Time-resolved Fluorescence Techniques Research Articles

Excited-State Proton Transfer to H2O in Mixtures of CH3CN-H2O of a Superphotoacid, Chlorobenzoate Phenol Cyanine Picolinium (CBCyP).

Steady-state and time-resolved fluorescence techniques were employed to study a superphotoacid with a p Ka* of ∼-7, the chlorobenzoate phenol cyanine picolinium salt (CBCyP) in acetonitrile-water mixtures. We found that the time-resolved fluorescence is bimodal. The amplitude of the short-time component depends on χwater; the larger χwater, the greater the amplitude. We found that the excited-state proton-transfer (ESPT) rate constant, kPT, is ≥5 × 1012 s-1 in mixtures of χwater ≥ 0.08, whereas in neat water, kPT = 6 × 1012 s-1. The long-time component has a lifetime of 50 ps at χwater = 0.75. We attribute this time component to the CBCyP molecules that are not hydrogen-bonded to H2O clusters. The results suggest that the ESPT rate constant to water in acetonitrile-water mixtures depends only slightly on the water cluster size and structure surrounding the CBCyP molecule. We attribute the independence of the ESPT rate on the average water-cluster size to the large photoacidity of CBCyP. QM TD-DFT calculations found that in the excited-state the RO-(S1) species that is formed by the ESPT process is more stable than the ROH(S1) species by -5 kcal/mol when four water molecules accept the proton, and when six water molecules accept the proton, the RO-(S1) drops to -10 kcal/mol. The calculations show that energy stabilities are kept constant in implicit CH3CN-H2O solvent mixtures of dielectric constant of ε ≥ 45.

Photoinduced electronic interactions between acridine derivatives and small gold nanoparticles: A spectroscopic insight

The central tricyclic moiety of acridine derivatives has fascinating redox properties. We take advantage of the electronic properties of two such acridine derivatives, 9-amino acridine (9AA-HCl) and acridine yellow, recognized so far as potential antimicrobial agents, since these drugs can act both as potential electron donor and acceptor, depending upon the reduction potential of its adjacent chemical entity. Precise timescale of formation of contact ion pair (CIP) and solvent separated ion pair in solution involving donors and acceptors has been much encountered and debated in literature till date. In our present communication, we like to decipher the mechanism and timescale of individual steps of photoinduced electron transfer (PET) between these drugs and gold nanoparticles (AuNPs) with dimension of 3-5 nm, which is important and presently our subject of interest since AuNPs could be used for shuttling of electrons between different donor and acceptor moieties. Steady-state and time-resolved absorption and fluorescence studies evidently reveal the signature of PET both in singlet and triplet states. Using ultrafast time-resolved fluorescence techniques we have tried to quantify precisely the time required for formation of CIP which is within fs-ps time regime. Moreover, 9AA-HCl, being a planar molecule, dimerises at excited state in presence of high concentration of AuNPs, due to its immobilisation over their surface. The occurrence of excited state proton transfer in triplet state for 9AA-HCl-AuNPs conjugate is depicted by laser flash photolysis experiments. The presence of all such charge transfer channels, provokes us to delineate the intricacies of the mechanisms involved which would help further to evaluate the possibility of making devices for energy storage using these drugs.

Solvatochromism of 1-naphthol-4-sulfonate photoacid and its encapsulation in cyclodextrin derivatives

Photophysical properties of the ground and excited states of 1-naphthol-4-sulfonate (1NO4S), photoacid, are investigated in different solvents and cyclodextrin derivatives of different size and structure. Fluorescence decay traces of 1NO4S are well fitted to double exponential decay function in all solvents. The short-lived traces with higher amplitude were assigned to emission of the neutral form (ROH*), while the long-lived traces with low amplitude were assigned to emission of the anionic form (RO−*). The photophysical properties of 1NO4S were correlated with a number of solvent parameters such as the solvent polarity parameters ETN and the reaction field factor Δf. Three linear solvation energy relationships developed by Kamlet-Taft, Catalán, and Laurence et al. were employed to assess the effect of non-specific and specific interactions on the photophysical properties of 1NO4S. All three solvation energy relationships showed that the non-specific interactions mainly control the photophysical properties of 1NO4S while the contribution of the specific interactions are weak. Molecular encapsulation of 1NO4S by α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), methyl-β-cyclodextrin (Mβ-CD), 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in aqueous solution has been studied by steady state and time resolved fluorescence techniques. Steady state fluorescence measurements show 1:1 inclusion of 1NO4S with different cyclodextrin derivatives with an association constant of 7.9 ± 4.0, 59.6 ± 10.0, 0.4 ± 0.5, 169.8 ± 25.0, and 359.0 ± 20.0 M−1 for 1NO4S complexes with α-CD, β-CD, γ-CD, Mβ-CD or HPβ-CD, respectively following the fluorescence intensity changes of the anionic form (RO−*) at 435.0 nm. Association constants obtained following the fluorescence enhancement of the neutral form (ROH*) were the same as determined following fluorescence intensity decrease of RO−* at 372 nm. Data derived from time resolved measurements gave very close value for the association constants as determined from steady state fluorescence measurements.

