Bichromophoric Compounds Research Articles

Spectral and generation properties of new coumarins – benzopyrylium derivatives

The spectral and generation parameters of coumarins, benzopyrylium derivatives, in ethanol and acetonitrile under coherent excitation were studied. It was found that the absorption spectra of the investigated compounds contain several maxima and the structuredness of the spectra is associated with their bichromophoric structure. It is shown that these organic dyes are bichromophoric compounds. It is noted that the nature of the substituents in positions 6 and 7 of the coumarin base and positions 6′ and 7′ of the benzopyrylium core determines their spectral-luminescent and lasing properties. It is demonstrated that the presence of substituents in certain positions of the coumarin base and the benzopyrylium core changes the conformation of the molecule and, accordingly, the dipole moments and oscillator strengths of the transition. The generation ability of coumarins of this class manifests itself only in the presence of the strongest donor substituents (for example, Et2N and OH) in position 7, and sufficiently strong acceptors in positions 6′ and 7′. Lasing was obtained in the spectral region of 730 –740 nm on dyes – benzopyrylium derivatives.

Impact of Optical Purity on the Light Harvesting Property in Supramolecular Nanofibers.

Supramolecular ordering and orientation of chromophores are tremendously accomplished in photosynthetic light harvesting complexes, which are crucial for long-range transfer of collected solar energy. We herein demonstrate the importance of optical purity on the organization of chromophoric chiral molecules for efficient energy migration. Enantiomeric bichromophoric compounds, which self-assemble into nanofibers capable of chiral recognition, were mixed to form supramolecular coassemblies with variable enantiopurity. The chiral molecules self-assembled into extended fibers regardless of enantiopurity, while their morphology was dependent on the enantiomeric excess. The optical purity of assemblies also had an effect on the emission efficiency; the nanofibers with higher enantiomeric excess afforded a larger emission quantum yield. The presence of an opposite enantiomer is considered to deteriorate the chiral molecular packing suitable for directional growth of the nanofiber, efficient exciton migration, and chiral guest recognition.

Bichromophoric Compounds with Orthogonally and Parallelly Arranged Chromophores Separated by Rigid Spacers.

Electronic energy transfer (EET) between chromophores is of fundamental importance for many biological processes and optoelectronic devices. However, common models fall short in fully describing the process, especially in bichromophoric model systems with a donor and acceptor connected by a rigid linker providing perpendicular geometries. Herein, we report a novel strategy for preparing bichromophores containing adamantane or 2-(2-adamantylidene)adamantane as rigid spacers, providing a fixed distance between chromophores, and their parallel or perpendicular arrangement without chromophore rotation. New fluorophores were developed and linked via spiroatoms. Bichromophores with identical (blue-blue) or different (blue-red) chromophores were synthesized, either in orthogonal or parallel geometry. These were characterized by absorption/fluorescence spectroscopy, time-resolved fluorescence anisotropy, and fluorescence antibunching measurements. Based on the Förster point-dipole approximation, EET efficiencies were estimated by using geometrical parameters from (time-dependent) density functional calculations. For bichromophores with parallel geometry, the predicted EET efficiencies were near unity and fit the measurements. In spite of estimated values around 0.4 and 0.5, 100 % efficiency was observed also for bichromophores with orthogonal geometry. The new rigid scaffolds presented here open new possibilities for the synthesis of bichormophores with well-defined parallel or perpendicular geometry.

Novel fluorescent anthracene–bodipy dyads displaying sensitivity to pH and turn-on behaviour towards Cu(ii) ions

Three bichromophoric compounds containing anthracene and bodipy moieties connected by different spacers have been synthesized and fully characterized. The set of spacers comprises a variety of bridges (short and flexible 1,2-diaminoethane in compound 1a, long and flexible 1,8-diaminooctane in compound 1b and short and rigid piperazine in compound 1c). The new compounds show interchromophoric communication via Electronic Energy Transfer (EET). When anthracene is excited, the energy is efficiently transferred to the bodipy moiety. The emission of the acceptor is partially quenched in dyad 1a but remarkably not in 1b and 1c. In an aqueous environment, the fluorescence of the new compounds display sensitivity to pH (emissive at acidic values and non-emissive at neutral ones), a typical behaviour of photoinduced electron-transfer (PET) probes, but with remarkable differences in the pKa of each compound, attributable to the nature of the linker. Additionally, the behaviour in the presence of several metallic cations (Cu2+, Ni2+, Cd2+, Zn2+) has been studied, and a turn-on response towards Cu2+ in water at neutral pH has been found.

