J. Pina

Excited-State Dynamics and Self-Organization of Poly(3-hexylthiophene) (P3HT) in Solution and Thin Films

The fluorescence decays of a stereoregular head-to-tail RR-HT poly(3-hexylthiophene), P3HT, in methylcyclohexane (MCH) are described by sums of three or four exponential terms, respectively above and below -10 °C. In the high-temperature region, the polymer lifetime (ca. 500 ps) is accompanied by two shorter decay times (ca. 20 and 120 ps), which are assigned to intrachain energy transfer from high to lower energy excitons on the basis of temperature and wavelength dependence of the fluorescence decays. The absence of conformational (torsional) relaxation is attributed to the small dihedral angle between monomers that is predicted for the stereoregular polymer in the ground state. Below -10 °C, the polymer forms excimer-like aggregates, showing vibrational structured absorption and emission bands similar to those observed in thin films. The vibrational structure is attributed to a deep minimum in the ground-state energy surface of the dimer or aggregate. Below -40 °C, the fluorescence measured at the aggregate emission wavelength (670 nm) basically results from direct excitation of the aggregate and decays with a sum of three exponential terms (decay times of ca. 0.14, 0.6, and 1.5 ns, with similar weights). Because the spectral similarities between film and aggregates indicate similar electronic first singlet excited states (and oscillator strengths), the much shorter decay times (0.05, 0.15, and 0.43 ns) and lower fluorescence quantum yield of P3HT in films are assigned to efficient exciton dissociation and/or phonon-induced internal conversion competing with radiative decay (>1 ns).

Multifaceted Regioregular Oligo(thieno[3,4-b]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap.

Thiophene-based materials have occupied a crucial position in the development of organic electronics. However, the energy band gaps of oligo- and polythiophenes are difficult to modulate without resorting to push-pull electronic effects. We describe herein a new series of monodisperse oligo(thieno[3,4-b]thiophene) derivatives with well-defined regioregular structures synthesized efficiently by direct C-H arylation. These compounds show a unique palette of colors and amphoteric redox properties with widely tunable energy band gaps. The capacity to stabilize both cations and anions results in both anodic and cathodic electrochromism. Under excitation, these compounds can produce photoionized states able to interconvert into neutral triplet or form these through singlet exciton fission or intersystem crossing. These features arise from a progressive increase in quinoidization on a fully planar platform making the largest effective conjugation length among hetero-oligomers. Oligo(thieno[3,4-b]thiophene)s might represent the more distinctive family of oligothiophenes of this decade.

Triphenylamine–Benzimidazole Derivatives: Synthesis, Excited-State Characterization, and DFT Studies

The synthesis and comprehensive characterization of the excited states of four novel triphenylamine-benzimidazole derivatives has been undertaken in solution (ethanol and methylcyclohexane) at room temperature. This includes the determination of the absorption, fluorescence, and triplet-triplet absorption spectra, together with quantum yields of fluorescence, internal conversion, intersystem crossing, and singlet oxygen. From the overall data the radiative and radiationless rate constants could be obtained, and it is shown that the compounds are highly emissive with the radiative decay dominating, with more than 70% of the quanta loss through this deactivation channel. The basic structure of the triphenylamine-benzimidazole derivatives (1a) was modified at position 5 of the heterocyclic moiety with electron-donating (OH (1b), OCH3 (1c)) or electron-withdrawing groups (CN, (1d)). It was found that the photophysical properties remain basically unchanged with the different substitutions, although a marked Stokes shift was observed with 1d. The presence and nature of a charge-transfer transition is discussed with the help of theoretical (DFT and TDFT) data. All compounds displayed exceptionally high thermal stability (between 399 and 454 °C) as seen by thermogravimetric analysis.

Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.

