Transient Absorption Spectroscopic Techniques Research Articles

Delineating the core and surface state heterogeneity of carbon dots during electron transfer.

Exploring the heterogeneity of carbon dots (C-Dots) is challenging because of the existence of complex structural diversity, and it is a demanding task for the development and designing of efficient C-Dots for various applications. Herein, we studied the role of the core state and surface state of C-Dots in heterogeneity via the successful investigation of the electron transfer (ET) process between different (blue, green, and red) emitting C-Dots and an electron acceptor methyl viologen (MV2+) using steady-state and time-resolved fluorescence and ultrafast transient absorption (TA) spectroscopic techniques. Selective excitation in the steady-state and time-resolved mode shows that the ET ability of the core state is higher than that of the surface state. Moreover, the kinetics of MV+˙ generation was probed using TA spectroscopy after the excitation of the core and surface state, where we observed that the surface state becomes less efficient due to the presence of an oxygen-containing functional group in the surface state, which acts as an electron scavenger. Moreover, the heterogeneity of the core and surface state was explored through the detection of the MV+˙ generation yield after the irradiation of UV and visible light (exciting the core and surface state). The result indicates that the graphitic nitrogen content in the core state and the oxygen-containing functional group in the surface state play an important role in the heterogeneity in the structure and the ET process. Our findings on the fundamental understanding of the heterogeneity of different emissive C-Dots will provide a new way of designing and developing a metal-free light-harvesting system.

Spectroscopic and computational characterization of new promising donors – Manganese(III) porphyrins bearing carbazole groups

UV–vis, IR, MALDI-TOF, and femtosecond transient absorption spectroscopic techniques together with DFT and TDDFT computations have been employed to explore new manganese(III) porphyrins bearing [3,6-di-tert-butyl-carbazol-9-yl-benzoyloxy]- (MnP1) and [3,6-bis(3΄,6΄-di(tert-butyl)-9΄H-carbazol)-9H-carbazolbenzoyloxy]phenyl (MnP2) groups. MnP1 and MnP2 demonstrate the significant deviation of the macrocycle from planarity, which can be due to the high spin (S = 2) state of the manganese(III). By studying MnP1 and MnP2 excited states, the formation of the trip-quintet ones decaying in 13.7 ps and 17.3 ps, respectively, was established. The comparisons of MnP1 and MnP2 with the Co and Zn analogs have shown that both the number of generations and metal ion influence excited-state dynamics and electronic/structural properties.

Length and Charge of the N-terminus Regulate the Lifetime of the Signaling State of Photoactive Yellow Protein.

Photoactive yellow protein (PYP) is one of the most extensively studied photoreceptors. Nevertheless, the role of the N-terminus in the photocycle and structural transitions is still elusive. Here, we attached additional amino acids to the N-terminus of PYP and investigated the effect of the length and charge of additional N-terminal residues using circular dichroism, two-dimensional nuclear magnetic resonance (2D-NMR), transient absorption (TA), and transient grating (TG) spectroscopic techniques. TA experiments showed that, except for negatively charged residues (5D-PYP), additional N-terminal residues of PYP generally enable faster dark recovery from the putative signaling state (pB2) to the ground state (pG). TG data showed that although the degree of structural changes can be controlled by adjusting specific amino acid residues in the extended N-terminus of N-terminal extended PYPs (NE-PYPs), the dark recovery times of wt-PYP and NE-PYPs, except for 5D-PYP, are independent of the structural differences between pG and pB2 states. These results demonstrate that the recovery time and the degree of structural change can be regulated by controlling the length and sequence of N-terminal residues of PYP. The findings in this study emphasize the need for careful attention to the remaining amino acid residues when designing recombinant proteins for genetic engineering purposes.

Excited State Kinetics of Benzo[a]pyrene Is Affected by Oxygen and DNA.

Benzo[a]pyrene is a widespread environmental pollutant and a strong carcinogen. It is important to understand its bio-toxicity and degradation mechanism. Herein, we studied the excited state dynamics of benzo[a]pyrene by using time-resolved fluorescence and transient absorption spectroscopic techniques. For the first time, it is identified that benzo[a]pyrene in its singlet excited state could react with oxygen, resulting in fluorescence quenching. Additionally, effective intersystem crossing can occur from its singlet state to the triplet state. Furthermore, the interaction between the excited benzo[a]pyrene and ct-DNA can be observed directly and charge transfer between benzo[a]pyrene and ct-DNA may be the reason. These results lay a foundation for further understanding of the carcinogenic mechanism of benzo[a]pyrene and provide insight into the photo-degradation mechanism of this molecule.