Localized operator partitioning method for electronic excitation energies in the time-dependent density functional formalism.

We extend the localized operator partitioning method (LOPM) [J. Nagesh, A. F. Izmaylov, and P. Brumer, J. Chem. Phys. 142, 084114 (2015)] to the time-dependent density functional theory framework to partition molecular electronic energies of excited states in a rigorous manner. A molecular fragment is defined as a collection of atoms using Becke's atomic partitioning. A numerically efficient scheme for evaluating the fragment excitation energy is derived employing a resolution of the identity to preserve standard one- and two-electron integrals in the final expressions. The utility of this partitioning approach is demonstrated by examining several excited states of two bichromophoric compounds: 9-((1- naphthyl)- methyl)- anthracene and 4-((2- naphthyl)- methyl)- benzaldehyde. The LOPM is found to provide nontrivial insights into the nature of electronic energy localization that is not accessible using a simple density difference analysis.

Photo-optical properties of amorphous and crystalline films of azobenzene-containing photochromes with bent-shaped molecular structure

Abstract A comparative study of the photo-optical properties and the photo-orientation processes in thin films of the bent-shaped azobenzene-containing compounds with different molecular structure was performed. For this purpose three new compounds were designed and prepared: two of these compounds with a symmetric bent-shaped molecular structure incorporate two azobenzene chromophores with different length of alkyl chains (6 and 8 methylene units), the third bent-shaped compound possesses only one azobenzene fragment. The effect of molecular and supramolecular structure, thermal prehistory of the films, wavelengths of the excitation light on the photo-optical properties, photoorientation processes and films morphology was revealed. It was shown that UV-irradiation leads to E – Z isomerization of azobenzene fragments in both amorphousized and crystalline films of the synthesized substances. This process is partially suppressed in crystalline films, but, nevertheless, UV-irradiation of the bichromophoric compounds results in transition from crystalline to amorphous state and decrease in the surface roughness. Irradiation of amorphousized films with polarized visible and UV-light induces the photoorientation of chromophores in direction perpendicular to the polarization plane of the incident light. Relatively high values of photoinduced dichroism in the bent-shaped compounds (D ∼ 0.60–0.65) are comparable with other azobenzene-containing rod-shaped systems. Photoinduced isothermal melting and photoorientation processes could be served for the photo-optical data recording and storage.

The synthesis of new fluorescent bichromophoric compounds as ratiometric pH probes for intracellular measurements.

Three different bichromophoric compounds (1-3) containing an aminomethyl anthracene moiety linked to a second chromophore (pyrene, 4-nitrobenzo-2-oxa-1,3-diazole (NBD) and dansyl) through a valine-derived pseudopeptidic spacer have been prepared and their fluorescent properties studied. The results obtained show that upon irradiation the photophysical behavior of these probes involves electronic energy transfer from the excited anthracene to the second chromophore and also intramolecular photoinduced electron transfer. The X-ray structure obtained for 3 reveals that the folding associated with the pseudopeptidic spacer favours a close proximity of the two chromophores. The emissive response of 3 is clearly dependent on the pH of the medium, hence this bichromophoric compound was shown to be an excellent ratiometric pH fluorescent sensor. The emission intensity due to the anthracene moiety exhibits a decrease at neutral-basic pH values that is concomitant with an increase in the intensity arising from the dansyl fluorophore. These properties make this compound a good candidate for biological pH sensing as has been confirmed by preliminary studies with RAW 264.7 macrophage cells imaged by means of confocal fluorescence microscopy with an average pH estimation of 5.4-5.8 for acidic organelles.