Excited-State Proton Transfer from the Photoacid 2-Naphthol-8-sulfonate to Acetonitrile/Water Mixtures.

Steady-state and time-resolved fluorescence techniques were used to study excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol-8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to χwater ≤ 0.12, we found a slow ESPT process on the order of nanoseconds. At χwater ≈ 0.15, the RO- fluorescence band intensity is at the minimum, whereas at χwater ≈ 0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO- fluorescence band at χwater ≈ 0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH3CN/H2O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S, and this enables the ESPT process between the two sites of the molecular structure of 2N8S. In mixtures of χwater ≥ 0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the χwater in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small, 4.5 × 109 s-1. TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8-sulfonate in the excited state. The activation energy for the ESPT reaction is about 9 kcal/mol, and the RO-(S1) species is energetically above the ROH(S1) species by about 1.6 kcal/mol.

Laser-induced linear and non-linear optical features in novel benzoxazole-based on donor-acceptor chromophores

The photophysical properties of donor-acceptor based benzoxazoles were investigated by means of steady-state and time-resolved fluorescence techniques. Additionally, laser stimulated second-order optical effects (preliminary second harmonic generation (SHG) were studied. The studies show that the nitro-benzoxazole derivatives decorated with different alkylo/arylo amino groups dissolved in medium polar aprotic solvents: tetrahydrofuran and butyl chloride, exhibit dual fluorescence from the locally excited state (1LE) and a highly polar charge transfer (CT) state. Ambient-temperature time-resolved fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction 1LE→1CT. The lack of any CT emissions in cyclohexane and highly polar acetonitrile is presumably due to an efficient inter-system crossing process or fast deactivation of a low-lying CT state. In addition, strong CT properties inspired us to check a possibility to vary the absorption of the first UV–Vis maxima under influence of the external bicolour coherent laser light and the related optical second harmonic generation (SHG). The two coherent beams of the infrared Er:glass laser at 1540 nm/770 nm formed gratings in the photopolymer solidified chromophore. The performed experiments are compared with the theoretically calculated electronic parameters within the B3LYP DFT approach. The presented data open novel type of materials, allowing to operate its transparency using external IR laser light in the near IR spectral range.

Ferrocene and triphenylamine appended boranils

Boranils containing ferrocene and triphenylamine groups were synthesized and well characterized by IR, $$^{1}\hbox {H}$$ , $$^{13}\hbox {C}$$ , $$^{11}\hbox {B}$$ and $$^{19}\hbox {F}$$ NMR and ESI-MS spectrometry. UV-Vis absorption, steady-state and time-resolved fluorescence techniques were employed to study their photophysical properties. The presence of electron rich substituents caused significant alterations in their absorption and emission spectra. As compared to the parent boranil compound ( $$\lambda _{\mathrm{abs}} = 342 \hbox { nm}$$ ), boranil-triphenylamine and boranil-ferrocene conjugates exhibited red shifted absorption maxima ( $$\lambda _{\mathrm{abs}} = 384$$ , 375 nm, respectively). The boranil-triphenylamine and boranil-ferrocene conjugates displayed red shifted emission spectra ( $$\lambda _{\mathrm{em}} = 524$$ , 489 nm, respectively), as compared to the parent boranil ( $$\lambda _{\mathrm{em}} = 471 \hbox { nm}$$ ). Also, large Stokes shifts ( $$6217{-}6958 \hbox { cm}^{-1}$$ ) were observed for both the conjugates in solution phase. SYNOPSIS Synthesis and optical properties of boranils containing ferrocene and triphenylamine groups are reported. The presence of substituents caused significant bathochromic shifts in their absorption and emission spectra. As compared to the parent boranil compound, boranil-triphenylamine and boranil-ferrocene conjugates exhibited 32–41 nm red shifted absorption spectra, and 18–53 nm red shifted emission spectra in solution. Also, large Stokes shifts (6200–6900 $$\hbox { cm}^{-1}$$ ) were observed for both the conjugates in solution.

