Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor–π–acceptor (D–π–A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump–probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

•fsCARS spectroscopy is employed to study OH stretching (υOH) of aqueous solutions.•Numerical evaluation was applied to confirm the distinctive OH stretching mode.•Average number of hydrogen bonds per water molecule was computed.•MD simulations were used for microscopic description of the CARS spectra.•The equilibrium distributions of hydrogen bonds was analyzed in the solvation shells.Femtosecond coherent anti-Stokes Raman scattering (fsCARS) spectroscopy, together with perturbation theory based numerical calculation, is employed to study OH stretching (υOH) of pure water and aqueous lithium chloride solutions. Vibrational OH stretching (υOH) modes of aqueous solutions are Raman-excited by a pair of ultrashort, femtosecond laser pulses, and then probed through inelastic scattering of a third, time-delayed laser field. In order to overcome limited spectral resolution of fsCARS, numerical evaluation of the CARS signal through vibrational wave packet propagation was employed in order to confirm the position of distinctive OH stretching mode that is complicated by intramolecular and intermolecular vibrational coupling. Moreover, in order to come to a microscopic description of the observed CARS spectra for aqueous solutions, we have performed molecular dynamics simulations of aqueous lithium chloride solutions with varying concentrations at ambient conditions. To this end we have analyzed the equilibrium distributions of hydrogen bonds in the first solvation shells of the ions as well as in bulk water and also computed the average number of hydrogen bonds per water molecule. According to our experimental and theoretical results on time evolution of Raman OH stretching band of water, it can be inferred that the dissolved ions mainly influence hydrogen bond strength and structure of water molecules in the first hydration shell, the addition of lithium chloride primarily breaks the tetrahedral hydrogen bonding, promotes formation of the donor hydrogen bonding in water, and slightly increases the amount of free OH bonds.

The exact interaction between Au cores and surface ligands remains largely unknown because of the complexity of the structure and chemistry of ligand/Au-core interfaces in ligand-protected Au nanoclusters (AuNCs), which are commonly found in many organic–inorganic complexes. Here, femtosecond transient absorption measurement of the excited-state dynamics of a newly synthesized phosphine-protected cluster [Au20(PPhpy2)10Cl4]Cl2 (1) is reported. Intramolecular charge transfer (ICT) from the Au core to the peripheral ligands was identified. Furthermore, we found that solvation strongly affected ICT at ligand/Au-core interfaces while by choosing several typical alcoholic solvents with different intrinsic solvation times, we successfully observed that excited-state relaxation dynamics together with displacive excited coherent oscillation of Au20 clusters were significantly modulated through the competition between solvation and surface trapping. The results provide a fundamental understanding of the structure–property relationships of the solvation-dependent core–shell interaction of AuNCs for the potential applications in catalysis, sensing and nanoelectronics.

Determination of the excited-state dynamics of carotenoids has attracted considerable interest, engendering a number of controversial hypotheses because of the strongly overlapping spectral peaks and complicated dynamics of transient species. In the present work, aiming for better understanding the complexity of excited-state processes in carotenoids, excited-state dynamics of all-trans-β-carotene in ethanol was investigated by femtosecond pump–probe spectroscopy. Following the excitation of the strongly allowed S2 state of the β-carotene, transient absorption spectra were recorded in the visible spectral range. For comparison, the time-resolved transient absorption spectra are analyzed in a conventional way, fitting kinetic traces with a multi-exponential function at chosen wavelengths from obtained spectra, and then again by means of the soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) method for modeling pure profiles and the generalized two-dimensional (2D) correlation spectroscopy data analysis for providing additional information on the dynamics of spectral features. MCR-ALS analysis shows that both the dynamics of the S* state, identified using the 2D correlation spectra, and the S1 state develop on a different timescale than the relaxation of the vibrationally hot S1v′ state. Hot S1v′ and S* states are shown to have different species associated difference spectra. Results of our analysis indicate that the S* state observed in this work is not the hot S1v′ state but instead a separate singlet state.

The rotational dynamics of coumarin 153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different alkyl chain lengths (alkyl = butyl, pentyl, hexyl, heptyl, octyl) ([Cnmim][PF6], n = 4–8) to examine the alkyl chain length dependent local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of [Cnmim][PF6] as a complementary measurement. The rotational dynamics of C153 show that at a certain length of the alkyl chain there are incompact and compact domains within [Cnmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosities in different structural domains of [Cnmim][PF6] with different alkyl chain lengths are investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that with an increase in the length of the alkyl chain, the microviscosity of [Cnmim][PF6] is obviously increased first and then slightly decreased. The steady state fluorescence measurements with the microviscosity probe of BPP further prove that the microviscosity is not increased as much as expected when ionic liquids [Cnmim][PF6] have a relatively long alkyl chain. The different heterogeneous structures of [Cnmim][PF6] with different lengths of the alkyl chain are proposed to interpret the unusual microviscosity behaviors.