Ultrafast Energy Transfer in Triptycene‐Grafted Bodipy Scaffoldings

A series of new compounds in which various Bodipy dyes are grafted logically on triptycene rigid structures are synthesized and characterized, and their absorption spectra and photophysical properties are studied, also by pump-probe transient absorption spectroscopy. The studied compounds are: the mono-Bodipy species TA, TB, and TC (where A, B, and C identify different Bodipy subunits absorbing and emitting at different wavelengths), the multichromophore species TA3 , which bears three identical A subunits, and the three multichromophoric species TAB, TBC, and TABC, all of them containing at least two different types of Bodipy subunits. The triptycene moiety plays the role of a rigid scaffold, keeping the various dyes at predetermined distances and allowing for a three-dimensional structural arrangement of the multichromophoric species. The absorption spectra of the multichromophoric Bodipy species are essentially additive, indicating that negligible inter-chromophoric interaction takes place at the ground state. Luminescence properties and transient absorption spectroscopy indicate that a very fast (on the picosecond time scale) and efficient photoinduced energy transfer occurs in all the multi-Bodipy species, with the lower-energy Bodipy subunits of each multi-Bodipy compounds playing the role of an electronic energy collector. In TAB, an energy transfer from the A-type Bodipy subunit to the B-type one takes place with a rate constant of 1.6×10(10) s(-1), whereas in TBC an energy transfer from the B-type Bodipy subunit to the C-type subunit is bi-exponential, exhibiting rate constants of 1.7×10(11) and 1.9×10(10) s(-1); the possible presence of different conformers with different donor-acceptor distances in this bichromophoric species is proposed to cause the bi-exponential energy-transfer process. Interpretation of the intricate energy-transfer pathways occurring in TABC is made with the help of the processes identified in the bichromophoric compounds. In all cases, the measured energy-transfer rate constants agree with a Förster mechanism for the energy-transfer processes.

Bichromophores bearing s-triazinyl ring spacer: A role of charge transfer states on fluorescence quenching of excitation energy acceptor moieties

Abstract Novel bichromophoric compounds bearing 2-aminoanthracene as the donor of excitation energy and 3-aminoperylene or 3-aminobenzanthrone as the acceptor of excitation energy and s-triazinyl ring as the spacer were synthesized and characterized. The UV/Vis absorption and fluorescence spectra as well as fluorescence quantum yields in different solvents were measured. On the basis of experimental results and semi-empirical quantum chemical calculations, the relationships between the chemical structure and photo-physical properties of prepared compounds were investigated.

New Tetrazines Functionalized with Electrochemically and Optically Active Groups: Electrochemical and Photoluminescence Properties.

Original new fluorescent and electroactive compounds have been prepared, where the fluorescent moiety is a chloroalkoxy-s-tetrazine. Besides the tetrazine, several of these compounds possess a second active group, electroactive or able to absorb light at a lower wavelength. The electrochemical properties and photophysical properties of these bichromophoric compounds have been investigated, especially focusing on the occurrence of energy or electron transfer to the tetrazine. In one case, where the primary absorber is a naphthalimide, a quasi-complete energy transfer, followed by the tetrazine fluorescence, is observed. This allows the preparation of remarkable transparent solutions displaying a yellow fluorescence.

BODIPY‐Tetrazine Multichromophoric Derivatives

AbstractNew dyes based on BODIPY and tetrazine fluorophores connected through a phenyl spacer have been synthesized and their absorption, emission and electrochemical properties characterized. BODIPY can be reversibly oxidized into a stable cation radical whereas tetrazine can be reduced to a stable anion radical. The electrochemical and absorption studies demonstrate that both fluorophores behave independently. The bichromophoric compounds show an expected very weak emission by the BODIPY core that is quenched by the phenoxytetrazine mainly through energy transfer. DFT calculations and spectroelectrochemistry experiments demonstrate that photoinduced electron transfer and energy transfer remain possible when the tetrazine moiety is reduced electrochemically, which prevents switching on of the fluorescence of the BODIPY unit.