The influence of the relative position of the thiophene and pyrrole rings in donor-acceptor thienylpyrrolyl benzothiazole derivatives. A photophysical and theoretical investigation

A detailed spectroscopic and photophysical study has been carried out on a series of heterocyclic compounds-known to display nonlinear optical properties-consisting on a electron donating thienylpyrrolyl pi-conjugated system functionalized with an electron acceptor benzothiazole moiety. The absorption, emission and triplet-triplet absorption together with all relevant quantum yields (fluorescence, intersystem crossing and internal conversion), excited state lifetimes and the overall set of deactivation rate constants (k(F), k(IC) and k(ISC)) were obtained in solution at room (293 K) and low (77 K) temperature. The optimized ground-state molecular geometries for the compounds together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours for the compounds were also determined using density functional theory (DFT) at the B3LYP/3-21G* level. The experimental results showed that the photophysical properties are influenced by the relative position of the pyrrole and thiophene relative to the benzothiazole group.

Unusual photophysical properties of conjugated, alternating indigo-fluorene copolymers†

Alternating indigo–fluorene copolymers have been synthesized by the coupling of didromoindigo and fluorendiboronic ester monomers. The low solubility of the copolymers only allowed for the synthesis of moderate molecular weight copolymers, with a degree of polymerization (DP) up to 11. The syntheses were accomplished through a 10% excess of the fluorene-based monomer component in an AA/BB-type polycondensation mixture. Next, a comprehensive spectroscopic (singlet–singlet and transient – from fs to μs – absorption, fluorescence and phosphorescence spectra) and photophysical investigation (fluorescence, phosphorescence and triplet lifetimes together with fluorescence and triplet occupation and singlet oxygen sensitization quantum yields) of the copolymers was carried out. The experiments were complemented with the spectroscopic results from a fluorene–indigo–fluorene model compound, as well as by TDDFT calculations. Based on our kinetics analysis, singlet energy transfer from the fluorene to indigo moieties is found to be inefficient. Besides the low energy indigo-related absorption band, an additional intermediate energy absorption band is also observed between 400 nm and 500 nm, both for the copolymer and for the model compound. Excitation into this band causes an emission of the indigo moiety. The triplet state is found to be mainly localized at the fluorene moiety; however, the decrease of the phosphorescence quantum yield (ϕPh) when going from the monomeric 9,9-bis(dodecyl)fluorene (0.075) to the model trimer (0.003) and copolymer (ϕPh = 0.008) suggests that excitation energy transfer occurs in the triplet state. This is further confirmed by the higher level of delocalization of the transient triplet–triplet absorption spectra of the copolymer relative to the monomeric 9,9-bis(dodecyl)fluorene.

Synthesis and characterization of the ground and excited states of tripodal-like oligothienyl-imidazoles.

Six new thiophene oligomers, here designated as tripodal-like oligothienyl-imidazoles, were synthesized and have been investigated in ethanol solution at room and low temperature. The oligomers bear a common core where two or more thiophenes are linked to one or more imidazole units that further links through its alpha-position to a different number of incremental thiophene units. The study involves a comprehensive spectral and photophysical investigation where the properties of the singlet and triplet states have been investigated regarding absorption, fluorescence and phosphorescence, transient triplet-triplet absorption together with all relevant quantum yields (fluorescence, phi(F), internal conversion, phi(IC), intersystem crossing, phi(T,) and singlet oxygen, phi(Delta)) and lifetimes. In addition, density functional theory quantum chemical calculations were performed to gain a detailed understanding of the molecular geometry and optical properties of the investigated oligomers. From the overall data, the radiative (k(F)) and radiationless (k(NR), k(IC), and k(ISC)) rate constants have been determined and it is shown that, in contrast with the parent oligothiophenes, the radiative competes with the radiationless deactivation channels. The results show that, by comparison with the oligothiophene counterparts, there is an augment of the relative contributions of the internal conversion and fluorescence processes relative to the S(1)~~-->T(1) intersystem crossing. Phosphosphorescence emission was found for the simplest member of the investigated compounds with a low quantum yield (phi(Ph) = 0.009) and a lifetime of 8 micros. The data also show that the introduction of a 4,5-dithienyl-imidazole moiety in a bi- or terthiophene oligomer results in, respectively, a 20-fold and a 3-fold increase of the fluorescence quantum yield relative to their oligothiophene counterparts. The synergy of the structural and photophysical properties, combined with the exceptional thermal stabilities, opens new perspectives related to the copolymerization of oligothiophenes with thienyl-imidazole units with potential application as organic light emitting devices.

