Intramolecular Excitation Energy Research Articles

Viscosity-modulated intramolecular excitation energy transfer for mitochondria-targeted sensing and photokilling

Viscosity is a key parameter of mitochondria that related to several diseases. To design a probe acting as both the mitochondria-targeted viscosity sensor and photosensitizer is meaningful for diagnosis and therapies. Herein, we report a ratiometric viscosity sensor with dual-emission mediated by excitation energy transfer (EET) effect, which can detect the mitochondrial viscosity and serve as an efficient photosensitizer to eradicate cancer cells. The probe comprises of a blue-emitting imidazole moiety as the energy donor and a green-emitting boron dipyrromethene (BODIPY) core as the energy acceptor, respectively. Via viscosity-modulated EET effect, the dual-emissive probe can sensitively detect the medium viscosity in a ratiometric model. Directed by imidazole as the targeting factor, the probe can specifically accumulate in mitochondria and ratiometrically determine the intracellular viscosity increase induced by Nystatin treatment. Furthermore, it also exhibits light-triggered reactive oxygen species (ROS) generation including singlet oxygen and superoxide radical, which can efficiently kill the cancer cells under white light illumination. Thus, this work provides a versatile and promising candidate for disease diagnosis and phototherapy.

Synthesis of chlorophyll-BODIPY conjugates and their intramolecular excitation energy transfer

Zinc 3-hydroxymethyl-pyropheophorbides-[Formula: see text] covalently linked with an 8-aryl-1,3,5,7-tetramethyl-BODIPY moiety in the 17-substituent were synthesized by chemically modifying naturally occurring chlorophyll(Chl)-[Formula: see text]. The synthetic Chl–BODIPY conjugates in tetrahydrofuran exhibited sharp visible absorption and weak circular dichroism spectra, indicating that their molecules were fully dispersed in the solution. In their ground states, no intramolecular interaction between the Chl and BODIPY parts was observed. By contrast, singlet excitation energy was efficiently transferred from the photoexcited BODIPY moiety to zinc chlorin moiety within their molecules. The intramolecular energy transfer efficiencies exceeded 90% and were independent of the linkages including ester and amide bonds as well as methylene and hexamethylene spacers.

Intramolecular Photoinduced Energy and Electron Transfer Reactions in Phenanthroimidazole-Boron Dipyromethane Donor-Acceptor Dyads.

Donor-acceptor systems in which a donor phenanthroimidazole (PhI) is directly connected to a BODIPY acceptor (Dyad1) and separated by an ethynyl bridge between PhI and BODIPY (Dyad2) have been designed, synthesized, and characterized by various spectroscopic and electrochemical techniques. Optical absorption and 1H NMR characteristics of both dyads with those of constituent individuals suggest that there exists a minimum π-π interaction between phenanthroimidazole and BODIPY. Quenched emission of both the dyads was observed when excited either at phenthaoimidazole absorption maxima or at BODIPY absorption maxima in all three investigated solvents. The detailed spectral analysis provided evidence for an intramolecular photoinduced excitation energy transfer (PEnT) from the singlet excited state of phenanthroimidazole to BODIPY and photoinduced electron transfer (PET) from the ground state of phenanthroimidazole to BODIPY. Transient absorption studies suggest that charge-separated species (PhI•+ - BODIPY•-) are generated at a rate constant of (1.16 ± 0.01) × 108 s-1 for the dyads Dyad1 and (5.15 ± 0.03) × 108 s-1 and for Dyad2 whereas energy transfer rate constants were much higher and were on the order of (1.1 ± 0.02) × 1010 s-1 and (1.6 ± 0.02) × 1010 s-1 for Dyad1 and Dyad2, respectively, signifying their usefulness in light energy harvesting applications.