Estimation of the solvent reorganization energy and the absolute energy of solvation of charge-transfer states from their emission spectra

We report herein the solvent and temperature effects on the emission of the intermolecular exciplexes 1-cyanonaphthalene/triethylamine and 1-methylnaphthalene/triethylamine and the intramolecular exciplexes formed by the bichromophoric compounds diethyl-(3-naphthalen-1-yl-propyl)-amine (I), diethyl-(2-naphthalen-1-yl-ethyl)-amine (II), 3-[ethyl-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (III) and 3-[(2-cyano-ethyl)-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (IV). The results are interpreted within the formalism of the semiclassical Marcus theory for radiative back electron transfer (BET) reactions. We show that, following a few simple assumptions, reliable values of the Gibbs free energy changes (DeltaG(epsilon)(-et)) and the solvent reorganization energies (lambda(epsilon)(s)) associated to the BET reactions (and their corresponding enthalpic and entropic contributions) can be estimated directly from the emission of the CT states. We also show that for the 1-cyanonaphthalene/triethylamine exciplex, which exhibits emission in the vapour phase, the experimental calculation of the absolute energy of solvation of the CT state (DeltaG(epsilon)(s)) is also possible. The calculated DeltaG(epsilon)(-et) correlate quite satisfactorily with the corresponding values obtained from independent electrochemical and kinetics experiments. The temperature effects on DeltaG(epsilon)(-et), lambda(epsilon)(s) and DeltaG(epsilon)(s) are discussed qualitatively using different solvation models. The limitations of the present approach for the estimation of DeltaG(epsilon)(-et) and lambda(epsilon)(s) and its possible application to more complex A/D systems are also examined.

Substituted Position Effect on Twisted intramolecular Charge Transfer of 1- and 2-anthracene aromatic Carboxamides as Chemosensors Based on Linear Polyether

Bi-chromophoric compounds linked to linear polyether N,N'-[oxybis(3-oxapentamethyleneoxy-2-phenyl)]-bis(1-anthracenecarboxamide) (1(4)) and its analogues (1(5), 2(4) and 2(5)) were synthesized. Their photochemical properties and complexation actions were investigated in acetonitrile. These fluoroionophores have shown weak emissions in the absence of guest ions, resulting in a twisted intramolecular charge transfer (TICT) quenching process (off state) of an anthracene aromatic amide moiety. After the formation of a complex with alkaline earth metal ions, a large enhancement of fluorescent intensities was obtained (on state). The efficiencies of controlling an off-on fluorescent signal using 1(4) and its analogues (1(5), 2(4) and 2(5)) were larger than that of the 9-positioned analogue. These results suggested that the TICT relaxation process was effectively able to control using 1(4) and its analogues (1(5), 2(4) and 2(5)) by complex formations.

The ultrafast energy transfer process in naphtole–nitrobenzofurazan bichromophoric molecular systems: A study by femtosecond UV–vis pump-probe spectroscopy

This work presents an experimental and computational study of the intramolecular electronic energy transfer process occurring in two newly synthesized bichromophoric species: N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino-bis-ethyl-2-[(4-chloro-1-naphthyl)oxy]acetate ( f-Bi ) and N-(7-nitrobenzo[c][1,2,5]oxadiazole-4-yl)-(3S, 4S)-pyrrolidin-3,4-bis-yl-2-[(4-chloro-1-naphthyl)oxy]acetate ( r-Bi ). In both f-Bi and r-Bi the donor chromophore is the [(4-chloro-1-naphthyl)oxy]acetate moiety, whereas the acceptor units belong to the family of the 4-dialkylaminonitrobenzoxadiazoles, well-known fluorescent probes. The two bichromophores differ in the structural flexibility. In f-Bi , acceptor and donors are linked by a diethanolamine moiety, whereas in r-Bi through a (3S, 4S)3,4-dihydroxypyrrolidine ring. By means of steady-state and time-resolved UV–vis spectroscopies we carried out a detailed analysis of the photo-response of donor and acceptor chromophores as individual molecules and when covalently linked in f-Bi and r-Bi . The intramolecular energy transfer process occurs very efficiently in both the bichromophores. The rate constant and the quantum efficiency of the process are k ET = (2.86 ± 0.16) × 10 11 s −1 and Q = 0.998 in f-Bi , and k ET = (1.25 ± 0.08) × 10 11 s −1 and Q = 0.996 in r-Bi . Semiempirical calculations were utilized to identify the energy and the nature of the electronic states in the isolated chromophores. Molecular mechanics calculations have been performed to identify the most stable structures of the bichromophoric compounds. The predictions of Förster theory are consistent with the experimental results and provide a suitable way to evaluate the structural differences between the two compounds.

