters with polar solvents.

Microsolvation effects on the excited state deactivation dynamics of 2-thiocytosine (2tC) were studied in hydrogen-bonded 2tC clusters with protic solvents using resonance Raman, FT-IR, FT-Raman, UV-vis spectroscopy combining with density functional theoretical calculation. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, acetonitrile (MeCN), were used to investigate the 2tC(H2O)1-5, 2tC(MeOH)1-5, and 2tC(MeCN)1-3 microsolvated clusters. In CH3OH and H2O solvents, most of the Raman shifts were due to the vibration modes of 2tC(solvent)n (solvent=H2O, CH3OH; n=1-4) clusters via intermolecular NH⋯O hydrogen bonds (HB). The intermolecular >NH⋯O hydrogen bond interactions, which are the key constituents of stable thione structure of 2tC, revealed the spectra difference of 2tC in CH3CN, CH3OH and H2O. With the aid of electronic structural and vibration frequency calculations, the observed Raman spectra were assigned to the low energy isomers of 2tC(solvent)2 (solvent=H2O, CH3OH) clusters in water and methanol and 2tC(CH3CN) in acetonitrile solvents. 2tC(solvent)2 clusters in water and methanol may prohibit or promote excited state proton transfer reaction from sulfur atom to neighbor nitrogen atom due to the hydrogen bonding chain between 2tC and protic solvent molecules. Our experimental and theoretical studies confirmed that the hydrogen bond sites were located on the specified functional group SCNH of 2tC with solvent molecules.

Multiple Anti-Kasha Emissions in Transition-Metal Complexes.

In this Letter, we present first-principles evidence that several higher-lying excited states are responsible for the emission spectrum of [M(CO)4(bpy)] (M = Cr, Mo, W and bpy = 2,2'-bipyrimidine) complexes. These results highlight the violation of Kasha's rule, which states that after irradiation, molecules emit light with appreciable yield only from their lowest energy excited state. Furthermore, in [W(CO)4(bpy)] and [Mo(CO)4(bpy)], the breaking of Kasha's rule is two-fold because at least two different excited states besides T1 are involved in emission. To our knowledge, these are the first transition-metal complexes unambiguously demonstrated to display simultaneous equilibrated and nonequilibrated anti-Kasha emissions. This work also highlights the complexity of the emissive processes of tetracarbonyl-diimine transition-metal complexes, which are controlled via a subtle interplay of electronic and geometrical effects along the excited-state deactivation dynamics.

Solvent viscosity induces twisted intramolecular charge transfer state lifetime tunable of Thioflavin-T

Excited-state deactivation dynamics of Thioflavin-T (ThT) in gradual viscosity solvents were investigated. Femtosecond transient absorption spectra and dynamic decay curves both present significant distinction of ThT in different volume ratios binary mixtures solvents. Dynamics fitting lifetime of twisted intramolecular charge transfer (TICT) state is strongly dependent on solvents viscosity. Compared to rotation corresponding time of ThT in low viscosity solvent (0.6 cp) experimentally coincident well with Stokes-Einstein-Debye (SED) equation, the relation between rotation corresponding time and relatively high viscosity (5.9 cp to 1091.2 cp) is more consistent with fractional SED equation. Combined with optimized geometric structures of ThT by density functional theory and time-dependent density functional theory, further understand TICT state lifetime increases with increasing solvents viscosity. Our work provides a comprehensive understanding of fluorescence molecular rotor (FMR) deactivation process in different viscosity solvents and is helpful to design new FMR.

Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents.

Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)1-6, Cy·(MeOH)1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H2O) n are 0.5 ps for n = 1 and ∼0.2-0.25 ps for n = 2-6. For Cy·(MeOH) n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy·THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Microhydration Effects on the Ultrafast Photodynamics of Cytosine: Evidences for a Possible Hydration‐Site Dependence

Ultrafast excited-state deactivation dynamics of small cytosine (Cy) and 1-methylcytosine (1mCy) microhydrates, Cy⋅(H2O)1-3 and 1mCy⋅(H2O)1,2, produced in a supersonic expansion have been studied by mass-selected femtosecond pump-probe photoionization spectroscopy at about 267 nm excitation. The seeded supersonic expansion of Ar/H2O gas mixtures allowed an extensive structural relaxation of Cy and 1mCy microhydrates to low-energy isomers. With the aid of electronic structure calculations, we assigned the observed ultrafast dynamics to the dominant microhydrate isomers of the amino-keto tautomer of Cy and 1mCy. Excited-state lifetimes of Cy⋅(H2O)1-3 measured here are 0.2-0.5 ps. Comparisons of the Cy⋅H2O and 1mCy⋅H2O transients suggest that monohydration at the amino Watson-Crick site induces a substantially stronger effect than at the sugar-edge site in accelerating excited-state deactivation of Cy.