Characterization and detection of adulterated whey protein supplements using stationary and time-resolved fluorescence spectroscopy

The objective of this work was to evaluate the efficacy of time-resolved and stationary fluorescence spectroscopic techniques in the detection of adulterations in Whey Protein Concentrate (WPC) powder, used in nutritional supplements. Adulterations were performed by the individual addition of caffeine, creatine and lactose in WPC in different levels (10%, 20% and 30% w/w). Samples were submitted to 275 nm excitation wavelength. Time-resolved fluorescence revealed that the adulterations changed the lifetimes of the tri-exponential decay curves obtained at 335 nm. The 30% adulterations demonstrated higher values of the mean emission lifetime, with caffeine treatment statistically significant. The fluorescence spectra showed an emission peak at approximately 335 nm with a broadband between 325 and 430 nm in the caffeine treatment. Euclidean distance analysis indicated the creatine and lactose treatments (10, 20 and 30% w/w) as similar to control WPC, while caffeine 20% and 30% treatments not. Hierarchical Cluster Analysis could differentiate all caffeine treatments from the other samples. Principal Component Analysis confirmed the differentiation of all adulterants from the control WPC. In general, the application of time-resolved fluorescence and stationary fluorescence spectroscopy techniques allowed the characterization and observation of differences among the samples, when allied to statistical tools.

Enhanced Excited-State Proton Transfer via a Mixed Water-Methanol Molecular Bridge of 1-Naphthol-5-Sulfonate in Methanol-Water Mixtures.

We used steady-state and time-resolved fluorescence techniques to study the excited-state proton transfer (ESPT) and the nonradiative properties of two irreversible photoacids, 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-5-sulfonate (1N5S). We found that the ESPT rate constant of 1N4S in water is 2.2 × 1010 s-1, whereas in methanol, it is smaller by about 3 orders of magnitude and is not observed. The ESPT process of 1N5S competes with a major nonradiative process of equal rate and kPT of 2.2 × 1010 s-1. In methanol-water mixtures of χH2O = 0.2, the fluorescence lifetime of the ROH form of 1N5S is lower by a factor of 10 than that in pure methanol. In the steady-state fluorescence spectra of 1N5S in methanol-water mixtures, there are two iso-emissive points, one for χH2O < 0.2 and one for χH2O > 0.3. This large reduction in fluorescence intensity and the two iso-emissive points are explained by the existence of a mixed water-methanol bridge of about three molecules that connects the proton donor 1-OH with the 5-sulfonate in mixtures of χH2O < 0.2. The bridge enhances both the ESPT and the nonradiative processes. For 1N4S in methanol-water mixtures at χH2O ≈0.2, the reduction in the fluorescence lifetime is only by ∼30%, and only one iso-emissive point exists in the steady-state fluorescence spectra for 0 <χH2O < 1. TD-DFT computations show that a mixed bridge of one water molecule and two methanol molecules that connects the 1-OH with 5-sulfonate is more stable by 7.7 kcal/mol than the 1-OH reactant in the S1 state, and the barrier is only 8.0 kcal/mol. The nonradiative channel is because the S2 dark state is about 4.6 kcal/mol higher than the S1 state.

Steady state and time resolved fluorescence studies of new indolizine derivatives with phenanthroline skeleton

Some indolizines with phenanthroline skeleton derivatives have been investigated using steady state absorption, emission and time resolved fluorescence techniques. The effects of the substituents on the electronic absorption spectra and fluorescence emission have been demonstrated. The fluorescence decay curves exhibit one or two decay times. The emission lifetime measurements revealed values of 3–5 ns under the excitation with 370 nm. The quantum yield was also determined. The quenching process of emission using picric acid as quencher was characterized by Stern-Volmer plots. The phenanthroline derivatives 2 and 5 possess high efficiency for the detection of picric acid.