Dual Chromophoric Meso‐Aryl BODIPYs with Aggregation Induced Red‐Emission Characteristics: Optical Properties and their Application in Fe3+ Detection

AbstractHerein, we report the design of meso‐aryl BODIPYs as a structural motif for aggregation‐caused quenching (ACQ) to aggregation‐induced emission (AIE) transformation. A series of meso‐aryl BODIPY derivatives were synthesized, by systematically increasing the size of the chromophore at the meso‐position from phenyl to pyrene. The effect of various factors, such as the aryl ring size, solvents, viscosity, and metal cations, on the photophysical properties was analyzed. The emission properties are well correlated with the flexibility of the aromatic ring for free rotation around the Caryl−CBODIPY bond. Accordingly, meso‐phenanthrene BODIPY (PhB) has the highest emission characteristics. The emission property of less bulky aryl‐substituted BODIPYs increases by increasing the solvent viscosity. The interaction of Fe3+ ions with aryl‐BODIPYs provides a prominent photophysical response based on Lewis‐acid supported decomplexation of BF2 in aryl‐BODIPYs. The bichromophoric meso‐aryl BODIPYs exhibit notable intramolecular excitation energy transfer from the aromatic ring to the BODIPY core, which is higher in meso‐anthracene BODIPY(AB). Hence, decorating BODIPYs with polycyclic aromatic systems generates a twisted structure, which inhibits the π‐π stacking between the planar aromatic molecules. This can be proposed as an effective approach at the molecular level to convert planar aryl luminophores having ACQ to AIEgens. Besides, the meso‐pyrene BODIPY derivative shows excellent mechanofluorochromic behaviour.

Intersystem crossing and intramolecular triplet excitation energy transfer in spiro[9,10-dihydro-9-oxoanthracene-10,2′-5′,6′-benzindan] investigated by DFT/MRCI methods

Recent experimental studies of a spiro-linked anthracenone (A)–naphthalene (N) compound (AN) in butyronitrile (BuCN) solution (Dobkowski et al., J. Phys. Chem. A 2019, 123, 6978) proposed an excited-state energy dissipation pathway [1ππ* (N) + 1ππ*(A)]↝1 nπ* (A)↝3 nπ* (A)↝3ππ*(N). However, a detailed theoretical study employing combined density functional theory and multireference configuration interaction methods, performed in the present work, suggests that the photoexcitation decay follows a different pathway. In BuCN solution, the intersystem crossing (ISC) follows the well-established El-Sayed rule and involves the 3ππ*(A) state which is found to be the lowest excited triplet state localized on the anthracenone moiety. Because the Dexter triplet excitation energy transfer (TEET) to the first excited triplet state of the naphthalene subunit is forbidden in C2 v symmetry, it is mandatory to go beyond the Condon approximation in modeling this process. Nonadiabatic coupling matrix elements were computed to obtain a TEET rate different from zero. Our calculations yield time constants of 5 ps for the 1 nπ* (A)↝3ππ* (A) ISC and of 3 ps for the subsequent 3ππ* (A)↝3ππ* (N) TEET in BuCN whereas the energy dissipation involving the 3 nπ*(A) state as an intermediate occurs on a much longer time scale.

Enhancement of polarization anisotropy in ultrafast carrier dynamics by intramolecular excitation in the organic conductor κ-(BEDT- TTF)2Cu(NCS)2

In this work, in order to investigate the origin of the anomalous polarization dynamics observed in the normal conducting state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, we performed optical pump probe measurements by varying the probe pulse energy from to 1.59 eV. We found that the polarization-dependent transient signal was most enhanced at , which was close to the intramolecular excitation energy of the BEDT-TTF molecules but is not exactly matched. This difference is linked to the formation of an energy gap at the Fermi energy at low temperatures, which results in an increase or decrease in pump-induced absorption between the states near the Fermi energy and the intramolecular excited states depending on probe polarization. In other words, the intramolecular excitation at 1.41 eV, which has a strong polarization dependence, serves as a probe of the dynamics involved in gap formation. A simple calculation shows that the size of the gap is , which is close to that of the BEDT-TTF based Mott insulators. It is suggested that the gap arises from the phase-separated Mott state caused by photoinduced modulation of effective electron correlation.

Computational Study on the Charge Transport and Optical Spectra of Anthracene Derivatives in Aggregates.