Photophysical and spectroscopic investigations on (oligo)thiophene-arylene step-ladder copolymers. The interplay of conformational relaxation and on-chain energy transfer.

An optical spectroscopy and photophysics study on four (oligo)thiophene-phenylene and (oligo)thiophene-naphthylene step-ladder type copolymers in solution (room and low temperature) and in the solid state (thin film) is presented. The study involves absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. The overall data allow for a determination of the rate constants for all decay processes and from these several conclusions could be drawn: (1) in solution the main deactivation channels are radiationless processes (S(1) approximately -->S(0) internal conversion and S(1) approximately -->T(1) intersystem crossing); (2) from time-resolved fluorescence decays in the picosecond time domain three decay components are seen: a fast decay (10-20 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (120-190 ps) most probably associated to isolated conjugated segments, and a third exponential related to the emission of the fully relaxed polymer. The assignment of the fast decay component to an on-chain energy transfer/migration is based of the dependence of the decay time on the solvent viscosity in combination with the investigation of an oligomeric model compound. Here, the absence of any significant changes of the decay parameters (decay times and pre-exponential factors) upon going from a less (toluene) to a more viscous (decalin) solvent together with the monoexponential fluorescence decay of the oligomeric model compound allow us to differentiate between deactivation of the singlet excited state by conformational relaxation and on-chain energy/transfer migration.

Effects of the Interaction Between β-Carboline-3-carboxylic acid N-Methylamide and Polynucleotides on Singlet Oxygen Quantum Yield and DNA Oxidative Damage

The complexation of β‐carboline‐3‐carboxylic acid N‐methylamide (βCMAM) with the sodium salts of the nucleotides polyadenylic (Poly A), polycytidylic (Poly C), polyguanylic (Poly G), polythymidylic (Poly T) and polyuridylic (Poly U) acids, and with double stranded (dsDNA) and single stranded deoxyribonucleic acids (ssDNA) was studied at pH 4, 6 and 9. Predominant 1:1 complex formation is indicated from Job plots. Association constants were determined using the Benesi–Hildebrand equation. βCMAM‐sensitized singlet oxygen quantum yields were determined at pH 4, 6 and 9, and the effects on this of adding oligonucleotides, dsDNA and ssDNA were studied at the three pH values. With dsDNA, the effect on βCMAM triplet state formation was also determined through triplet–triplet transient absorption spectra. To evaluate possible oxidative damage of DNA following singlet oxygen βCMAM photosensitization, we used thiobarbituric acid‐reactivity assays and electrophoretic separation of DNA assays. The results showed no oxidative damage at the level of DNA degradation or strand break.

Excited-State Proton Transfer in Indigo

Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle point, which is enhanced by tunneling through the barrier. From the temperature dependence of the steady-state and time-resolved fluorescence data, the activation energy for the process was found to be ∼11 kJ mol-1 for Ind and ∼5 kJ mol-1 for NHxInd, in close agreement with the values calculated by TDDFT: 12.3 kJ mol-1 (Ind) and 3.1 kJ mol-1 (NHxInd). From time-resolved data, the rate constants for the ESPT process in dimethyl sulfoxide were found to be 9.24 × 1010 s-1 (Ind) and 7.12 × 1010 s-1 (NHxInd). The proximity between the two values suggests that the proton transfer mechanism in indigo is very similar to that found in NHxInd, where a single proton is involved. In addition, with NHxInd, the TDDFT calculations, together with the viscosity dependence of the fast component, and differences in the activation energy values between the steady-state and time-resolved data indicate that an additional nonradiative process is involved, which competes with ESPT. This is attributed to rotation about the central carbon-carbon bond, which brings the system close to a conical intersection (CI). The CI is of the sloped type, where the seam is reached through an OH stretching vibration.