Excited-State Proton Transfer of Phenol Cyanine Picolinium Photoacid.

Steady-state and time-resolved fluorescence techniques as well as quantum-mechanical calculations were used to study the photophysics and photochemistry of a newly synthesized photoacid—the phenol cyanine picolinium salt. We found that the nonradiative rate constant knr of the excited protonated form of the photoacid is larger than that of the excited-state proton transfer (ESPT) to the solvent, kESPT. We estimate that the quantum efficiency of the ESPT process is about 0.16. The nonradiative process is explained by a partial trans–cis isomerization reaction, which leads to the formation of a “dark” excited state that can cross to the ground state by nonadiabatic coupling. Moreover, the ESPT process is coupled to the photo-isomerization reaction, as this latter reaction enhances the photoacidity of the studied compound, as a result of photoinduced charge transfer. To prevent trans–cis isomerization of the cyanine bridge, we conducted experiments of PCyP adsorbed on cellulose in the presence of water. We found that the steady-state fluorescence intensity increased by about a factor of 50 and the lifetime of the ROH band increased by the same factor. The fluorescence intensity of the RO– band with respect to that of the ROH band was the same as in aqueous solution. This explains why inhibiting the photo-isomerization reaction by adsorbing the PCyP on cellulose does not lead to a higher ESPT rate.

Characterization of F2N12S in Cell Membranes using Time-Resolved Fluorescence Techniques

Characterization of F2N12S in Cell Membranes using Time-Resolved Fluorescence Techniques

Single Molecular Level Probing of Structure and Dynamics of Papain Under Denaturation.

Papain is a cysteine protease enzyme present in papaya and known to help in digesting peptide. Thus the structure and function of the active site of papain is of interest. The objective of present study is to unveil the overall structural transformation and the local structural change around the active site of papain as a function of chemical denaturant. Papain has been tagged at Cys-25 with a thiol specific fluorescence probe N-(7- dimethylamino-4-methylcoumarin-3-yl) iodoacetamide (DACIA). Guanidine hydrochloride (GnHCl) has been used as the chemical denaturant. Steady state, time-resolved, and single molecular level fluorescence techniques was applied to map the change in the local environment. It is found that papain undergoes a two-step denaturation in the presence of GnHCl. Fluorescence correlation spectroscopic (FCS) data indicate that the size (hydrodynamic diameter) of native papain is ~36.8 Å, which steadily increases to ~53 Å in the presence of 6M GnHCl. FCS study also reveals that the conformational fluctuation time of papain is 6.3 µs in its native state, which decreased to 2.7 µs in the presence of 0.75 M GnHCl. Upon further increase in GnHCl concentration the conformational fluctuation time increase monotonically till 6 M GnHCl, where the time constant is measured as 14 µs. On the other hand, the measurement of ellipticity, hence the helical structure, by circular dichroism spectroscopy is found to be incapable to capture such structural transformation. It is concluded that in the presence of small amount of GnHCl the active site of papain takes up a more compact structure (although the overall size increases) than in the native state, which has been designated as the intermediate state.

Selective binding of pyrene in subdomain IB of human serum albumin: Combining energy transfer spectroscopy and molecular modelling to understand protein binding flexibility

The ability of human serum albumin (HSA) to bind medium-sized hydrophobic molecules is important for the distribution, metabolism, and efficacy of many drugs. Herein, the interaction between pyrene, a hydrophobic fluorescent probe, and HSA was thoroughly investigated using steady-state and time-resolved fluorescence techniques, ligand docking, and molecular dynamics (MD) simulations. A slight quenching of the fluorescence signal from Trp214 (the sole tryptophan residue in the protein) in the presence of pyrene was used to determine the ligand binding site in the protein, using Förster's resonance energy transfer (FRET) theory. The estimated FRET apparent distance between pyrene and Trp214 was 27Å, which was closely reproduced by the docking analysis (29Å) and MD simulation (32Å). The highest affinity site for pyrene was found to be in subdomain IB from the docking results. The calculated equilibrium structure of the complex using MD simulation shows that the ligand is largely stabilized by hydrophobic interaction with Phe165, Phe127, and the nonpolar moieties of Tyr138 and Tyr161. The fluorescence vibronic peak ratio I1/I3 of bound pyrene inside HSA indicates the presence of polar effect in the local environment of pyrene which is less than that of free pyrene in buffer. This was clarified by the MD simulation results in which an average of 5.7 water molecules were found within 0.5nm of pyrene in the binding site. Comparing the fluorescence signals and lifetimes of pyrene inside HSA to that free in buffer, the high tendency of pyrene to form dimer was almost completely suppressed inside HSA, indicating a high selectivity of the binding pocket toward pyrene monomer. The current results emphasize the ability of HSA, as a major carrier of several drugs and ligands in blood, to bind hydrophobic molecules in cavities other than subdomain IIA which is known to bind most hydrophobic drugs. This ability stems from the nature of the amino acids forming the binding sites of the protein that can easily adapt their shape to accommodate a variety of molecular structures.