A recent experiment [Angew. Chem. Int. Ed. 2017, 56, 722-727] found that a (1 : 9) blend film of two anthracene derivatives, 2-fluorenyl-2-anthracene (FlAnt) and 2-anthryl-2-anthracence (2 A), exhibit both efficient white light emission and high hole mobility, thus promising for organic light-emitting transistors (OLETs). Employing quantum chemistry at the polarizable continuum model (PCM) and the quantum mechanics/molecular mechanics (QM/MM) levels, we investigated the excited-state structures, optical spectra, band structure and the carrier mobility for FlAnt and 2 A from solution to aggregate phases. We suggest using the ratio of intermolecular excitonic coupling J and intramolecular excited state relaxation energy E to judge the bathochromic shift in optical emission in aggregates. For FlAnt, ρ=J/E is calculated to be less than 0.17, a critical value we identified earlier, and the spectra in solution and aggregate phases present quite similar features (blue emission). However, ρ is ∼0.5 for 2 A systems, and the calculated emission in the aggregate phase exhibits a remarkable bathochromic shift. In addition, the 0-0 emission is strongly suppressed in the herringbone stacking. These observations justify the experimental findings that (i) 2 A is blue emissive in solution but yellow-green in the aggregate phase, whereas FlAnt is always blue, and (ii) the blend of them show white emission. By using the "quantum nuclear tunneling" model we proposed earlier, we found the hole mobility for FlAnt and 2 A are 0.5 and 4.2 cm2 V-1 s-1 , respectively, indicating both are good hole transport materials.

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

AbstractInvitedInvited for the cover of this issue are AnneàKathrine Junker and ThomasàJustàSørensen from University of Copenhagen, Denmark. The cover image shows the fireworks of lanthanide luminescence and illustrates that intramolecular excited state energy transfer happens with a rate constant similar to that of the intermolecular process.

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

Front Cover: Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions (Eur. J. Inorg. Chem. 9/2019)

Illuminating the Intermolecular vs. Intramolecular Excited State Energy Transfer Quenching by Europium(III) Ions

Bright lanthanide‐based probes rely on sensitised lanthanide luminescence. The outcome of the excited state energy transfer cascade – from sensitiser chromophore to lanthanide(III) ion – should be the population of the lanthanide excited state with an efficiency equal to unity. This is rarely the case, and this is a particular problem for europium(III) due to the low energy of the Eu(II)/Eu(III) redox couple. Thus, photoinduced electron transfer quenching can be an efficient drain for the excited state energy, reducing the brightness of a probe. Here, the effect of intramolecular PeT in a lanthanide‐based probe is investigated by contrasting intramolecular processes in coumarin‐appended 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetate (DO3A) complexes with intermolecular processes induced through collisional quenching. We find that the excited energy transfer processes involving terbium(III) and europium(III) are slow in the complex when compared to collisional quenching. Further, we show that the quenching event may lead to lanthanide luminescence and suggest that the fate of a PeT quenching event may not be non‐radiative loss of the excitation energy.

Thermoactivated heat transfer mechanism in molecular crystals: Thermal conductivity of benzophenone single crystals

Thermal conductivities of two benzophenone single crystals have been measured at temperatures from 4.7 to 270 K. The experimental data for both are consistent for temperatures above 15 K. The thermal conductivity of benzophenone can be represented as a sum of two contributions: κ1 + κTA where κ1 is due to the standard phonon mechanisms accepted for ordered crystals and κTA takes into account the heat flow due to intermolecular hopping of thermally activated intramolecular vibrational modes. The thermal activation contribution in unsubstituted benzophenone is substantially smaller when compared to that in any of the two para-bromobenzophenone polymorphs studied previously. Unlike in the 4-bromobenzophenone crystals, the microscopic agent responsible for intramolecular excitation(s) was not determined. The characteristic intramolecular excitation energy was evaluated to be 220 K, about three times less compared to 4-bromobenzophenone.

Establishing the Critical Role of Oriented Aggregation in Molecular Solid State Fluorescence Enhancement.

The general occurrence of fluorescence emission quenching in molecular aggregates is circumvented in select classes of molecules. This has largely been attributed to the rigidification of the molecule and its environment, which hinders non-radiative excited state energy loss through structural relaxation; since such an effect should in principle apply to most aggregates and crystals, there must clearly be other critical factors that make the select molecules exceptional. Discovery of three crystalline structures of a new push-pull molecule in its enantiomorphic and racemic forms, exhibiting not only very high, but distinctly different solid state fluorescence enhancements, has now allowed a systematic investigation of the role of intramolecular and intermolecular excited state energy loss pathways. Crystallographic, spectroscopic and computational investigations provide a detailed appraisal of the assembly patterns in the crystals, and rigorous establishment of an inverse correlation between intermolecular energy transfer and solid state fluorescence. The study provides a clear visualization of the critical role of oriented molecular aggregation in solid state fluorescence efficiency enhancement.