It is accepted that the monolayer ligand shell in monolayer-protected gold nanoclusters (MPCs) plays an important role in stabilizing the metal core structure. Previous reports have shown that the core and shell do not interact chemically, and very few studies investigating the intramolecular charge transfer (ICT) between the core and ligand shell in clusters have been reported. The underlying excited state relaxation mechanisms about the influence of solvents, the optically excited vibration, and the roles of the core and shell in charge transfer remain unknown to a large extent. Here we report a femtosecond transient absorption study of a Au20(SR)16 (R = CH2CH2Ph) cluster in toluene and tetrahydrofuran. The ICT from the outside shell to the inside core upon excitation in Au20(SR)16 is identified. The observed solvation-dependent oscillations in different solvents further confirm the photoinduced ICT formation in Au20(SR)16. The results provide a fundamental understanding of the structure–property relationships about the solvation-dependent core–shell interaction in Au MPCs.

Hofmeister ion-specific effects on optical properties of a water-soluble cationic polymer, poly(3-alkoxy-4-methylthiophene) (PMNT), are investigated by means of absorption, resonance Raman spectroscopy, and molecular dynamic simulations. It is found that the ionochromism of conjugated polyelectrolytes PMNT obeys Hofmeister series with high optical sensitivity, while the spectral changes result from the different electrostatic interactions and the conformational change of the cationic PMNT in different salt solutions. As a result, UV–vis absorption spectra exhibit almost no shift of absorption of PMNT in the presence of SO42–, F–, etc., whereas a red-shifted absorption of PMNT with I–, SCN– is clearly observed. To gain a deeper understanding of the nature of these anion-dependent chromic phenomena, ab initio calculations and molecular dynamics (MD) simulations are carried out to present the microscopic insights, that the Hofmeister effect occurs at the PMNT/water interface through the direct (hydrophobic, hydrophilic, and electrostatic) interactions between the anions and PMNT backbone. It is found that the salting-in anions like I– strongly suppress the hydrophobic collapse of PMNT backbone, leading to more extended and ordered PMNT backbone with red-shifted absorption, and the salting-out anions like F– strongly avoid the hydrophobic PMNT backbone, keeping a random-coiled configuration of PMNT backbone without obvious absorption changes in KF solution. The existence of ordered and disordered backbone configurations is further verified by monitoring the main in-plane skeleton Raman modes (C═C and C–C stretch) of PMNT in various salt solutions. The results presented here could provide a fundamental understanding of salt effects on chemical and biological processes occurring at the macromolecular/water interface, and then may potentially stimulate many chemical and biological applications.

In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor–acceptor–donor (D–A–D) electronic push–pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (−C═C– or −C≡C−) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.

The rotational dynamics of coumarin 153 (C153) in imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and tetraethylene glycol dimethyl ether (TEGDME) mixtures across all mole fractions have been investigated to determine the local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of the mixtures as a complementary measurement. The rotational dynamics of C153 show that there are incompact and compact domains within the heterogeneous structural [bmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosity in different structural domains of [bmim][PF6] before and after adding cosolvent TEGDME with different mole fractions is further investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that the microviscosity of [bmim][PF6] is enhanced after mixing with a certain amount of TEGDME, although the bulk viscosity of TEGDME itself is much lower than that of the ionic liquid. This unusual behavior of microviscosity enhancement is further proven by the steady state fluorescence measurement with the microviscosity probe of BPP. The microviscosity enhancement is reasonably demonstrated by the fast time constant of C153 rotational dynamics and the departure between the experimentally observed and calculated ratio of IE/IM of BPP, which shows that this effect is most pronounced at intermediate mole fractions of the [bmim][PF6] and TEGDME mixtures. The strengthening effects caused by the molecular interactions between TEGDME and structural heterogeneous ionic liquid [bmim][PF6] are proposed to interpret the unusual microviscosity behaviors.