Anomalous Rate of H+ and D+ Excited-State Proton Transfer (ESPT) in H2O/D2O Mixtures: Irreversible ESPT in 1-Naphthol-4-sulfonate.

We employed steady-state and time-resolved fluorescence techniques to study the rates of excited-state proton and deuteron transfer (ESPT) from an irreversible photoacid, 1-naphthol-4-sulfonate, to solvent mixtures of H2O and D2O. We found that the overall ESPT rate to the solvent mixture does not follow a linear relation with the H2O mole ratio. We used a chemical kinetic model to explain the deviation of the ESPT rate constant from linear behavior with H2O mole ratio. There are three water species in the H2O-D2O mixtures, H2O, D2O, and HOD. There are six rate constants of H+ and D+ transfers to the three species. When the H2O mole ratio before mixing is 0.5, HOD mole ratio in the mixture is 0.5. The ESPT rate to HOD is much smaller than that of H+ transfer to neat H2O and hence the concave shape of the plot of ESPT rate constants versus the H2O molar ratio of the mixtures.

The photoacidity of phenol chloro benzoate cyanine picolinium salt photoacid in alkanols

Abstract Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) rate to methanol, ethanol and propanol of a new photoacid, the chloro benzoate phenol cyanine picolinium salt (CBCyP). We found that the ESPT rate constants for methanol, ethanol and propanol are about 3 × 1012 s−1, 2 × 1012 s−1 and 1.2 × 1012 s−1 respectively, whereas for water it is 6 × 1012 s−1. The photoacid pKa* in water is about pKa ∼ −7. The kinetic isotope effect as measured from the fluorescence decay rate of the protonated form is 1.5 and 1.25 for methanol and ethanol respectively, whereas in water it is 1.7. We suggest that a nonradiative process takes place and reduces the measured kinetic isotope effect in both methanol and ethanol.

Mesoporous silica for drug delivery: Interactions with model fluorescent lipid vesicles and live cells

Formulated mesoporous silica nanoparticle (MSN) systems offer the best possible drug delivery system through the release of drug molecules from the accessible pores. In the present investigation, steady state and time resolved fluorescence techniques along with the fluorescence imaging were applied to investigate the interactions of dye loaded MSN with fluorescent unilamellar vesicles and live cells. Here 1,2-dimyristoyl-sn-glycero-3-phospocholine (DMPC) was used to prepare Small Unilamellar Vesicles (SUVs) as the model membrane with fluorescent 1,6-diphenyl-1,3,5-hexatriene (DPH) molecule incorporated inside the lipid bilayer. The interaction of DPH incorporated DMPC membrane with Fluorescein loaded MSN lead to the release of Fluorescein (Fl) dye from the interior pores of MSN systems. The extent of release of Fl and spatial distribution of the DPH molecule has been explored by monitoring steady-state fluorescence intensity and fluorescence lifetime at physiological condition. To investigate the fate of drug molecule released from MSN, fluorescence anisotropy has been used. The drug delivery efficiency of the MSN as a carrier for doxorubicin (DOX), a fluorescent chemotherapeutic drug, has also been investigated at physiological conditions. The study gives a definite confirmation for high uptake and steady release of DOX in primary oral mucosal non-keratinized squamous cells in comparison to naked DOX treatment.