Dynamics of Intramolecular Energy Hopping in Multi-Bodipy Self-Assembled Metallocyclic Species: A Tool for Probing Subtle Structural Distortions in Solution

The intramolecular excitation energy transfer (EET) processes in a series of fluorescent-unquenched, self-assembled metallocycles consisting of spatially fixed-separated and parallel-aligned Bodipy chromophores, are investigated here by steady-state and femtosecond-fluorescence upconversion measurements in the solution phase. These multi-Bodipy macrocycles, namely, the rhomboid (A1), the tetragon (A2) and the hexagon (A3), are formed via temperature-regulated Pt(II)–pyridyl coordination and consist, respectively, of two, four, and six Bodipy subunits, which are locked at the corners and aligned with their long molecular axes perpendicular to the rigid polygonal frame formed by the alternating B···Pt(II) connectivities. Extensive simulations and fits to the experimental fluorescence anisotropy decays r(t) show that EET within the cyclic scaffolds is quite uniform and much faster than the intrinsic decay rate of the Bodipy’s. The equalization of the excitation survival probabilities over time of all chromop...

Triphenylamine corrole dyads: Synthesis, characterization and substitution effect on photophysical properties

We present our results on the effect of substitution on the photophysical properties of donor-acceptor (D-A) systems in which triphenylamine is the donor and substituted corroles i.e., 5,15-phenyl-10-triphenylaminecorrole TPACor 1, 5,15-di(3,5-ditertbutylphenyl)-10-triphenylaminecorrole TPACor 2, and 5,15-(4-nitrophenyl)-10-triphenylaminecorrole TPACor 3 is the acceptor. All three dyads have been characterized by elemental analysis, MALDI-MS, cyclic voltammetry, UV-Vis and fluorescence (steady state and time-resolved) spectroscopies. Both Soret and Q bands of TPACor 3 are red-shifted when compared to other two dyads due to the presence of electron withdrawing nitro group. Similarly, redox properties of TPACor 3 are altered, when correlated to TPACor 1 and TPACor 2 dyads. However, the fluorescence emission of triphenylamine in all three dyads was quenched significantly (>90%) compared to its monomeric unit. The presence of either electron releasing or electron withdrawing group on corrole moiety has not much effect on the photophysical properties. The quenched emission was attributed to intramolecular excitation energy transfer and the photoinduced electron transfer reactions contested in these dyads. Intramolecular photoinduced reactions were monitored in three 5,15-diaryl-10-triphenylaminecorroles, represented as TPACor 1 (phenyl), TPACor 2 (3,5-ditertiarybutylphenyl), and TPACor 3 (p-nitrophenyl). In these dyad systems, energy transfer from triphenylamine to corrole is the major pathway, which is explained with optical, electrochemical, and DFT studies.

Light induced intramolecular electron and energy transfer events in rigidly linked borondipyrromethene: Corrole Dyad

We have designed and synthesized a photo-induced energy/electron donor–acceptor conjugate comprising of corrole linked to BODIPY at the 5-position via ester linkage. The dyad was characterized by elemental analysis, MALDI-MS, UV-Visible, 1H NMR fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical methods. A comparison of the UV–visible and 1H NMR spectra of the dyad with those of the corresponding individual model compounds (i.e., BODIPY-CO2H and BPFC-OH) reveal that there exist minimum π–π interactions between BODIPY and corrole π-planes. Quenched emission of BODIPY and corrole part of the dyad has been observed in five different solvents. Excitation spectral data provided evidence for an intramolecular excitation energy transfer (EET) from the singlet BODIPY to the corrole and an intramolecular photoinduced electron transfer (PET) from singlet state of corrole to ground state of BODIPY. Detailed analysis of the data suggests that Forster׳s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET.

Boron Arylations of Subporphyrins with Aryl Zinc Reagents.

Boron arylations of B-(methoxo)triphenylsubporphyrin have been developed with a combined use of ArZnI⋅LiCl and trimethylsilyl chloride. Aryl zinc reagents bearing bromo, cyano, amide, and ester groups can be employed for the B-arylation reaction to provide the corresponding B-arylated subporphyrins in moderate yields. Postmodifications of B-arylated subporphyrins have been demonstrated without loss of the B-C bond. These modifications include conversion of the cyano group into a benzoyl group with PhMgBr, hydrolysis of the ester group to give B-(4-carboxyphenyl)subporphyrin, and Pd-catalyzed Suzuki-Miyaura coupling of the 4-bromophenyl group to give a 1,4-phenylene-bridged subporphyrin-ZnII porphyrin hybrid that displays intramolecular excitation energy transfer from the subporphyrin to the porphyrin. The newly synthesized B-arylated subporphyrins have been fully characterized by NMR, UV/Vis absorption and fluorescence spectroscopies, mass spectrometry, electrochemical measurements, and X-ray diffraction analysis.