Bichromophoric Naphthalene Derivatives of Ethylenediaminetetraacetate: Fluorescence from Intramolecular Excimer, Protonation and Complexation with Zn2+ and Cd2+

Chelating water-soluble bichromophoric compounds were designed by linking two naphthalene rings with an ethylenediaminetetraacetate (edta) chain through amide linkages, and characterized by fluorescence and NMR spectroscopies: the bichromophores studied were 1,4-bis(methylenecarboxy)-1,4-bis(N-1-naphthylacetamide)-1,4-diazabutane, abbreviated as (edta1nap)H2, and the corresponding 2-naphthylacetamide, abbreviated as (edta2nap)H2. Completely deprotonated species M2 − that formed in basic solution exhibited an intense emission from intramolecular excimer at 440 nm for (edta1nap)2 − and 404 nm for (edta2nap)2 − , while emission from monomeric naphthyl group was comparatively very weak. Geometry optimization based on Density Functional Theory showed that two naphthyl groups in an M2 − molecule faced each other in such a way that an intramolecular excimer was readily formed by light excitation. Protonation on amino nitrogen affected monomer–excimer interconversion in different modes for the two compounds: the excimer emission was strengthened in the resulting (edta1nap)H− , but was weakened in (edta2nap)H− . Upon coordination with Zn2+, the excimer band of (edta1nap)2 − as well as (edta2nap)2 − was strengthened in a linear manner with [Zn]/[L]; concurrently monomer emission was weakened. Coordination of (edta2nap)2 − with Cd2+ resulted in a large decrease in the excimer emission, while the emission of (edta1nap)2 − was almost unchanged. The formation of intramolecular excimer, which is highly sensitive to protonation and complexation, is due to the strictly defined molecular conformation.

Intramolecular Triplet Energy Transfer in Donor–Acceptor Molecules Linked by a Crown Ether Bridge

Bichromophoric compounds BP-C-NP and BP-C-NBD were synthesized with benzophenone chromophore (BP) as the donor, and 2-naphthyl (NP) and norbornadiene group (NBD) as the acceptor, respectively. Their intramolecular triplet energy transfer was examined. The bridges linking the donor and acceptors in these molecules involve a crown ether moiety complexing a sodium ion. Phosphorescence quenching, flash photolysis and photosensitized isomerization experiments indicate that intramolecular triplet energy transfer occurs with rate constants of about 3.3 x 10(5) and 4.8 x 10(5) s(-1) and efficiencies of about 33 and 42 % for BP-C-NP and BP-C-NBD, respectively. Theoretical calculations indicate that these molecules adopt conformations below room temperature which allow their two-end chromophores conducive to through-space energy transfer.

Ab initio studies on excited state intramolecular electron transfer in 4‐amino‐N‐methylphthalimide and 3‐amino‐N‐methylphthalimide

AbstractThe molecules 4‐amino‐N‐methylphthalimide (4AMP) and 3‐amino‐N‐methylphthalimide (3AMP) are bichromophoric compounds composed of both electron donors and acceptor groups. They undergo intramolecular electron transfer (ET), and these types of compounds are frequently used to probe microenvironments in proteins, micelles, membranes, polymer, surfaces, etc. Ab initio calculations using restricted Hartree–Fock (RHF), Møller–Plesset second‐order perturbation theory (MP2) and the multi‐configurational self‐consistent field (MCSCF) methods with the 6‐31G** basis set have been performed to characterize the intermediates of the ET process. Analysis of geometrical and electronic parameters, e.g., changes in bond lengths, angles, dihedrals, and charge density, generally used to predict the ET pathway, is not always sufficient to understand the through‐bond intramolecular charge‐transfer (ICT) process. Hence we have attempted to support the predicted ET pathway in a very unique way by analyzing normal modes of vibrations for molecules in both the ground and excited states. Our results predict a planar ICT model that exhibits a conformational change of the amino nitrogen from nearly sp3 in the ground state to approximately sp2 type in the ICT state. A through‐bond ET occurs from the amino group of both molecules to the proximal carbonyl group in 4AMP and to the distant carbonyl group in 3AMP. Besides ET, the proximal carbonyl group in 3AMP also takes part in hydrogen bonding with the same amino group especially in excited state, which may also contribute toward ET. Such a hydrogen bond, which demands a planar amino group, is not observed in ground‐state 3AMP. These results indicate that nonplanar amino group geometry in the vicinity of a conjugated system is favored, rather than the conventional planar geometry, even in the presence of a suitable hydrogen bond acceptor. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Geometrical Effects on the Intramolecular Quenching of π,π* Aromatic Ketones by Phenols and Indoles