Femtosecond relaxation dynamics of two‐dimensional BiOI nanoplatelets as efficient photocatalysts

AbstractThe structural and optical properties of pristine and photoactivated surfaces of facet [001] of 2D BiOI nanoplatelet powders synthesized with an antisolvent method were studied using X‐ray diffraction (XRD), a scanning electron microscope, X‐ray photoelectron spectra (XPS), visible–near infrared (vis–NIR) absorption, and transient absorption spectroscopic techniques. The synthesized BiOI nanoplatelets possessed a tetragonal structure with length 200–400 nm and thickness less than 50 nm. XRD analysis showed that the photoactivation did not affect the crystal structure. In contrast, the XPS analysis showed vacancies associated with elements Bi, I, and O. The band gaps estimated from the diffuse reflectance spectra of pristine and photoactivated samples are 1.83 and 1.80 eV, respectively. Femtosecond transient‐absorption spectra measured in the vis–NIR (2.25–0.9 eV) region showed photobleach bands associated with the band edge, shallow trap, and deep trap states. A global fit of the transient spectral profiles showed that all bands decay synchronously for both pristine and photoactivated samples, but the photoactivated samples showed a greater magnitude of carrier‐carrier annihilation following photoexcitation due to photodoping caused by defects. A carrier‐relaxation model involving thermal equilibrium between band‐edge, shallow, and deep trap states is proposed to explain the synchronous decay of all bands.

Nature of Linear Spectral Properties and Fast Electronic Relaxations in Green Fluorescent Pyrrolo[3,4-c]Pyridine Derivative.

The electronic nature of 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) was comprehensively investigated in liquid media at room temperature using steady-state and time-resolved femtosecond transient absorption spectroscopic techniques. The analysis of the linear photophysical and photochemical parameters of HPPT, including steady-state absorption, fluorescence and excitation anisotropy spectra, along with the lifetimes of fluorescence emission and photodecomposition quantum yields, revealed the nature of its large Stokes shift, specific changes in the permanent dipole moments under electronic excitation, weak dipole transitions with partially anisotropic character, and high photostability. Transient absorption spectra of HPPT were obtained with femtosecond resolution and no characteristic solvate relaxation processes in protic (methanol) solvent were revealed. Efficient light amplification (gain) was observed in the fluorescence spectral range of HPPT, but no super-luminescence and lasing phenomena were detected. The electronic structure of HPPT was also analyzed with quantum-chemical calculations using a DFT/B3LYP method and good agreement with experimental data was shown. The development and investigation of new pyrrolo[3,4-c]pyridine derivatives are important due to their promising fluorescent properties and potential for use in physiological applications.

Morphological Evolution of Strongly Fluorescent Water Soluble AIEEgen-Triblock Copolymer Mixed Aggregates with Shape-Dependent Cell Permeability.

Dimethyl-2,5-bis(4-methoxyphenylamino)terephthalate (DBMPT) is a water-insoluble fluorogenic molecule, which has been rendered water-soluble in physiological conditions, by the addition of triblock copolymers (TBPs), P123 (PEO19PPO69PEO19), and F127 (PEO100PPO65PEO100). DBMPT-TBP mixed aggregates, formed in the process, exhibit significant aggregation-induced enhancement of emission, with nanosecond fluorescence lifetimes. Dynamics involved in suppression of nonradiative pathways and consequent enhancement of fluorescence are followed by femtosecond transient absorption and time-resolved fluorescence spectroscopic techniques. Interestingly, shapes of the aggregates formed with the two TBPs are found to be very different, even though they differ only in the length of hydrophilic blocks. DBMPT-P123 aggregates are micrometer-sized and spherical, while DBMPT-F127 aggregates form nanorods. Evolution of their morphologies, as a function of TBP concentration, is monitored using cryo-TEM, FESEM, and fluorescence lifetime imaging microscopy. Fluorescence lifetime distribution provides useful insight into microheterogeneity in these mixed aggregates. Excellent cell permeability is observed for DBMPT-F127 nanorods, in contrast to DBMPT-P123 microspheres. These fluorescent nanorods exhibit the ability to mark lipid droplets within the cell and hence bear the promise for application in intracellular imaging.

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.

Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron-Donating and -Withdrawing Moieties.

