Deducing the conformational space for an octa-proline helix

A molecular dyad, PY-P8-PER, comprising a proline octamer sandwiched between pyrene and perylene terminals has been synthesized in order to address the dynamics of electronic energy transfer (EET) along the oligo-proline chain. A simple pyrene-based control compound equipped with a bis-proline attachment serves as a reference for spectroscopic studies. The N-H NMR signal at the terminal pyrene allows distinction between cis and trans amides and, although the crystal structure for the control has the trans conformation, temperature-dependent NMR studies provide clear evidence for trans/cis isomerisation in D6-DMSO. Polar solvents tend to stabilise the trans structure for the pyrene amide group, even for longer oligo-proline units. Circular dichroism shows that the proline spacer for PY-P8-PER exists mainly in the all-trans geometry in methanol. Preferential excitation of the pyrene chromophore is possible at wavelengths in the 320-350 nm range and, for the dyad, is followed by efficacious EET to the perylene emitter. The probability for intramolecular EET, obtained from analysis of steady-state spectroscopic data, is ca. 80-90% in solvents of disparate polarity. Comparison with the Förster critical distance suggests the terminals are ca. 18 Å apart. Time-resolved fluorescence spectroscopy, in conjunction with DFT calculations, indicates the dyad exists as a handful of conformers displaying a narrow range of EET rates. Optimisation of a distributive model allows accurate simulation of the EET dynamics in terms of reasonable structures based on isomerisation of certain amide groups.

Dynamic exciton localisation in a pyrene-BODIPY-pyrene dye conjugate

The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.

Energy transfer and charge separation dynamics in photoexcited pyrene-bodipy molecular dyads

The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2,6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2-dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the pyrene (Py) subunit leads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5-5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with ability to undergo CS.

Rational molecular design enhancing the photonic performance of red-emitting perylene bisimide dyes

We report the synthesis of novel multichromophoric organic architectures, where perylene red is decorated with BODIPY and/or hydroxycoumarin dyes acting as light harvesters and energy donors. The computationally-aided photophysical study of these molecular assemblies reveals a broadband absorption which, regardless of the excitation wavelength, leads solely to a bright red-edge emission from perylene bisimide after efficient intramolecular energy transfer hops. The increase of the absorbance of these molecular antennas at key pumping wavelengths enhances the laser action of the commercial perylene red. The herein applied strategy based on energy transfer dye lasers should boost the use of perylene-based dyes as active media for red-emitting lasers.

Electronic Communication in Closely Connected BODIPY-Based Bichromophores

A small series of closely spaced, bichromophoric boron dipyrromethene (BODIPY) derivatives has been examined by optical spectroscopy and compared to the corresponding mononuclear dyes. The compounds vary according to the site of attachment and also by the nature of alkyl or aryl substituents incorporated into the dipyrrin backbone. Excitonic coupling splits the lowest-energy absorption transition in each case, but to highly variable degrees. There are also marked changes in the fluorescence quantum yields across the series but much less variation in the excited-state lifetimes. After comparing different models, it is concluded that the ideal dipole approximation gives a crude qualitative representation of the observed splitting of the absorption transition, but the extended dipole approach is not applicable to these systems. Agreement is substantially improved by employing a model that takes into account the dihedral angle between the planes of the two dipyrrin units. The large variation in radiative rate constants, and those for the accompanying nonradiative processes, is accountable in terms of electronic coupling and/or intensity borrowing between the two excitonic states. In all cases, the dihedral angle between the two BODIPY units plays a key role.

Synthesis and spectroscopic properties of β,β'-dibenzo-3,5,8-triaryl-BODIPYs

A series of β,β'-bicyclo-3,5-diaryl-BODIPYs were synthesized from the corresponding β,β'-bicyclo-3,5-diiodo-BODIPYs (1a,b) via Pd(0)-mediated Suzuki cross-coupling reactions in 82-92% yields. Subsequent aromatization with DDQ afforded the corresponding β,β'-dibenzo-aryl-BODIPYs, which showed red-shifted absorptions and emissions in the near-IR range. The dibenzo-appended BODIPYs showed characteristic 1H-, 13C-, 11B- and 19F-NMR shifts, and nearly planar conformations by X-ray crystallography.

BODIPY Dye, the Most Versatile Fluorophore Ever?

BODIPY laser dyes constitute a fascinating topic of research in modern photochemistry due to the large variety of options its chromophore offers, which is ready available for a multitude of synthetic routes. Indeed, in the literature one can find a huge battery of compounds based on the indacene core. The possibility of modulating the spectroscopic properties or inducing new photophysical processes by the substitution pattern of the BODIPY dyes has boosted the number of scientific and technological applications for these fluorophores. Along the following lines, I will overview the main results achieved in our laboratory with BODIPYs oriented to optoelectronic as well to biophotonic applications, stressing the more relevant photophysical issues to be considered in the design of a tailor-made BODIPY for a certain application and pointing out some of the remaining challenges.

