Effect of the Polymer Chain Arrangement on Exciton and Polaron Dynamics in P3HT and P3HT:PCBM Films

Arrangement of polymer chains at the interface with an acceptor is one of the key issues which influences the interfacial charge transfer. Films of P3HT and P3HT:PCBM blends with the different polymer chain arrangement have been prepared from one-component and binary solvents representing a mixture of chloroform and different poor solvents with high boiling temperature. Electronic absorption and photoluminescence spectra evidenced in favor of reduced disorder of P3HT films as a result of use of poor solvents; the ordering was displayed through structuring and narrowing the spectral bands, indicative of decreasing width of Gaussian distribution of molecular transition frequencies. At the same time, transient absorption of singlet excitons showed that exciton decay in highly ordered P3HT films slows down as compared to the disordered film, and this effect was reproduced for the different poor solvents used. A more pronounced effect was revealed in P3HT:PCBM blends where much faster decay of excitons was found in disordered as-prepared P3HT:PCBM film from chloroform as compared to the annealed film or films prepared from the binary solvents. A complex behavior of polarons in the different regions of the blend film, i.e., within crystalline domains of P3HT and at the interface of P3HT/PCBM, was observed. The conclusion was drawn that chain disorder induces easier exciton dissociation and charge transfer at the P3HT/PCBM interface

Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine

Thin films of vapor-deposited metal-free phthalocyanine (H₂Pc) were studied using ultrafast transient absorption spectroscopy in the visible region. Following photoexcitation, an excited state absorption feature located near 532 nm was observed which served as a probe of the excited state. For exciton densities larger than 5 × 10¹⁸ excitons/cm³ the time-dependent measurements of the excited state absorption included the presence of nonexponential decay kinetics attributed to exciton–exciton annihilation. At exciton densities less than 5 × 10¹⁸ excitons/cm³ annihilation was negligible, and the decay kinetics appeared single exponential within the signal-to-noise. The fitted time constant, 239 ± 24 ps, was attributed to the lifetime decay of the singlet excitons. When the H₂Pc was diluted into a wide energy gap host via vapor deposition, the observed lifetime was significantly reduced, reaching 87 ± 9 ps for a concentration of 25% H₂Pc. The decreased exciton lifetime with dilution was remarkable since it has been commonly reported that excited state lifetimes decrease as the chromophore concentration is increased. The reduced lifetime was correlated to the loss of α-phase ordering as indicated in the UV/vis spectra of the films. Within the context of photovoltaic applications this highlights the importance of both molecular level ordering and chromophore concentration when trying to engineer fundamental material properties such as exciton diffusion length

Intramolecular Exciton Diffusion in Poly(3-hexylthiophene)

Emission quenching by fullerenes covalently attached to both ends of a series of size-selected regioregular poly­(3-hexylthiophene) samples was quantified and used to determine the intrachain exciton diffusion length. The diffusion length was found to be <i>L</i>_D = 7.0 ± 0.8 nm. When the distance dependence of the quenching mechanism is considered, this is the same value that has been reported for emissive excitons in thin films. This result indicates that intrachain exciton transport is more facile for excitons localized to single chains than that for excitons that are delocalized between chains. In the context of solar cells, the result indicates additional complexity and the potential for competing issues when considering morphological design of the film to enhance both exciton and charge transport

Evaluation of the Intramolecular Charge-Transfer Properties in Solvatochromic and Electrochromic Zinc Octa(carbazolyl)phthalocyanines

2,3,9,10,16,17,23·24-Octakis-(9<i>H</i>-carbazol-9-yl) phthalocyaninato zinc­(II) (<b>3</b>) and 2,3,9,10,16,17,23·24-octakis-(3,6-di-<i>tert</i>-butyl-9<i>H</i>-carbazole) phthalocyaninato zinc­(II) (<b>4</b>) complexes were prepared and characterized by NMR and UV–vis spectroscopies, magnetic circular dichroism (MCD), matrix-assisted laser desorption ionization mass spectrometry, and X-ray crystallography. UV–vis and MCD data are indicative of the interligand charge-transfer nature of the broad band observed in 450–500 nm range for <b>3</b> and <b>4</b>. The redox properties of <b>3</b> and <b>4</b> were probed by electrochemical and spectro-electrochemical methods, which are suggestive of phthalocyanine-centered first oxidation and reduction processes. Photophysics of <b>3</b> and <b>4</b> were investigated by steady-state fluorescence and time-resolved transient absorption spectroscopy demonstrating the influence of the carbazole substituents on deactivation from the first excited state in <b>3</b> and <b>4</b>. Protonation of the <i>meso</i>-nitrogen atoms in <b>3</b> results in much faster deactivation kinetics from the first excited state. Spectroscopic data were correlated with density functional theory (DFT) and time-dependent DFT calculations on <b>3</b> and <b>4</b>