Charge-transfer processes from photoexcited CdSe quantum dots (QDs) to phenol derivatives with electron- donating (4-methoxy) and -withdrawing (4-nitro) moieties have been demonstrated by using steady-state and time- resolved emission and femtosecond transient absorption spectroscopy. Steady-state and time-resolved emission studies suggest that in the presence of both 4-nitrophenol (4NP) and 4-methoxyphenol (4MP) CdSe QDs luminescence is quenched. Stern-Volmer analysis suggests both static and dynamic mechanisms are active for both the QD/phenol composites. Cyclic voltammetric analysis recommends that photoexcited CdSe QDs can donate electrons to 4NP and holes to 4MP. To reconfirm both electron- and hole-transfer mechanisms, CdSe/CdS quasi-type II and CdSe/CdTe type II core-shell nanocrystals were synthesized and photoluminescence quenching was monitored in the absence and presence of both 4NP and 4MP, for which hole and electron transfer were systematically restricted. Results suggest that indeed electron and hole transfer take place from photoexcited CdSe to 4NP and 4MP, respectively. To monitor the charge-transfer dynamics in both systems on an early timescale, femtosecond transient absorption spectroscopic techniques have been employed. Electron and hole transfer and charge-recombination dynamics are discussed and the effect of electron-donating and -withdrawing groups has been demonstrated.

Ultrafast Time-Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy.

A comprehensive study of ultrafast molecular relaxation processes of isomeric meso-(pyridyl) porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solvent mixture. By combined studies of fluorescence up-conversion, time-correlated single photon counting, and transient absorption spectroscopic techniques, a complete model with different microscopic rate constants associated with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is observed at low-frequency mode, ca. 26 cm(-1) region. Fluorescence up-conversion studies constitute ultrafast time-resolved emission spectra (TRES) over the whole emission range (430-710 nm) starting from S2 state to Qx state via Qy state. Careful analysis of time profiles of up-converted signals at different emission wavelengths helps to reveal detail molecular dynamics. The observed lifetimes are as indicated: A very fast decay component with 80 ± 20 fs observed at ∼435 nm is assigned to the lifetime of S2 (B) state, whereas being a rise component in the region of between 550 and 710 nm emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B → Qy and B → Qx as well. Two distinct components of Qy emission decay with ∼200-300 fs and ∼1-1.5 ps time constants are due to intramolecular vibrational redistribution (IVR) induced by solute-solvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision, respectively. The weighted average of these two decay components is assigned as the characteristic lifetime of Qy, and it ranges between 0.3 and 0.5 ps. An additional ∼20 ± 2 ps rise component is observed in Qx emission, and it is assigned to the formation time of thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent. This relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm), and Qx (590 nm) along with extensive global and target model analysis of TA data exclusively generate the true spectra of each excited species/state with their respective lifetimes along with microscopic rate constants associated with each state. The following five exponential components with lifetime values of 65-70 fs, ∼0.3-0.5 ps, ∼20 ± 2 ps, ∼7 ± 1 ns, and 1-2 μs are observed which are associated with S2, Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states, respectively, when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx, and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx), respectively, as expected. The TA results parallel the fluorescence up-conversion studies, and both the results not only compliment each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx, and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.

Electron Transfer in Bacterial Reaction Centers with the Photoactive Bacteriopheophytin Replaced by a Bacteriochlorophyll through Coordinating Ligand Substitution

The influence of amino acid substitutions at position M214 (M-subunit, residue 214) on the rate and pathway of electron transfer involving the bacteriopheophytin cofactor, HA, in a bacterial photosynthetic reaction center has been explored in a series of Rhodobacter sphaeroides mutants. The M214 leucine (L) residue of the wild type was replaced with histidine (H), glutamine (Q), and asparagine (N), creating the mutants M214LH, M214LQ, and M214LN, respectively. As has been reported previously for M214LH, each of these mutations resulted in a bacteriochlorophyll molecule in place of a bacteriopheophytin in the HA pocket, forming so-called β-type mutants (in which the HA cofactor is called βA). In addition, these mutations changed the properties of the surrounding protein environment in terms of charge distribution and the amino acid side chain volume. Electron transfer reactions from the excited primary donor P to the acceptor QA were characterized using ultrafast transient absorption spectroscopic techniques. Similar to that of the previously characterized M214LH (β mutant), the strong energetic mixing of the P(+)BA(-) and P(+)βA(-) states (the mixed anion is denoted I(-)) increased the rate of charge recombination between P(+) and I(-) in competition with the I(-) → QA forward reaction. This reduced the overall yield of charge separation forming the P(+)QA(-) state. While the kinetics of the primary electron transfer forming P(+)I(-) were essentially identical in all three β mutants, the rates of the βA(-) (I(-)) → QA electron transfer in M214LQ and M214LH were very similar but quite different from that of the M214LN mutant. The observed yield changes and the differences in kinetics are correlated more closely with the volume of the mutated amino acid than with their charge characteristics. These results are consistent with those of previous studies of a series of M214 mutants with different sizes of amino acid side chains that did not alter the HA cofactor composition [Pan, J., et al. (2013) J. Phys. Chem. B 117, 7179-7189]. Both studies indicate that protein relaxation in this region of the reaction center plays a key role in stabilizing charge-separated states involving the HA or βA cofactor. The effect is particularly pronounced for reactions occurring on time scales of tens and hundreds of picoseconds (forward transfer to the QA and charge recombination).