The absorption spectra and intramolecular charge transfer (CT) properties of terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes (DPE) and α,ω-diphenylpolyynes (DPY) molecules with different conjugated bridge length and substitution modes were investigated by using quantum chemical calculations. We calculated the ground state structures and energy of two series of terminal donor/acceptor DPE and DPY by DFT method. The dependence of conjugation length and substitution modes of the electronic absorption spectra was obtained by TDDFT calculation. The hybrid-GGA XC-functional PBE0 employed in this work was selected from several functionals by comparing the calculated electronic spectral data with experimental value. The CIS-based generalized Mulliken-Hush (GMH) approach was further used to calculate coupling values HAD of the CT process. The calculation shows that both the HOMO–LUMO energy gaps and average bond length alternations between unsaturated multiple (CC and CC) and saturated single bonds (C–C) decrease regularly with the extension of conjugation. The effective conjugated length (ECL) of DPE and DPY with the same order MM > MP/PM > PP is found together with the regular red shift of the electronic absorption spectra with the extension of conjugation, resulting from the different π-electron delocalization and conjugation efficiency. The GMH analysis further suggests that the CT process in both DPE and DPY is predominated by the through-bond mechanism. The remarkable difference of the conjugated length dependence of squared CT coupling between substituted DPE and DPY is the result of the energetic matching degree of the frontier molecular orbitals between donor/acceptor and the conjugated bridge.

The fluorescence depletion dynamics of Rhodamine 700 (R-700) molecules in room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([HOemim][BF4]) were investigated to determine the local viscosity of the microenvironment surrounding the fluorescent molecules, which is induced by strong hydrogen bonding interaction between cationic and anionic components in RTILs. The solvation and rotation dynamics of R-700 molecules in RTILs show slower time constants relative to that in conventional protic solvents with the same bulk viscosity, indicating that the probe molecule is facing a more viscous microenvironment in RTILs than in conventional solvents because of the strong hydrogen bonding interaction between cationic and anionic components. In addition, this effect is more pronounced in hydroxyl-functionalized ionic liquid than in the regular RTIL due to the presence of a hydroxyl group as a strong hydrogen bonding donor. The hydrogen-bonding-induced local viscosity enhancement effect related to the heterogeneity character of RTILs is confirmed by the nonexponential rotational relaxation of R-700 determined by time-correlated single photon counting (TCSPC). The geometry of hydrogen bonding complexes with different components and sizes are further optimized by density functional theory methods to show the possible hydrogen-bond networks. A model of the hydrogen-bonding network in RTILs is further proposed to interpret the observed specific solvation and local viscosity enhancement effect in RTILs, where most of the fluoroprobes exist as the free nonbonding species in the RTIL solutions and are surrounded by the hydrogen-bonding network formed by the strong hydrogen-bonding between the cationic and anionic components in RTIL. The optimized geometry of hydrogen bonding complexes with different components and sizes by density functional theory methods confirms the local viscosity enhancement effect deduced from fluorescence depletion and TCSPC experiments. The calculated interaction energies reveal the existence of the stronger hydrogen bonding network in RTILs (especially in hydroxyl-functionalized ionic liquid) than that in conventional protic solvent, which leads to the enhancement effect of local microviscosity, and therefore leads to the slow solvation and rotation dynamics of probe molecules observed in RTILs.

The rotational dynamics of coumarin 153 (C153) have been investigated in a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with different alkyl chain lengths (alkyl = butyl, pentyl, hexyl, heptyl, octyl) ([Cnmim][PF6], n = 4–8) to examine the alkyl chain length dependent local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of [Cnmim][PF6] as a complementary measurement. The rotational dynamics of C153 show that at a certain length of the alkyl chain there are incompact and compact domains within [Cnmim][PF6], resulting in fast and slow components of C153 rotational dynamics. The microviscosities in different structural domains of [Cnmim][PF6] with different alkyl chain lengths are investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that with an increase in the length of the alkyl chain, the microviscosity of [Cnmim][PF6] is obviously increased first and then slightly decreased. The steady state fluorescence measurements with the microviscosity probe of BPP further prove that the microviscosity is not increased as much as expected when ionic liquids [Cnmim][PF6] have a relatively long alkyl chain. The different heterogeneous structures of [Cnmim][PF6] with different lengths of the alkyl chain are proposed to interpret the unusual microviscosity behaviors.