Cyclic 2,12-porphyrinylene nanorings as a porphyrin analogue of cycloparaphenylenes.

β-to-β Directly linked cyclic Ni(II) porphyrin trimer, tetramer, and pentamer ([3]CP, [4]CP, and [5]CP) have been synthesized by reaction of a 2,12-diborylated Ni(II) porphyrin with Pt(cod)Cl2 followed by reductive elimination. The structures of these cyclic porphyrin arrays have been revealed by X-ray diffraction analysis. The strain energies of these cyclic oligomers are calculated to be 77, 57, and 47 kcal/mol for [3]CP, [4]CP, and [5]CP, respectively. Intramolecular excitation energy hopping was observed between the (3)(d,d) states of the Ni(II) porphyrins with rates of 3.0, 4.4, and 4.6 ps for [3]CP, [4]CP, and [5]CP, respectively, reflecting the close proximity of the Ni(II) centers.

Photoinduced intramolecular reactions in triphenylamine–corrole dyads

Abstract Donor–Acceptor systems, in which a donor triphenylamine (TPA) directly connected to a corrole (Cor) acceptor (TPA-Cor) and separated by an ethynylphenyl bridge between TPA and Cor (TPA-E-Cor) have been designed, synthesized and fully characterized by elemental analysis, MALDI-MS, UV–Visible and 1H NMR spectroscopic methods. A comparison of the UV–Visible and 1H NMR features of these D–A systems with those of the corresponding individual model compounds (i.e., TPA and Cor) reveal that there exist minimum π–π interactions between triphenylamine and corrole π-planes. Quenched emission of triphenylamine (but not corrole) part of both the dyads have been observed in three different solvents. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet triphenylamine to the corrole. Detailed analysis of the data suggests that Forster’s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET.

Two-photon antenna-core oxygen probe with enhanced performance.

Recent development of two-photon phosphorescence lifetime microscopy (2PLM) of oxygen enabled first noninvasive high-resolution measurements of tissue oxygenation in vivo in 3D, providing valuable physiological information. The so far developed two-photon-enhanced phosphorescent probes comprise antenna-core constructs, in which two-photon absorbing chromophores (antenna) capture and channel excitation energy to a phosphorescent core (metalloporphyrin) via intramolecular excitation energy transfer (EET). These probes allowed demonstration of the methods’ potential; however, they suffer from a number of limitations, such as partial loss of emissivity to competing triplet state deactivation pathways (e.g., electron transfer) and suboptimal sensitivity to oxygen, thereby limiting spatial and temporal resolution of the method. Here we present a new probe, PtTCHP-C307, designed to overcome these limitations. The key improvements include significant increase in the phosphorescence quantum yield, higher efficiency of the antenna-core energy transfer, minimized quenching of the phosphorescence by electron transfer and increased signal dynamic range. For the same excitation flux, the new probe is able to produce up to 6-fold higher signal output than previously reported molecules. Performance of PtTCHP-C307 was demonstrated in vivo in pO2 measurements through the intact mouse skull into the bone marrow, where all blood cells are made from hematopoietic stem cells.

Intramolecular photoinduced reactions in corrole–pyrene and corrole–fluorene dyad systems

We have designed and synthesized donor–acceptor conjugates in which donor polycyclic aromatic hydrocarbons such as either pyrene or fluorene linked at the pyrrole-β position of a corrole using vinylene spacer. Both the dyads are characterized by elemental analysis, MALDI-MS, 1H NMR, UV–vis and fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical method. The Soret absorption band of corrole was split in both the dyads and more over both Soret and Q bands are red-shifted by 15–25 nm as compared to the unsubstituted corrole. Ground state properties indicate that there is a moderate π–π interactions in these dyad systems. However, fluorescence emission of polycyclic aromatic hydrocarbons of both the dyads quenched significantly (88–98%) compared to their monomeric units. The quenched emission was attributed in terms of intramolecular excitation energy transfer, which competes with the photoinduced electron transfer reaction in these dyads.