Excited state relaxation pathways of 4-dimethylamino-β-nitrostyrene: Effect of solvent polarity and donor–acceptor conjugation

Abstract Solvent polarity dependent relaxation pathways of the excited state of 4-dimethylamino-β-nitrostyrene (DANS) have been investigated using steady state and transient absorption spectroscopic techniques in femtosecond and nanosecond timescale. The molecule undergoes ultrafast twisted intramolecular charge transfer (TICT) relaxation involving the rotation of dimethylaniline group in polar solvents. In the medium and low polarity solvents, TICT relaxation rate is retarded due to larger barrier of twisting along TICT coordinate. In nonpolar solvents, the S1 state population undergo efficient intersystem crossing (ISC) to triplet surface without any conformational change. However, trans → cis isomerisation in nonpolar solvents takes place from the excited triplet manifold, as monitored by steady state photo-irradiation experiments. The ISC efficiency decreases with the increase in solvent polarity, which is explained by invoking solvent polarity dependent stabilization of charge transfer S1 state relative to a receiver triplet state. Comparison of the excited state dynamics of DANS with less conjugated para-dialkylamino nitrobenzene derivatives, reported previously, suggests that additional π-conjugation between the donor and acceptor groups stabilizes planar ICT state resulting to slower TICT relaxation.

Investigation of the Influence of Nitro-Substitution on the Reaction of Biphenyl-Nitrene and Nitrenium Ion by Transient Absorption and Resonance Raman Spectroscopic Techniques

Investigation of the Influence of Nitro-Substitution on the Reaction of Biphenyl-Nitrene and Nitrenium Ion by Transient Absorption and Resonance Raman Spectroscopic Techniques

Nature of Excited States of Ruthenium-Based Solar Cell Dyes in Solution: A Comprehensive Spectroscopic Study.

The photophysical properties of a number of ruthenium complexes of the general structure [Ru(L1)(L2)(NCS)2], related to the prominent solar cell dye [Ru(dcb)2(NCS)2] (dcb = 4,4'-dicarboxylato-2,2'-bipyridine) are investigated. For L1 = dcb and dmb (dmb = 4,4'-dimethyl-2,2'-bipyridine), several variations of L2 show very little difference in the lowest energy absorption peak. Resonance Raman and density functional theory calculations have been used to assign the corresponding transitions as {Ru(NCS)2} → dcb with significant contributions of the NCS ligands. Transient absorption, time-resolved infrared, and transient resonance Raman spectroscopic techniques were used to probe the photophysics of the complexes and relatively short-lived {Ru(NCS)2} → dcb/dpb (dpb = 4,4'-diphenylethenyl-2,2'-bipyridine) excited states were observed with the exception of [Ru(dcb)(dab)(NCS)2] (dab = 4,4'-dianthracenethenyl-2,2'-bipyridine), which showed a long-lived excited state assigned as ligand centered charge separated.

Intramolecular energy transfer dynamics in differently linked zinc porphyrin–dithiaporphyrin dyads

Intramolecular energy transfer dynamics in two molecular dyads, in which zinc porphyrin and dithiaporphyrin units were linked covalently, were studied by ultrafast time-resolved transient absorption and fluorescence spectroscopic techniques.

Pyrene Schiff base: photophysics, aggregation induced emission, and antimicrobial properties.