The exact interaction between Au cores and surface ligands remains largely unknown because of the complexity of the structure and chemistry of ligand/Au-core interfaces in ligand-protected Au nanoclusters (AuNCs), which are commonly found in many organic–inorganic complexes. Here, femtosecond transient absorption measurement of the excited-state dynamics of a newly synthesized phosphine-protected cluster [Au20(PPhpy2)10Cl4]Cl2 (1) is reported. Intramolecular charge transfer (ICT) from the Au core to the peripheral ligands was identified. Furthermore, we found that solvation strongly affected ICT at ligand/Au-core interfaces while by choosing several typical alcoholic solvents with different intrinsic solvation times, we successfully observed that excited-state relaxation dynamics together with displacive excited coherent oscillation of Au20 clusters were significantly modulated through the competition between solvation and surface trapping. The results provide a fundamental understanding of the structure–property relationships of the solvation-dependent core–shell interaction of AuNCs for the potential applications in catalysis, sensing and nanoelectronics.

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump–probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

Excited state solvation plays a very important role in modulating the emission behavior of fluorophores upon excitation. Here, the solvation effects on the local micro-environment around a fluorophore are proposed by investigating the fantastic emission behavior of a novel amyloid fibril marker, NIAD-4, in different alcoholic and aprotic solvents. In alcoholic solvents, high solvent viscosity causes an obvious enhancement of fluorescence because of the restriction of torsion of NIAD-4, where the formation of a non-fluorescent twist intramolecular charge transfer (TICT) state is suppressed. In aprotic solvents, high solvent polarity leads to a remarkable redshift of the emission spectra suggesting strong solvation. Surprisingly, an abnormal fluorescence enhancement of NIAD-4 is observed with increasing solvent polarity of the aprotic solvents, whereas solvent viscosity plays little role in influencing the fluorescence intensity. We conclude that such an abnormal phenomenon is originated from a solvation induced micro-viscosity enhancement around the fluorophore upon excitation which restricts the torsion of NIAD-4. Femtosecond transient absorption results further prove such a micro-viscosity increasing mechanism. We believe that this solvation induced micro-viscosity enhancement effect on fluorescence could widely exist for most donor–π–acceptor (D–π–A) compounds in polar solvents, which should be carefully taken into consideration when probing the micro-viscosity in polar environments, especially in complex bioenvironments.

The absorption spectra and intramolecular charge transfer (CT) properties of terminal donor/acceptor-substituted all-trans-α,ω-diphenylpolyenes (DPE) and α,ω-diphenylpolyynes (DPY) molecules with different conjugated bridge length and substitution modes were investigated by using quantum chemical calculations. We calculated the ground state structures and energy of two series of terminal donor/acceptor DPE and DPY by DFT method. The dependence of conjugation length and substitution modes of the electronic absorption spectra was obtained by TDDFT calculation. The hybrid-GGA XC-functional PBE0 employed in this work was selected from several functionals by comparing the calculated electronic spectral data with experimental value. The CIS-based generalized Mulliken-Hush (GMH) approach was further used to calculate coupling values HAD of the CT process. The calculation shows that both the HOMO–LUMO energy gaps and average bond length alternations between unsaturated multiple (CC and CC) and saturated single bonds (C–C) decrease regularly with the extension of conjugation. The effective conjugated length (ECL) of DPE and DPY with the same order MM > MP/PM > PP is found together with the regular red shift of the electronic absorption spectra with the extension of conjugation, resulting from the different π-electron delocalization and conjugation efficiency. The GMH analysis further suggests that the CT process in both DPE and DPY is predominated by the through-bond mechanism. The remarkable difference of the conjugated length dependence of squared CT coupling between substituted DPE and DPY is the result of the energetic matching degree of the frontier molecular orbitals between donor/acceptor and the conjugated bridge.

Determination of the excited-state dynamics of carotenoids has attracted considerable interest, engendering a number of controversial hypotheses because of the strongly overlapping spectral peaks and complicated dynamics of transient species. In the present work, aiming for better understanding the complexity of excited-state processes in carotenoids, excited-state dynamics of all-trans-β-carotene in ethanol was investigated by femtosecond pump–probe spectroscopy. Following the excitation of the strongly allowed S2 state of the β-carotene, transient absorption spectra were recorded in the visible spectral range. For comparison, the time-resolved transient absorption spectra are analyzed in a conventional way, fitting kinetic traces with a multi-exponential function at chosen wavelengths from obtained spectra, and then again by means of the soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) method for modeling pure profiles and the generalized two-dimensional (2D) correlation spectroscopy data analysis for providing additional information on the dynamics of spectral features. MCR-ALS analysis shows that both the dynamics of the S* state, identified using the 2D correlation spectra, and the S1 state develop on a different timescale than the relaxation of the vibrationally hot S1v′ state. Hot S1v′ and S* states are shown to have different species associated difference spectra. Results of our analysis indicate that the S* state observed in this work is not the hot S1v′ state but instead a separate singlet state.