Energy Flow in a Purpose‐Built Cascade Molecule Bearing Three Distinct Chromophores Attached to the Terminal Acceptor

A multicomponent cluster has been synthesised in which four disparate chromophores have been covalently linked through a logical arrangement that favours efficient photon collection and migration to a terminal emitter. The primary energy acceptor is a boron dipyrromethene (Bodipy) dye and different polycyclic aryl hydrocarbons have been substituted in place of the regular fluorine atoms attached to the boron centre. The first such unit is perylene, linked to boron through a 1,4‐diethynylphenyl unit, which collects photons in the 320–490 nm region. The other photon collector is pyrene, also connected to the boron centre by a 1,4‐diethynylphenyl spacer and absorbing strongly in the 280–420 nm region, which itself is equipped with an ethynylfluorene residue that absorbs in the UV region. Illumination into any of the polycyclic aryl hydrocarbons results in emission from the Bodipy unit. The rates of intramolecular electronic energy transfer have been determined from time‐correlated, single‐photon counting studies and compared with the rates for Coulombic interactions computed from the Förster expression. It has been necessary to allow for i) a more complex screening potential, ii) multipole–multipole coupling, iii) an extended transition dipole moment vector and iv) bridge‐mediated energy transfer. The bridge‐mediated energy transfer includes both modulation of the donor transition dipole vector by bridge states and Dexter‐type electron exchange. The latter is a consequence of the excellent electronic coupling properties of the 1,4‐diethynylphenyl spacer unit. The net result is a large antenna effect that localises the photon density at the primary acceptor without detracting from its highly favourable photophysical properties.     

Synthesis of multi-branched dipyrromethene dyes with soluble diethynylphenyl links

Multi-branched dyes were prepared using a convergent step-by-step procedure. The key dibutoxydiethynylphenyl spacing units were first linked to a difluoroboradipyrromethene (yellow fluorophore) dye via the pseudo-meso position and then selectively cross-coupled to a di-(4-phenyliodo)ethynylborodiisoindolomethene (red emitter via the boron). When ethynylpyrene units were connected at the periphery to the boron of the yellow emitter, efficient cascade singlet energy transfer events occurred from the pyrene to the yellow and to the red emitters.     

Photochemical Bleaching of an Elaborate Artificial Light‐Harvesting Antenna

The target artificial light‐harvesting antenna, comprising 21 discrete chromophores arranged in a logical order, undergoes photochemical bleaching when dispersed in a thin plastic film. The lowest‐energy component, which has an absorption maximum at 660 nm, bleaches through first‐order kinetics at a relatively fast rate. The other components bleach more slowly, in part, because their excited‐state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor. Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy‐transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy‐transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by a series of eight pyrene‐based chromophores, does not bleach to any significant degree. Confirmation of the various bleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalytic route does not exist and bleaching is very slow.     

Enhanced Light‐Harvesting Capacity by Micellar Assembly of Free Accessory Chromophores and LH1‐like Antennas

Biohybrid light‐harvesting antennas are an emerging platform technology with versatile tailorability for solar‐energy conversion. These systems combine the proven peptide scaffold unit utilized for light harvesting by purple photosynthetic bacteria with attached synthetic chromophores to extend solar coverage beyond that of the natural systems. Herein, synthetic unattached chromophores are employed that partition into the organized milieu (e.g. detergent micelles) that house the LH1‐like biohybrid architectures. The synthetic chromophores include a hydrophobic boron‐dipyrrin dye (A1) and an amphiphilic bacteriochlorin (A2), which transfer energy with reasonable efficiency to the bacteriochlorophyll acceptor array (B875) of the LH1‐like cyclic oligomers. The energy‐transfer efficiencies are markedly increased upon covalent attachment of a bacteriochlorin (B1 or B2) to the peptide scaffold, where the latter likely acts as an energy‐transfer relay site for the (potentially diffusing) free chromophores. The efficiencies are consistent with a Förster (through‐space) mechanism for energy transfer. The overall energy‐transfer efficiency from the free chromophores via the relay to the target site can approach those obtained previously by relay‐assisted energy transfer from chromophores attached at distant sites on the peptides. Thus, the use of free accessory chromophores affords a simple design to enhance the overall light‐harvesting capacity of biohybrid LH1‐like architectures.     