Excited-State Quenching Mechanism of a Terthiophene Acid Dye Bound to Monodisperse CdS Nanocrystals: Electron Transfer versus Concentration Quenching

Oleate-capped CdS nanocrystals (NCs) dispersed in dichloromethane were found to quench the excited-state fluorescence of the terthiophene derivative 3′,4′-dibutyl-5″-phenyl-[2,2′:5′,2″-terthiophene]-5-carboxylic acid (<b>1-CO</b>_<b>2</b><b>H</b>). Infrared and ¹H NMR spectroscopies provided evidence that <b>1-CO</b>_<b>2</b><b>H</b> substitutes for oleate on the surface of the CdS NCs. Upon binding, the fluorescence of <b>1-CO</b>_<b>2</b><b>H</b> is quenched, and the ¹H NMR lines from <b>1-CO</b>_<b>2</b><b>H</b> are broadened. The importance of the carboxylate group in binding to the CdS NC was further established by examining the behavior of a similar fluorophore where the carboxylic acid group was replaced with a bromo substituent (<b>1-Br</b>). The CdS NCs had no influence on the fluorescence intensity or NMR line shapes of <b>1-Br</b>. For <b>1-CO</b>_<b>2</b><b>H</b>, Stern–Volmer plots indicated a nearly linear increase in <i>I</i>₀/<i>I</i> as the CdS NCs’ concentration was increased, but as the dye/NC ratio reached ∼20/1, <i>I</i>₀/<i>I</i> reached a maximum of ∼8 and began to decrease. By a dye/NC ratio of 2/1, the <i>I</i>₀/<i>I</i> reached a steady value of ∼2.5. The peak in the Stern–Volmer plot at a 20/1 ratio was consistent with a maximum in the contribution from concentration quenching at this coverage. On the basis of the appearance of the dye’s radical cation spectrum at low dye/NC ratios, ultrafast transient absorption spectroscopy confirmed electron transfer from the singlet excited state of the dye to the CdS NC with a lifetime of 16 ps. At higher dye/NC ratios, the signal from the radical cation was much less dominant, and the decay of the singlet excited state was dominated by the concentration quenching process having a 1 ps lifetime

Outsourcing Intersystem Crossing without Heavy Atoms: Energy Transfer Dynamics in PyridoneBODIPY–C₆₀ Complexes

The excited state dynamics in two fully characterized pyridoneBODIPY–fullerene complexes were investigated using time-resolved spectroscopy. Photoexcitation was initially localized on the pyridoneBODIPY chromophore. The energy was rapidly transferred to the fullerene, which subsequently underwent ISC to form a triplet state and returned the energy to the pyridoneBODIPY via triplet–triplet energy transfer. This ping-pong energy transfer mechanism resulted in efficient (>85%) overall conversion of the excited state pyridoneBODIPY constituent despite a complete lack of ISC in the pyridoneBODIPY in the absence of the fullerene partner. The small difference in attachment chemistry for the fullerene did not impact the initial singlet energy transfer. However, the N-methylpyrrolidine bridge did slow both the triplet–triplet energy transfer and the ultimate relaxation rate of the final triplet state when compared to an isoxazole-based bridge. The rates of each step were quantified, and computational predictions were used to complement the proposed mechanism and energetics. The result demonstrated efficient triplet sensitization of a strong chromophore that lacks significant spin–orbit coupling

Redox and Photoinduced Electron-Transfer Properties in Short Distance Organoboryl Ferrocene-Subphthalocyanine Dyads

Reaction between ferrocene lithium or ethynylferrocene magnesium bromide and (chloro)­boronsubphthalocyanine leads to formation of ferrocene- (<b>2</b>) and ethynylferrocene- (<b>3</b>) containing subphthalocyanine dyads with a direct organometallic B–C bond. New donor–acceptor dyads were characterized using UV–vis and magnetic circular dichroism (MCD) spectroscopies, NMR method, and X-ray crystallography. Redox potentials of the rigid donor–acceptor dyads <b>2</b> and <b>3</b> were studied using the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) approaches and compared to the parent subphthalocyanine <b>1</b> and conformationally flexible subphthalocyanine ferrocenenylmethoxide (<b>4</b>) and ferrocenyl carboxylate (<b>5</b>) dyads reported earlier. It was found that the first oxidation process in dyads <b>2</b> and <b>3</b> is ferrocene-centered, while the first reduction as well as the second oxidation are centered at the subphthalocyanine ligand. Density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to probe the electronic structures and explain the UV–vis and MCD spectra of complexes <b>1</b>–<b>5</b>. DFT-PCM calculations suggest that the LUMO, LUMO+1, and HOMO-3 in new dyads <b>2</b> and <b>3</b> are centered at the subphthalocyanine ligand, while the HOMO to HOMO-2 in both dyads are predominantly ferrocene-centered. TDDFT-PCM calculations on compounds <b>1</b>–<b>5</b> are indicative of the π → π* transitions dominance in their UV–vis spectra, which is consistent with the experimental data. The excited state dynamics of the parent subphthalocyanine <b>1</b> and dyads <b>2</b>–<b>5</b> were investigated using time-resolved transient spectroscopy. In the dyads <b>2</b>–<b>5</b>, the initially excited state is rapidly (<2 ps) quenched by electron transfer from the ferrocene ligand. The lifetime of the charge transfer state demonstrates a systematic dependence on the structure of the bridge between the subphthalocyanine and ferrocene

