Photophysics and Dynamics of Dye-Doped Conjugated Polymer Nanoparticles by Time-Resolved and Fluorescence Correlation Spectroscopy

Fluorescent dye encapsulated conjugated polymer nanoparticles have been paid significant attention for potential applications in photonics and biophotonics due to their high brightness and better photostability. Bright, photostable, and monodispersed Nile Red (NR) dye encapsulated poly-<i>N</i>-vinylcarbazole (PVK) fluorescent polymer nanoparticles have been prepared to understand the influence of size of particles and the concentration of dye inside the particles on the photophysical properties by using steady-state, time-resolved fluorescence spectroscopy and fluorescence correlation spectroscopy (FCS). Here, we have quantitatively analyzed the hydrodynamic diameter, particle brightness, and population of NR molecules inside the particle with varying the particle size and NR concentration by using fluorescence correlation spectroscopy (FCS). The average fluorescence intensity of a single nanoparticle, i.e., per particle brightness (PPB) value, increases from 80 to 500 kHz, and the number of NR molecules per nanoparticle increases from 5 to 22 by increasing the concentration of NR from 0.5 to 1.8 wt % at the time of nanoparticle preparation. Fluorescence anisotropy study has been undertaken to understand the rotational dynamics of encapsulated NR molecules with varying particle size and NR concentration inside the nanoparticle. The particle brightness and quantum yield are enhanced due to increasing the radiative decay rate. Higher brightness (almost one order of magnitude higher with respect to free dye) and better photostability (15-fold enhancement) of these polymer nanoparticles are found to be efficient for bioimaging purposes

Synthesis and Ultrafast Dynamics of a Donor–Acceptor–Donor Molecule Having Optoelectronic Properties

The use of push–pull molecules having donor (D) and acceptor (A) parts arranged in different shapes are being widely studied for application in various optoelectronic devices. In this study three covalently linked D–A–D molecules containing three different carbazole derivatives as donor, anthracene as acceptor, and thiophene as spacer have been synthesized and characterized. A detailed stepwise study has been carried out using anthracene, thiophene–anthracene, and carbazole–thiophene–anthracene derivatives so as to indicate the role of each moiety in the molecule. Steady state fluorescence, time-resolved fluorescence, transient absorption, and cyclic voltammetric methods have been employed to understand the intramolecular charge separation (CS) and charge recombination (CR) dynamics in solvents of different polarity. The thermodynamic free-energy obtained by measuring the redox potential and singlet state energy suggested the possibility of electron transfer from the excited singlet state of carbazole moiety to the anthracene entity. Steady state and time-resolved fluorescence studies showed fluorescence quenching of anthracene moiety upon addition of thiophene while highly efficient fluorescence quenching of anthracene moiety was observed on addition of carbazole derivatives. Femtosecond transient absorption studies confirmed the electron transfer to be the mechanism of fluorescence quenching, in which formation and recombination dynamics of electron-transfer products, anthracene radical anion and carbazole radical cation, were analyzed. The rate of charge separation, <i>k</i>_CS, was found to be very high for all the three molecules, and it was on the order of 10¹⁰–10¹¹ s^–1, while the rate of charge recombination, <i>k</i>_CR, was observed to be much slower, and it was on the order of 10⁸–10⁹ s^–1. The stepwise structure–property relationship leading to the efficient charge separated state established in the systems studied would help in the improved design of optoelectronic materials that use these moieties

Ultrafast Relaxation Dynamics of 5,10,15,20-<i>meso-</i>Tetrakis Pentafluorophenyl Porphyrin Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy

The ultrafast photophysical characterization of 5,10,15,20-<i>meso</i>-tetrakis pentafluorophenyl porphyrin (H₂F₂₀TPP) in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solution has been done in the femtosecond–picosecond time domain, by combining fluorescence up-conversion and femtosecond transient absorption spectroscopy. Fluorescence up-conversion studies on H₂F₂₀TPP were done demonstrating fluorescence dynamics over the whole spectral range from 440 to 650 nm when excited at 405 nm, 360.5 cm^–1 excess vibrational energy of Soret band (411 nm). Single-exponential decay with ∼160 ± 50 fs lifetime of Soret fluorescence (also called S₂ fluorescence or B band fluorescence) at around 440 nm was observed. On going from 440 nm, S₂ fluorescence to S₁ fluorescence, (Q-band) around 640 nm (wavelength of 0–0 transition in the stationary spectrum), single-exponential fluorescence time profile turns into a multiexponential time profile and it could be resolved critically into five-exponential components. An ultrafast rise component with ∼160 ± 50 fs followed by two decay components: a very fast decay component with 200 ± 50 fs time constant and another relatively slower 1.8 ± 0.5 ps decay component. Next, a very prominent rise component with 105 ± 30 ps lifetime followed by long-lived 10 ns decay component. The initial rise of S₁ (Q-band) fluorescence around 640 nm agreed with the decay time of S₂ (Soret or B band) fluorescence indicates that internal conversion (IC) from relaxed S₂ to vibrationally excited S₁ occurs in the ∼160 fs time scale and subsequent very fast decay with 200 fs time constant, which is assigned to be intramolecular vibrational dephasing or redistribution. The 1.8 ps decay component of S₁ fluorescence is attributed to be “hot” fluorescence from vibrationally excited S₁ state, and it reveals the vibrational relaxation time induced by elastic or quasi-elastic collision with solvent molecules. The 105 ps rise component is the creation time of the thermally equilibrated S₁ state population, and it could be attributed either to an excited state conformational relaxation/intramolecular charge transfer or a molecular cooling process by dissipation of excess energy within the solvent by inelastic collision. Finally, the decay of equilibrated S₁(Q_<i>x</i> state) occurs on 10 ns to S₀ by fluorescence. Femtosecond resolved transient absorption studies on H₂F₂₀TPP in the spectral range 390–620 nm following both S₂ (Soret band) and S₁ (Q_<i>x</i>) band excitation have been done and they complement the observations found in fluorescence up-conversion studies. The stimulated emission (SE) kinetics observed at 640 nm, S₁ emission peak, in 2–10 ps time domain rebuilds a dynamic similar to that observed by fluorescence up-conversion study. The transient absorption kinetics upon S₁ excitation were observed mainly to be biexponential with decay constants 105 ps and 10 ns, respectively. At a long time window (6 ns), a long-lived rise component could be predicted followed by two long-lived decay components for both the excitations in between 450 and 500 nm probe wavelengths. The lifetimes of these components were longer-lived than were possible to exactly measure using our existing femtosecond transient absorption system. However, this apparent rise component is assigned to be a T_n ← T₁ transition, and the longest decay component is attributed to the lifetime of the T₁ state

Ultrafast Fluorescence Photoswitch Incorporating Diketopyrrolopyrrole and Benzo[1,3]oxazine