Synthesis of a highly luminescent three-dimensional pyrene dye based on the spirobifluorene skeleton

We have synthesized a highly luminescent (log e > 5.0, F > 0.9) pyrene dye based on a spirobifluorene skeleton [2,2',7,7'-tetrakis(7-tert-butyl-1-pyrenyl)-9,9'-spirobi[9H-fluorene; 4-PySBF]. The use of spirobifluorene prevents fluorescence quenching by intramolecular energy transfer and/or electron transfer among the chromophores in the excited state. The emission spectra of 4-PySBF exhibited a red shift of 20 nm in comparison to a model compound [9,9'-dioctyl-2,7-bis(7-tert-butyl-1-pyrenyl)-9H-fluorene; 2-PyF], but its UV-Vis spectrum remained unchanged.     

DNA‐Based Oligochromophores as Light‐Harvesting Systems

The chromophores ethynyl pyrene as blue, ethynyl perylene as green and ethynyl Nile red as red emitter were conjugated to the 5‐position of 2′‐deoxyuridine via an acetylene bridge. Using phosphoramidite chemistry on solid phase labelled DNA duplexes were prepared that bear single chromophore modifications, and binary and ternary combinations of these chromophore modifications. The steady‐state and time‐resolved fluorescence spectra of all three chromophores were studied in these modified DNA duplexes. An energy‐transfer cascade occurs from ethynyl pyrene over ethynyl perylene to ethynyl Nile red and subsequently an electron‐transfer cascade in the opposite direction (from ethynyl Nile red to ethynyl perylene or ethynyl pyrene, but not from ethynyl perylene to ethynyl pyrene). The electron‐transfer processes finally provide charge separation. The efficiencies by these energy and electron‐transfer processes can be tuned by the distances between the chromophores and the sequences. Most importantly, excitation at any wavelength between 350 and 700 nm finally leads to charge separated states which make these DNA samples promising candidates for light‐harvesting systems.     

Rapid intersystem crossing in closely-spaced but orthogonal molecular dyads.

A borondipyrromethene (bodipy) dye is equipped with a 4-pyridine residue attached via the meso position. The strong fluorescence inherent to this class of dye is extinguished on protonation of the pyridine N atom. For the corresponding N-methylpyridinium derivative, fluorescence from the dye fragment is also extensively quenched due to the onset of a light-induced charge-shift reaction. The resultant charge-transfer state (CTS) is weakly fluorescent and decays primarily by way of population of the triplet excited state localized on the bodipy dye. Time-resolved spectral studies provide rate constants for all the steps involved in the forward and reverse charge-shift reactions. An interesting feature is that the lifetime of the CTS, around 1 ns, correlates with the viscosity of the solvent as might be expected if the rate-limiting step involves a substantial change in geometry. There is an unexpectedly small activation energy for the reverse charge-shift reaction, even allowing for the fact that this involves triplet formation. Local fluorescence is restored on cooling to 77 K.     

Facile pathways to multichromophoric arrays based on a truxene platform

Multi-branched dyes were synthesized using a truxene platform appropriately functionalized with iodo residues, the key step here being a controlled iodination process giving access to mono-, di-, tri- and hexa-derivatives. Conversion of the iodo compounds to the corresponding alkynes followed by cross-coupling with iodophenylindacenes catalysed by Pd(0) or with chloro-Pt(II) complexes catalysed by Cu(I) enabled the introduction of multiple chromophore/luminophore units. Both neutral and cationic species involving Pt(II) centres were obtained in excellent yields. Connection of ethynylpyrene units to boron within the hexa-grafted truxene Bodipys provided very efficient cascade singlet energy transfer from the pyrene to the yellow emitter. Irradiation in the truxene core also resulted in quantitative energy transfer to the organic dyes.     