Laser flash photolysis of a series of bichromophoric compounds 1-12 containing the 2-benzoylthiophene (BT) and phenol (PhOH) or indole (InH) moieties has been used to determine the possible geometrical effects in the intramolecular quenching of triplet excited ketones, resulting in formal hydrogen abstraction. The results are compared with those obtained in the intermolecular process. In both cases, substitution either at the thienyl or the phenyl moiety has a marked influence on the photoreactivity. Time-resolved experiments showed that the rate constants for bimolecular quenching by phenol and indole of 2-benzoylthiophene substituted at the thienyl 5-position were lower than those for BT substituted at the phenyl p-position, which agrees with the higher energy found for the excited triplet state of the latter compounds. However, the rate constant for hydrogen abstraction in the bichromophoric compounds by the pi,pi* triplet state of the derivatives with the spacer linked to the thienyl 5-position are higher than those of their regioisomers. These results indicate a possible geometry-dependence in the intramolecular quenching process. Theoretical DFT studies have been carried out in order to estimate the optimum conformation for hydrogen abstraction in two pairs of phenolic and indolic bichromophoric regioisomers. The energy profile for photoactivation/deactivation of the aromatic ketone and the structures of the triplet states and biradicals involved in the process have been determined. The observed regiodifferentiation in the experimental studies is consistent with a dependence of the rate constant on orbital overlap between the carbonyl oxygen and the X-H bonds.

Intramolecular Triplet-Triplet Energy Transfer in Short Flexible Bichromophoric Amino Acids, Dipeptides and Carboxylic Acid Diester

Efficiencies of the intramolecular triplet-triplet energy transfer (ITET) in various bichromophoric amino acids (glycine, valine, phenylalanine, and sarcosine), dipeptides (glycylglycine, phenylalanylphenylalanine), and a simple diester, with the benzoyl and naphthyl terminal groups serving as donor and acceptor, respectively, have been determined by the steady-state photokinetic measurements. The magnitude of the transfer rate constants (>108 s-1) and the number of bonds separating the chromophores (8 or 11 atoms) suggest a through-space exothermic exchange mechanism in all cases. The influence of interchromophore distance, the character of the connecting chain as well as of side chains, was evaluated. While the most efficient energy transfer was found in a flexible diester and in valine- and sarcosine-based molecules due to the steric effect of the side hydrocarbon groups, the benzyl groups in the phenylalanine and phenylalanylphenylalanine-based bichromophores had a suppressing effect on ITET. Rigidity of the peptide bond in short bichromophoric compounds causes that a large number of favorable geometries preexist already before excitation; thus the intramolecular processes are controlled by ground-state conformational distribution. Replacing this bond by a less rigid ester moiety would allow that certain unfavorable conformations may coil to favorable ones within the excited-state lifetime (a rotation-controlled photochemical model). Some conclusions were supported by a conformational search of the potential energy surface and molecular dynamics simulations.

Stereodifferentiation in the decay of triplets and biradicals involved in intramolecular hydrogen transfer from phenols or indoles to pi,pi aromatic ketones.

Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds 1-6 have been used to investigate the possible chiral discrimination in the quenching of triplet excited ketones, resulting in formal hydrogen abstraction. Deuterium isotopic effects show that triplet deactivation in these bichromophores is dominated by hydrogen atom transfer. A remarkable stereodifferentiation is found in the intramolecular quenching of the ketone triplets of 1-3 and 5 by the phenolic or indolic moieties, either in methanol or acetonitrile as solvents. This indicates the existence of specific structural requirements for hydrogen transfer. On the other hand, the lifetimes of the generated biradicals show large solvent dependence; solvation appears to slow their reversion to the starting ketone. The considerable stereodifferentiation observed for the biradical lifetimes suggests that the kinetics of biradical decay is faster when the approach of the two radical termini becomes easier.