With the objective of developing ultrafast fluorescent switch molecules, we have designed and synthesized fluorescence switch molecules incorporating two oxazine photochromes (OX) at the two end of single diketopyrrolopyrrole (DPP) fluorophore giving the shape of the dyad molecule as OX-DPP-OX. For precise characterization, steady-state photophysical properties, acid–base-induced spectroscopic studies and ultrafast transient absorption spectroscopic studies are performed. In acetonitrile (ACN) solution, the benzo­[1,3]­oxazine ring of studied oxazine derivatives in OX-DPP-OX opens up and reduces the fluorescence intensity of DPP by 66% upon addition of 50 equiv of trifluoroacetic acid (CF₃COOH, TFA) and addition of an equivalent amount of base, tetrabutylammonium hydroxide ((C₄H₉)₄NOH, TBAOH), closes the oxazine ring, reverting the fluorescence intensity of the DPP unit back to its original intensity. Likewise, upon 330 nm laser excitation, the oxazine ring opens up in less than 135 ps in ACN solution, reducing the DPP fluorescence by 90%. Both the processes, acidochromic effect and 330 nm laser excitation, generate a 3<i>H</i>-indolium cation, <i>p</i>-nitrophenolate (protonated) and <i>p</i>-nitrophenolate anion, respectively. The photogenerated isomer lives for 1.5–1.9 ns in room temperature and reverts to its original conformer with first-order kinetics. This photochromic dyad tolerates thousands of switching cycles with no sign of degradation. However, the Gibbs free energy of the cationic fragment of their photogenerated isomer and DPP fluorophore is exergonic (Δ<i>G</i>⁰ < −0.8) and ultrafast intramolecular electron transfer occurs very fast (in 16 ps time) from DPP moiety to 3<i>H</i>-indolium cation. As a result, the photoinduced transformation of the photochromic component within this dyad results in the effective quenching of the DPP emission. The fluorescence of this photoswitchable compound is modulated on a nanosecond time scale with excellent fatigue resistance under femtosecond (fwhm ∼100 fs) photoexcitation. Thus, the choice of OX as a photochromic component and DPP as fluorescence component can ultimately lead to the development of valuable ultrafast photoswitchable fluorescent probes for designing ultrafast switching devices. Such valuable mechanistic insights into their excitation dynamics can guide the design of novel members of this family of photochromic compounds with improved photochemical properties

Photochemical <i>E</i>(<i>trans</i>)–<i>Z</i>(<i>cis</i>)–<i>E</i> Isomerization of an Amphiphilic Cholest-5-en-3β-yl(<i>E</i>)-9-anthraceneprop-2-enoate on Solid Substrate

Surface morphology and photochemical isomerization properties of monolayers of anthrancene acrylic acid derivative with cholesterol (a new class of bistable compound), cholest-5-en-3β-yl(<i>E</i>)-9-anthraceneprop-2-enoate (CAE), transferred onto quartz substrates were studied. The spectroscopic and photochromic behavior of CAE on solid substrates and in solution are compared keeping in mind the possible application of CAE in constructing molecular electronic devices. Monolayers of the <i>trans</i>(<i>E</i>)-isomer of CAE transferred from the air–water interface onto quartz plates show regular distribution of “holes” in the film, whereas similar monolayers of the <i>cis</i>(<i>Z</i>)-isomer of CAE (∼96%) show very smooth surfaces, free from any definite structures. The surface pressure–area (<i>π</i>–<i>A</i>) isotherms of both monolayers at the air–water interface are found to be irreversible, indicating formation of 2D/3D aggregates for both isomers. The surface potential–area (Δ<i>V</i>–<i>A</i>) isotherms of the two isomers predict the orientation of their molecular dipoles to be different. The fluorescence peak intensity of the <i>E</i>-isomer of CAE in transferred monolayers shows a sharp decrease upon irradiation with 405 nm light, indicating the successful <i>E</i>-to-<i>Z</i> isomerization in the monolayer. Fluorescence excitation and emission polarization studies on the solid substrate also confirm the change of molecular orientation resulting from the <i>E</i>-to-<i>Z</i> isomerization. The isomerization rate is found to be faster in solid substrates than that in the solution phase. Six alternate monolayers of <i>E</i>-CAE and triplet sensitizer (liphophilic porphyrin) film shows 5% efficiency of <i>Z</i>-to-<i>E</i> isomerization upon exciting on 550 nm, where porphyrin has substantial absorbance where as film of 24 monolayers of mixture solution of the <i>E</i>-isomer of CAE (1 mM) and liphophilic porphyrin (1 mM) in chloroform increases 5-fold efficiency of <i>Z</i>-to-<i>E</i> conversion. These results suggest that the <i>E</i>-CAE has the potential to be used in making optical data storage devices employing the <i>trans</i>–<i>cis</i>–<i>trans</i> isomerization process