Light‐Harvesting Three‐Chromophore Systems Based on Biphenyl‐Bridged Periodic Mesoporous Organosilica

Three‐chromophore systems with light‐harvesting behavior were prepared, which are based on periodic mesoporous organosilica (PMO) with crystal‐like ordered structure. The organic bridges of biphenyl‐PMO in the pore walls act as donors and two types of dye are incorporated in the one‐dimensional channels. Consecutive two‐step‐Förster resonance energy transfer is observed from the biphenyl moieties to mediators (diethyl‐aminocoumarin or aminoacridone), followed by energy transfer from mediators to acceptors (dibenzothiacarbocyanine, indodicarbocyanine, sulforhodamine G). High energy‐transfer efficiencies ranging from 70 to 80 % are obtained for two‐step‐FRET, indicating that the mesochannel structure with one‐dimensional ordering provides spatial arrangement of chromophore pairs for an efficient direct energy transfer. The emission wavelength can be tuned by a choice of acceptor dye: 477 nm (diethylaminocoumarin), 519 nm (aminoacridone), 567 nm (sulforhodamine G), 630 nm (dibenzothiacarbocyanine), and 692 nm (indodicarbocyanine).     

Evidence for Dual Pathway in Through‐Space Singlet Energy Transfers in Flexible Cofacial Bisporphyrin Dyads

Flexible “pacman” scaffolds built upon a calix[4]arene platform bearing a [18]crown‐6 ether and either two OH functions or two OPr groups at the lower rim have been used to generate donor–acceptor (D–A) dyads incorporating a zinc–porphyrin donor and a free‐base porphyrin acceptor. Through‐space singlet energy transfer (SET) in the D–A dyads was studied by time‐resolved fluorescence spectroscopy. Although the effects of conformational changes are well documented when the chromophores switch from a non‐cofacial to a cofacial arrangement, little is known about flexible pacman scaffolds in which the changes are limited to the distance between the chromophores. The known SET rates for reported, geometrically well‐defined, rigid pacman D–A dyads were used as calibration to estimate the D–A distances in the flexible pacman dyads. Due to the flexibility of the calix[4]arene spacer, the D–A dyads adopt a “closed” or “open” geometry that is tuned by intramolecular hydrogen bonds (O? H???[18]crown‐6 ether) and by solvent interactions. Changes in the SET rates between the open and closed geometries were surprisingly less dramatic than expected, and are explained by a dual SET pathway that is specific to the calix[4]arene platform. Time‐resolved fluorescence studies support the hypothesis that, for the “open” conformer, the preferred through space SET pathway (i.e., at the shortest distance) is located within the calix[4]arene cavity through the cofacial phenyl groups. For the “closed” conformer, the preferred through space SET route is located between the zinc and free‐base porphyrins.     

Emulsion copolymerization of vinyl acetate and butyl acrylate in the presence of fluorescent dyes

This article describes our first experiments for preparing dye‐labeled latex particles by the emulsion copolymerization of a 4/1 (w/w) mixture of vinyl acetate‐butylacrylate (VAc‐BA). We discuss the synthesis of acrylate derivatives of phenanthrene, anthracene, and pyrene [9‐acryloxymethyl phenanthrene ( 7 ), 9‐acryloxymethyl‐10‐methyl anthracene ( 8 ), and 1‐acryloxymethyl pyrene ( 10 )] and an allyl ether derivative of anthracene [9‐allyoxymethyl‐10‐methyl anthracene ( 9 )]. Although the phenanthrene derivative 7 gave latex particles with high monomer conversion and good dye incorporation, the pyrene acrylate and both anthracene comonomers strongly inhibited the free‐radical reaction. To assist our search for a dye that would serve as a useful energy acceptor for phenanthrene and without suppressing VAc‐BA polymerization, we also examined batch emulsion polymerization in the presence of a variety of dye derivatives—substituted anthracenes, acridines, a coumarin, and two benzophenone derivatives. All of the anthracene derivatives, as well as acridine, strongly inhibited monomer polymerization. The coumarin dye 7‐hydroxy‐4‐methyl coumarin ( 22 ) that had only limited solubility allowed more than 90% monomer conversion. Most promising were 2‐hydroxy‐5‐methyl benzophenone ( 23 ) and 4‐N,N‐dimethylamino benzophenone ( 24 ) that at 1 mol % in the monomer mixture permitted virtually quantitative monomer conversion to latex. 4′‐Dimethylamino‐2‐acryloxy‐5‐methyl benzophenone ( 25 ) copolymerized well with the VAc‐BA mixture, yielding latex particles in high yield and with a narrow size distribution. These dyes appear to be useful acceptor dyes for energy‐transfer experiments with phenanthrene. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1594–1607, 2002     

Through‐Space Conjugated Molecular Wire Comprising Three π‐Electron Systems

A [2.2]paracyclophane‐based through‐space conjugated oligomer comprising three π‐electron systems was designed and synthesized. The arrangement of three π‐conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Förster mechanism. The energy transfer efficiency and rate constants were estimated to be >0.999 and >10¹² s^?1, respectively. The key point for the efficient energy transfer is the orientation of the transition dipole moments. The time‐dependent density functional theory (TD‐DFT) studies revealed the transition dipole moments of each stacked π‐electron system; each dipole moment was located on the long axis of each stacked π‐electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).     