Pyrene containing Schiff base molecule, namely 4-[(pyren-1-ylmethylene)amino]phenol (KB-1), was successfully synthesized and well characterized by using (1)H, (13)C NMR, FT-IR, and EI-MS spectrometry. UV-visible absorption, steady-state fluorescence, time-resolved fluorescence, and transient absorption spectroscopic techniques have been employed to elucidate the photophysical processes of KB-1. It has been demonstrated that the absorption characteristics of KB-1 have been bathochromatically tuned to the visible region by extending the π-conjugation. The extended π-conjugation is evidently confirmed by DFT calculations and reveals that π→π* transition is the major factor responsible for electronic absorption of KB-1. The photophysical property of KB-1 was carefully examined in different organic solvents at different concentrations and the results show that the fluorescence of this molecule is completely quenched due to photoinduced electron transfer. Intriguingly, the fluorescence intensity of KB-1 increases enormously by the gradual addition of water up to 90% with concomitant increase in fluorescence lifetime. This clearly signifies that this molecule has aggregation-induced emission (AIE) property. The mechanism of AIE of this molecule is suppression of photoinduced electron transfer (PET) due to hydrogen bonding interaction of imine donor with water. A direct evidence of PET process has been presented by using nanosecond transient absorption measurements. Further, KB-1 was successfully used for antimicrobial and bioimaging studies. The antimicrobial studies were carried out through disc diffusion method. KB-1 is used against both Gram-positive (Rhodococcus rhodochrous and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial species and also fungal species (Candida albicans). The result shows KB-1 can act as an excellent antimicrobial agent and as a photolabeling agent. S. aureus, P. aeruginosa, and C. albicans were found to be the most susceptible microorganisms at 1 mM concentration among the bacteria used in the present investigation.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2'-bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited (3)MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the (3)MLCT state is about 2.5 ps, vibrational cooling of the pap-localized(3)MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge localization dynamics observed here may have important consequences in artificial solar energy harvesting systems, which employ heteroleptic ruthenium complexes.

Time-resolved laser spectroscopy of nonreactive processes in ionic liquids and their binary mixtures with organic solvents and CO2

Nonreactive dynamics of molecules in ionic liquids (ILs) have been investigated in various environments, such as in neat aprotic and protic ILs, as well as in binary mixtures of ILs with polar organic solvents and CO2. Transient broadband absorption and fluorescence spectroscopic techniques on timescales from femtoseconds to milliseconds have been applied to elucidate the importance of different types of solute–solvent interactions in these media. Molecular probes such a 12′-apo-β-carotenoic-12′-acid and Coumarin C153 have been used as reporter molecules for ionic liquid solvation dynamics, micropolarity, hydrogen-bonding and collisional relaxation. With Coumarin C153 in ILs, the relationship between the complete solvation response and microwave-terahertz absorption measurements of the neat liquid was explored quantitatively. A reduced value for the conductivity plays a key role in such solvation experiments. Aprotic ILs exhibit a micropolarity larger than expected from their static dielectric constants, whereas the local polarity of protic alkylammonium formate ILs appears to be similar to that of alcohols having comparable dielectric constants. In IL mixtures with organic solvents or CO2, there is no indication for differences in the bulk composition and the local composition around the molecular probe. Collisional relaxation processes in ILs take place on similar timescales as in dipolar organic solvents.

Dynamics of solvent controlled excited state intramolecular proton transfer coupled charge transfer reactions

Coupled intramolecular proton and charge transfer reactions play an important role in many biological and chemical reactions. In this article, we report the relaxation dynamics of the excited states of a donor substituted 1,3-diketone (DMADK, ) using steady state and ultrafast transient absorption and fluorescence spectroscopic techniques. Dramatic dependence of the fluorescence quantum yield, nonradiative rate as well as the excited state relaxation pathways on solvent polarity reveals the solvent controlled excited state intramolecular proton transfer (ESIPT) directed intramolecular charge transfer (ICT) dynamics. The molecules in the ground state coexist in two possible cis-enol (Enol-A and Enol-B) forms. Time-dependent density functional theory (TDDFT) calculations reveal solvent polarity controlled thermodynamic stabilization of one of the tautomeric structures in the S1 state, dictating the direction of proton transfer and subsequent structural relaxation. In low and medium polarity solvents, the S1 state of Enol-B (Enol-B*) undergoes ultrafast ESIPT leading to the population of Enol-A*, followed by the ICT process. In polar solvents, the ESIPT process is reversed to populate Enol-B*, which undergoes a twisted intramolecular charge transfer (TICT) process via twisting of the N,N-dimethylaniline group. This work demonstrates that the strength of electronic coupling between the donor and acceptor group determines the structure of the ICT state.

Excited state dynamics of a push–pull stilbene: A femtosecond transient absorption spectroscopic study

Abstract The excited state dynamics of a structurally flexible push–pull stilbene, namely, trans -4-(N,N-dimethylamino)-4′-nitrostilbene (DMANS), in solvents of varying polarities, H-bonding abilities and viscosities, has been investigated using femtosecond transient absorption spectroscopic technique. Results of DFT and TDDFT calculations have been used to explain the experimental data. Following photoexcitation of the planar DMANS molecule to the excited singlet (S 1 ) state, it undergoes cis–trans isomerization via twisting of the double bond. However, in solvents of medium and large polarities, the twisted intramolecular charge transfer (TICT) process involving only the rotation of the nitrophenyl group, but not the rotation of the dimethyl aniline or nitro group, is the dominant relaxation process in the excited state of DMANS, in polar solvents.