Time-resolved fine structure mixing of cesium induced by helium and argon

The rates for fine structure mixing and quenching of the moderately excited and states of cesium under collision with argon and the more impulsive helium have been measured, using two-photon pulsed excitation and time-resolved fluorescence techniques. The pressure dependence of the eigenvalues of the rate matrix yields very rapid rate coefficients of for He and for Ar, about two–three times faster than for the Cs relaxation. The quenching rates are also rapid, and for He and Ar respectively. The rapid fine structure rates are explained by the highly impulsive nature of the collisions, and the large average distance of the valence electron from the nucleus. Quenching rates (intra-multiplet transfer) are likely enhanced by the closely spaced, levels. The observed rates are compared with the predictions from time-dependent perturbation theory.

Two-Photon Macromolecular Probe Based on a Quadrupolar Anthracenyl Scaffold for Sensitive Recognition of Serum Proteins under Simulated Physiological Conditions.

The binding interaction of a biocompatible water-soluble polycationic two-photon fluorophore (Ant-PIm) toward human serum albumin (HSA) was thoroughly investigated under simulated physiological conditions using a combination of steady-state, time-resolved, and two-photon excited fluorescence techniques. The emission properties of both Ant-PIm and the fluorescent amino acid residues in HSA undergo remarkable changes upon complexation allowing the thermodynamic profile associated with Ant-PIm–HSA complexation to be accurately established. The marked increase in Ant-PIm fluorescence intensity and quantum yield in the proteinous environment seems to be the outcome of the attenuation of radiationless decay pathways resulting from motional restriction imposed on the fluorophore. Fluorescence resonance energy transfer and site-marker competitive experiments provide conclusive evidence that the binding of Ant-PIm preferentially occurs within the subdomain IIA. The pronounced hypsochromic effect and increased fluorescence enhancement upon association with HSA, compared to that of bovine serum albumin (BSA) and other biological interferents, makes the polymeric Ant-PIm probe a valuable sensing agent in rather complex biological environments, allowing facile discrimination between the closely related HSA and BSA. Furthermore, the strong two-photon absorption (TPA) with a maximum located at 820 nm along with a TPA cross section σ2 > 800 GM, and the marked changes in the position and intensity of the band upon complexation definitely make Ant-PIm a promising probe for two-photon excited fluorescence-based discrimination of HSA from BSA.

Chloro benzoate cyanine picolinium photoacid excited-state proton transfer to water

Abstract Steady-state and time-resolved fluorescence techniques were employed to study the photoacidity of a newly synthesized photoacid, the chloro benzoate cyanine picolinium salt. It was found that the ESPT rate constant is ultrafast in water ( k PT = 6.2 × 10 12 s −1 ). We also found that the kinetic isotope effect is ∼1.7. The deprotonated form of the photoacid has a short lifetime of about 95 ps.

Effects of terthiophene as the end-groups in triblock copolymers consisting of poly(fluorene vinylene) and oligo(phenylene vinylene): Time-resolved fluorescence and its anisotropy

Effects of terthiophene (3T) as the end-groups in poly(9,9-di-n-octyl-fluorene-2,7-vinylene)s (PFVs) were studied by means of time-resolved fluorescence and anisotropy techniques. In a terthiophene end-modified triblock copolymer consisting PFV and oligo(phenylene vinylene) (PV, as the middle segment), [(10PFV)2-7PV](3T)2, steady-state fluorescence spectrum shows much intense vibronic bands compared to those of the reference copolymer without terthiophene groups. In contrast, the absorption spectrum of [(10PFV)2-7PV](3T)2 was almost identical to that of the reference copolymer. Fluorescence quantum yield of the 3T end-modified polymer is significantly lower than that of the reference copolymer: ϕf = 0.62 vs 0.83. It was found that fluorescence spectral profile of [(10PFV)2-7PV](3T)2 is time-dependent: As the delay time increases, relative intensities of the higher vibronic bands increase. Time-resolved signals were analyzed by two decay components, 430 ps (shorter wavelength region) and 630 ps (longer wavelength region). On the other hand, fluorescence anisotropy ratios decrease as the monitored wavelength becomes longer. It is highly likely that the terthiophene end-modified copolymer undergoes structural change in the excited state. The longer decay component can be ascribed to a species of which π-system of the excited PFV part is expanded over the end-group after coplanarization of the terthiophene unit upon the excitation.