Pyrenoimidazole‐Based Deep‐Blue‐Emitting Materials: Optical,Electrochemical, and Electroluminescent Characteristics

A series of pyrenoimidazoles that contained various functional chromophores, such as anthracene, pyrene, triphenylamine, carbazole, and fluorene, were synthesized and characterized by optical, electrochemical, and theoretical studies. The absorption spectra of the dyes are dominated by electronic transitions that arise from the pyrenoimidazole core and the additional chromophore. All of the dyes exhibited blue‐light photoluminescence with moderate‐to‐high quantum efficiencies. They also displayed high thermal stability and their thermal‐decomposition temperatures fell within the range 462–512 °C; the highest decomposition temperature was recorded for a carbazole‐containing dye. The oxidation propensity of the dyes increased on the introduction of electron‐rich chromophores, such as triphenylamine or carbazole. The application of selected dyes that featured additional chromophores such as pyrene, carbazole, and triphenylamine as blue‐emissive dopants into multilayered organic light‐emitting diodes with a 4,4′‐bis(9H‐carbazol‐9‐yl)biphenyl (CBP) host was investigated. Devices that were based on triphenylamine‐ and carbazole‐containing dyes exhibited deep‐blue emission (CIE 0.157, 0.054 and 0.163, 0.041), whereas a device that was based on a pyrene‐containing dye showed a bright‐blue emission (CIE 0.156, 0.135).     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

Energy transfer in multichromophoric self-assembled molecular squares

Metal-ion directed self-assembly afforded well-defined molecular squares with a mass of 12 kDa incorporating four perylene bisimide and sixteen pyrene chromophores organized in a circular way. Energy transfer from the pyrene to the perylene dye manifold is investigated by emission and excitation spectroscopy.     

Nitroanilines as Quenchers of Pyrene Fluorescence

The quenching of pyrene and 1‐methylpyrene fluorescence by nitroanilines (NAs), such as 2‐, 3‐, and 4‐nitroaniline (2‐NA, 3‐NA, and 4‐NA, respectively), 4‐methyl‐3‐nitroaniline (4‐M ‐3‐NA), 2‐methyl‐4‐nitroaniline (2‐M‐4‐NA), and 4‐methyl‐3,5‐dinitroaniline (4‐M‐3,5‐DNA), are studied in toluene and 1,4‐dioxane. Steady‐state fluorescence data show the higher efficiency of the 4‐NAs as quenchers and fit with a sphere‐of‐action model. This suggests a 4‐NA tendency of being in close proximity to the fluorophore, which could be connected with their high polarity/hyperpolarizability. In addition, emission and excitation spectra evidence the formation of emissive pyrene—NA ground‐state complexes in the case of the 4‐NAs and, in a minor degree, in the 2‐NA. Moreover, time‐resolved fluorescence experiments show that increasing amounts of NA decrease the pyrene fluorescence lifetime to a degree that depends on the NA nature and is larger in the less viscous solvent (toluene). Although the NA absorption and the pyrene (Py) emission overlap, we found no evidence of dipole–dipole energy transfer from the pyrene singlet excited state (¹Py) to the NAs; this could be due to the low NA concentration used in these experiments. Transient absorption spectra show that the formation of the pyrene triplet excited state (³Py) is barely affected by the presence of the NAs in spite of their efficiency in ¹Py quenching, suggesting the involvement of ¹Py—NA exciplexes which—after intersystem crossing—decay efficiently into ³Py.     

Excitation Energy Transfer from Semi‐Conducting Polymer Nanoparticles to Surface‐Bound Fluorescent Dyes

Summary: The first examples of the dye‐coated semi‐conducting polymer nanoparticles as well as experiments to demonstrate the excitation energy transfer from the excited chromophor of the nanoparticle to the fluorescent dye are described. We have demonstrated that the blue fluorescence of the dye‐coated polyfluorene nanoparticles is only slightly quenched after dye deposition. However, a new emission band of the surface‐bound dye (Rhodamine 6G or Rhodamine TM) appears in the wavelength region of 530–600 nm. These results clearly indicate an effective excitation energy transfer from the excited PF chromophores to the fluorescent dye.