Ultrafast excited-state dynamics and fluorescence deactivation of near-infrared fluorescent proteins engineered from bacteriophytochromes.

Near-infrared fluorescent proteins, iRFPs, are recently developed genetically encoded fluorescent probes for deep-tissue in vivo imaging. Their functions depend on the corresponding fluorescence efficiencies and electronic excited state properties. Here we report the electronic excited state deactivation dynamics of the most red-shifted iRFPs: iRFP702, iRFP713 and iRFP720. Complementary measurements by ultrafast broadband fluorescence and absorption spectroscopy show that single exponential decays of the excited state with 600 ~ 700 ps dominate in all three iRFPs, while photoinduced isomerization was completely inhibited. Significant kinetic isotope effects (KIE) were observed with a factor of ~1.8 in D2O, and are interpreted in terms of an excited-state proton transfer (ESPT) process that deactivates the excited state in competition with fluorescence and chromophore mobility. On this basis, new approaches for rational molecular engineering may be applied to iRFPs to improve their fluorescence.

Photophysics of H- and J-Aggregates of Indole-Based Squaraines in Solid State

The photodynamics of six types of indole-based squaraines (SQs) molecules deposited onto quartz substrates by using the spin-coating method have been studied by means of steady-state and time-resolved techniques. Enhanced scattering signal at the resonance wavelengths together with the broadening and shift of the absorption bands to the red side with respect to those in solution prove the formation of H- and J-aggregates in the thin film samples. Excited state deactivation dynamics were studied by the femtosecond (fs) transient absorption spectroscopy and nanosecond (ns) flash photolysis. Faster kinetics were obtained for the solid state samples as compared to those in solution. Singular value decomposition (SVD) analysis of the fs transient signal proves the presence of monomers and aggregates, enabling the separation of the kinetics for each one. For the SQ monomers, the dynamics of the singlet excited state does not depend on the pump fluence, and we thus propose that the quenching of the excited monom...

Ultrafast Energy Transfer Dynamics of a Bioinspired Dyad Molecule

A caroteno-purpurin dyad molecule was studied by steady-state and pump-probe spectroscopies to resolve the excited-state deactivation dynamics of the different energy levels as well as the connecting energy flow pathways and corresponding rate constants. The data were analyzed with a two-step multi-parameter global fitting procedure that makes use of an evolutionary algorithm. We found that following ultrafast excitation of the donor (carotenoid) chromophore to its S2 state, the energy flows via two channels: energy transfer (70%) and internal conversion (30%) with time constants of 54 and 110 fs, respectively. Additionally, some of the initial excitation is found to populate the hot ground state, revealing another limitation to the functional efficiency. At later times, a back transfer occurs from the purpurin to the carotenoid triplet state in nanosecond timescales. Details of the energy flow within the dyad as well as species associated spectra are disentangled for all excited-state and ground-state species for the first time. We also observe oscillations with the most pronounced peak on the Fourier transform spectrum having a frequency of 530 cm(-1). The dyad mimics the dynamics of the natural light-harvesting complex LH2 from Rhodopseudomonas acidophila and is hence a good model system to be used in studies aimed to further explain previous work in which the branching ratio between the competing pathways of energy loss and energy transfer could be manipulated by adaptive femtosecond pulse shaping.

Vibrational Coherence of Bis-imidazole Derivative, BDPI-2Y, Observed by Ultrafast Spectroscopy

ABSTRACT Ultrafast pump-probe measurement was applied to investigate the excited state deactivation dynamics of bisimidazolyl radical, 1,4-bis-(4,5-diphenylimidazol-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y). BDPI-2Y is non-fluorescent and the recovery of the ground state took place in the time scale of femtoseconds to picoseconds, indicating a direct ultrafast nonradiative decay from the excited state. Coherent intermolecular vibrations were also observed and the low-frequency mode at 150 cm−1 and 128 cm−1 had a phase shift of ∼ π compared to the mode at 290 cm−1.