Synthesis and Charge-Transfer Dynamics in a Ferrocene-Containing Organoboryl aza-BODIPY Donor–Acceptor Triad with Boron as the Hub

A <i>N</i>,<i>N</i>′-bis­(ferroceneacetylene)­boryl complex of 3,3′-diphenylazadiisoindolylmethene was synthesized by the reaction of an <i>N</i>,<i>N</i>′-difluoroboryl complex of 3,3′-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 μs

Tuning Electronic Structure, Redox, and Photophysical Properties in Asymmetric NIR-Absorbing Organometallic BODIPYs

Stepwise modification of the methyl groups at the α positions of BODIPY <b>1</b> was used for preparation of a series of mono- (<b>2</b>, <b>4</b>, and <b>6</b>) and diferrocene (<b>3</b>) substituted donor–acceptor dyads in which the organometallic substituents are fully conjugated with the BODIPY π system. All donor–acceptor complexes have strong absorption in the NIR region and quenched steady-state fluorescence, which can be partially restored upon oxidation of organometallic group(s). X-ray crystallography of complexes <b>2</b>–<b>4</b> and <b>6</b> confirms the nearly coplanar arrangement of the ferrocene groups and the BODIPY π system. Redox properties of the target systems were studied using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the first oxidation process in all dyads is ferrocene centered, while the separation between the first and the second ferrocene-centered oxidation potentials in diferrocenyl-containing dyad <b>3</b> is ∼150 mV. The density functional theory-polarized continuum model (DFT-PCM) and time-dependent (TD) DFT-PCM methods were used to investigate the electronic structure as well as explain the UV–vis and redox properties of organometallic compounds <b>2</b>–<b>4</b> and <b>6</b>. TDDFT calculations allow for assignment of the charge-transfer and π → π* transitions in the target compounds. The excited state dynamics of the parent BODIPY <b>1</b> and dyads <b>2</b>–<b>4</b> and <b>6</b> were investigated using time-resolved transient spectroscopy. In all organometallic dyads <b>2</b>–<b>4</b> and <b>6</b> the initially excited state is rapidly quenched by electron transfer from the ferrocene ligand. The lifetime of the charge-separated state was found to be between 136 and 260 ps and demonstrates a systematic dependence on the electronic structure and geometry of BODIPYs <b>2</b>–<b>4</b> and <b>6</b>

Nitrodibenzofuran: A One- and Two-Photon Sensitive Protecting Group That Is Superior to Brominated Hydroxycoumarin for Thiol Caging in Peptides

Photoremovable protecting groups are important for a wide range of applications in peptide chemistry. Using Fmoc-Cys­(Bhc-MOM)-OH, peptides containing a Bhc-protected cysteine residue can be easily prepared. However, such protected thiols can undergo isomerization to a dead-end product (a 4-methylcoumarin-3-yl thioether) upon photolysis. To circumvent that photoisomerization problem, we explored the use of nitrodibenzofuran (NDBF) for thiol protection by preparing cysteine-containing peptides where the thiol is masked with an NDBF group. This was accomplished by synthesizing Fmoc-Cys­(NDBF)-OH and incorporating that residue into peptides by standard solid-phase peptide synthesis procedures. Irradiation with 365 nm light or two-photon excitation with 800 nm light resulted in efficient deprotection. To probe biological utility, thiol group uncaging was carried out using a peptide derived from the protein K-Ras4B to yield a sequence that is a known substrate for protein farnesyltransferase; irradiation of the NDBF-caged peptide in the presence of the enzyme resulted in the formation of the farnesylated product. Additionally, incubation of human ovarian carcinoma (SKOV3) cells with an NDBF-caged version of a farnesylated peptide followed by UV irradiation resulted in migration of the peptide from the cytosol/Golgi to the plasma membrane due to enzymatic palmitoylation. Overall, the high cleavage efficiency devoid of side reactions and significant two-photon cross-section of NDBF render it superior to Bhc for thiol group caging. This protecting group should be useful for a plethora of applications ranging from the development of light-activatable cysteine-containing peptides to the development of light-sensitive biomaterials