Emission spectra of PF2/6 nanoparticle dispersion and of Rhodamine 6G‐coated nanoparticle dispersion.     

Energy transfer in calixarene-based cofacial-positioned perylene bisimide arrays

The synthesis of multichromophoric perylene bisimide-calix[4]arene arrays with up to five perylene units (containing orange, violet, and green perylene bisimide chromophores) and of monochromophoric model compounds was achieved by subsequent imidization of mono-Boc functionalized calix[4]arene linkers with three different types of perylene bisimide dye units. The optical properties of all compounds were studied with UV/vis absorption and steady state and time-resolved fluorescence spectroscopy. Upon excitation of the inner orange dye at 490 nm of array 3, strong fluorescence emission of the outer green perylene bisimide (PBI) chromophore at 744 nm is observed. The fluorescence excitation spectra of compounds 3 and 4 (lambdadet = 850 nm) show all absorption bands of the parent chromophores (e.g., all perylene units contribute to the emission from S1 state of the green PBI). Thus, the fluorescence emission and excitation spectra as well as time-resolved data of fluorescence lifetimes in the absence (tauD = 5.1 ns) and in the presence of an acceptor (tauDA = 0.8 ns) suggest efficient energy transfer processes between the perylene bisimide dye units. For the bichromophoric array 4, the energy transfer rate is calculated to a value of 1.05 x 109 s-1. These results demonstrate highly efficient energy transfer in cofacially assembled dye arrays.     

Solvent and Media Effects on the Photophysics of Naphthoxazole Derivatives

The photophysical properties of 2‐phenyl‐naphtho[1,2‐d][1,3]oxazole, 2(4‐N,N‐dimethylaminophenyl)naphtho[1,2‐d][1,3]oxazole and 2(4‐N,N‐diphenylaminophenyl) naphtho[1,2‐d][1,3]oxazole were studied in a series of solvents. UV–Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV–Vis absorption and fluorescence spectra in terms of the Lippert–Mataga equation shows a large increase in the excited‐state dipole moment, which is also compatible with the formation of an intramolecular charge‐transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1–3 ns as well as their high photostability.     

Optical,Redox, and DNA‐Binding Properties of Phenanthridinium Chromophores: Elucidating the Role of the Phenyl Substituent for Fluorescence Enhancement of Ethidium in the Presence of DNA

The phenanthridinium chromophores 5‐ethyl‐6‐phenylphenanthridinium ( 1 ), 5‐ethyl‐6‐methylphenanthridinium ( 2 ), 3,8‐diamino‐5‐ethyl‐6‐methylphenanthridinium ( 3 ), and 3,8‐diamino‐5‐ethyl‐6‐(4‐N,N‐diethylaminophenyl)phenanthridinium ( 4 ) were characterized by their optical and redox properties. All dyes were applied in titration experiments with a random‐sequence 17mer DNA duplex and their binding affinities were determined. The results were compared to well‐known ethidium bromide ( E ). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of E to be elucidated. Especially, compound 4 was used to compare the weak electron‐donating character of the phenyl substituent at position 6 of E with the more electron‐donating 4‐N,N‐diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of 1 and 2 , lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with E . In the presence of DNA, a significant fluorescence quenching was observed. Their binding affinity to DNA is reduced by nearly one order of magnitude. The electronic effect of the phenyl group in position 6 on this type of dye is rather small. The properties of the second set, 3 and 4 , are similar to E due to the presence of the two strongly electron‐donating amino groups in positions 3 and 8. However, in contrast to 1 and 2 , the electron‐donating character of the substituent in position 6 of 3 and 4 is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron‐donating effect of this substituent. Accordingly, compound 4 shows the strongest binding affinity and the strongest fluorescence enhancement. Quantum chemical calculations reveal a general mechanism related to the twisted intramolecular charge transfer (TICT) model. Accordingly, an increase of the twist angle between the phenyl ring in position 6 and the phenanthridinium core opens a nonradiative channel in the excited state that depends on the electron‐donating character of the phenyl group. Access to this channel is hindered upon binding to DNA.