Coumarin-BODIPY hybrids by heteroatom linkage: versatile, tunable and photostable dye lasers for UV irradiation

Linking amino and hydroxycoumarins to BODIPYs through the amino or hydroxyl group lets the easy construction of unprecedented photostable coumarin-BODIPY hybrids with broadened and enhanced absorption in the UV spectral region, and outstanding wavelength-tunable laser action within the green-to-red spectral region (∼520-680 nm). These laser dyes allow the generation of a valuable tunable UV (∼260-350 nm) laser source by frequency doubling, which is essential to study accurately the photochemistry of biological molecules under solar irradiation. The tunability is achieved by selecting the substitution pattern of the hybrid. Key factors are the linking heteroatom (nitrogen vs. oxygen), the number of coumarin units joined to the BODIPY framework and the involved linking positions.

Artificial light-harvesting arrays for solar energy conversion

Following natures' blueprint, the concept of artificial light-harvesting antennae is discussed in terms of sophisticated molecular arrays displaying a tailored cascade of electronic energy transfer steps.

Hole-transfer induced energy transfer in perylene diimide dyads with a donor–spacer–acceptor motif

Pump–probe spectroscopy, time resolved fluorescence, chemical variation and quantum chemical calculations reveal an efficient energy transfer mechanism enabled by a bright charge transfer state located on the spacer.

Design, synthesis and photophysical studies of dipyrromethene-based materials: insights into their applications in organic photovoltaic devices

This review article presents the most recent developments in the use of materials based on dipyrromethene (DPM) and azadipyrromethenes (ADPM) for organic photovoltaic (OPV) applications. These chromophores and their corresponding BF2-chelated derivatives BODIPY and aza-BODIPY, respectively, are well known for fluorescence-based applications but are relatively new in the field of photovoltaic research. This review examines the variety of relevant designs, synthetic methodologies and photophysical studies related to materials that incorporate these porphyrinoid-related dyes in their architecture. The main idea is to inspire readers to explore new avenues in the design of next generation small-molecule and bulk-heterojunction solar cell (BHJSC) OPV materials based on DPM chromophores. The main concepts are briefly explained, along with the main challenges that are to be resolved in order to take full advantage of solar energy.

Fluorophore incorporation allows nanomolar guest sensing and white-light emission in M4L6 cage complexes

M₄L₆ cages built from BODIPY- and pyrene-containing subcomponents perform multiple functions: sensing anions and amino acids, and forming a white-light emitting complex with perylene.

Photophysical and laser properties of cassettes based on a BODIPY and rhodamine pair

This work deals with the synthesis and the photophysical and laser properties of new BODIPY-rhodamine cassettes. These dyads differ in their rigid and conjugated spacer group (phenyl or acetylenephenyl) and in their linking positions (meta or para). The photophysical properties of these cassettes are controlled by the formation/opening of the spirolactone ring, which, in turn, switches off/on an energy-transfer process between the chromophores. Herein, we thoroughly describe the influence of the attached spacer group, as well as the distance and orientation between the donor-acceptor pair, on the excitation energy transfer. The observed fast dynamics and efficiency suggest that the process mainly takes place "through-bond", although the "through-space" mechanism also contributes to the whole process. As a result, efficient laser emission from the rhodamine is achieved upon excitation of the BODIPY, in particular for the cassette that contains an acetylenephenyl spacer group in a para disposition.

Providing power for miniaturized medical implants: triplet sensitization of semiconductor surfaces

Here, we recognize the growing significance of miniaturized devices as medical diagnostic tools and highlight the need to provide a convenient means of powering such instruments when implanted into the body. One of the most promising approaches to this end involves using a light-collection facility to absorb incident white light and transfer the photonic energy to a tiny semiconductor embedded on the device. Although fluorescent organic molecules offer strong potential as modules for such solar collectors, we emphasize the promise offered by transition metal complexes. Thus, an extended series of binuclear Ru(II)/Os(II) poly(pyridine) complexes has been shown to be highly promising sensitizers for amorphous silicon solar cells. These materials absorb a high fraction of visible light while the Ru(II)-based units possess triplet energies that are comparable to those of the naphthalene-based bridge. The metal complex injects a triplet exciton into the bridge and this, in turn, is trapped by the Os(II)-based terminal. The result is extremely efficacious triplet-energy transfer; at room temperature the rate of energy transfer is independent of distance over some 6 nm and only weakly dependent on temperature.

Influence of applied pressure on the probability of electronic energy transfer across a molecular dyad

A pair of covalently linked molecular dyads is described in which two disparate boron dipyrromethene dyes are separated by a tolane-like spacer. Efficient electronic energy transfer (EET) occurs across the dyad; the mechanism involves important contributions from both Förster-type coulombic interactions and Dexter-type electron exchange processes. The energy acceptor is equipped with long paraffinic chains that favor aggregation at high concentration or at low temperature. The aggregate displays red-shifted absorption and emission spectral profiles, relative to the monomer, such that EET is less efficient because of a weaker overlap integral. The donor unit is insensitive to applied pressure but this is not so for the acceptor, which has extended π-conjugation associated with appended styryl groups. Here, pressure reduces the effective π-conjugation length, leading to a new absorption band at higher energy. With increasing pressure, the overall EET probability falls but this effect is nonlinear and at modest pressure there is only a small recovery of donor fluorescence. This situation likely arises from compensatory phenomena such as restricted rotation and decreased dipole screening by the solvent. However, the probability of EET falls dramatically over the regime where the π-conjugation length is reduced owing to the presumed conformational exchange. It appears that the pressure-induced conformer is a poor energy acceptor.