Coupled Oscillators for Tuning Fluorescence Properties of Squaraine Dyes

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.

Squaraine Appended with Benzodipyrrole for Fluorescent Sensing of Methanol: Exciton Coupling Controls Photophysical Properties

The photophysical properties of dyes composed of two squaraine chromophores fused with a benzodipyrrole central moiety (BS1 and BS2), were investigated using steady-state absorption, fluorescence, and transient absorption spectroscopy. The dyes exhibit solvent-independent split electronic absorption due to exciton-coupling. Interestingly significant solvent-dependent fluorescence properties were observed. In toluene, they emit from the lowest excited state, while in methanol, they show weak emission in the higher energy region. In the low-temperature glass matrix, emission from the lowest excited state dominates similarly to that in toluene. The transient absorption spectra exhibit similar ground-state bleaching in toluene and methanol, revealing the formation of delocalized excited states by exciton coupling independent of solvent. However, the excited state deactivates rapidly in ultrafast time scale in methanol, likely due to solvent interaction, leading to rapid non-radiative deactivation. The PEG film doped with the exciton-coupled bis-squaraine shows a distinct fluorescence response to methanol vapor.

Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads

Strong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, and fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.

Ultrafast Intermolecular Interaction Dynamics between NIR-Absorbing Unsymmetrical Squaraines and PCBM: Effects of Halogen Substitution

Among near-infrared (NIR) dyes, squaraine derivatives are applied as efficient sensitizers in optoelectronic and biomedical devices due to their simple synthesis, intense absorption, and emission and exceptional photochemical stability. The fundamental understanding of the structure-property relationships of sensitizers provides the insight to increase the efficiency of such devices. Here, unsymmetrical squaraine derivatives (ABSQs) with donor-acceptor-donor (D-A-D') architectures having N,N-dimethyl amino anthracene and benzothiazole (ABSQ-H) halogenated with fluoride (ABSQ-F), chloride (ABSQ-Cl), and bromide (ABSQ-Br) were synthesized to understand the effect of halogen on the photophysical properties and intermolecular interaction dynamics with phenyl-C₆₁-butyric acid methyl ester (PCBM), which is used widely as an electron acceptor in bulk heterojunction-based devices. Interestingly, ABSQ-H exhibited intense absorption (ε ∼ 6.72 × 10⁴ M^-1 cm^-1) spectra centered at ∼660 nm. Upon halogen substitution, a bathochromic shift in the absorption spectra with an increase of molar absorptivity was observed (ε ∼ 8.59 × 10⁴ M^-1 cm^-1), which is beneficial for NIR light harvesting. The femtosecond transient absorption spectra of ABSQs revealed that the polarity of the solvent controlled the excited-state relaxation dynamics. Upon addition of PCBM, the fluorescence intensity and dynamics of halogenated ABSQs were quenched, and the formation of a squaraine radical cation was observed, reflecting the occurrence of intermolecular charge-transfer dynamics between ABSQs and PCBM. Thus, the observation of a bathochromic shift with intense absorption and an efficient intermolecular interaction with PCBM upon halogenation of ABSQs provide a design strategy for the development of unsymmetrical squaraine derivatives for bulk heterojunction-based optoelectronic devices.

Transforming Dyes into Fluorophores: Exciton-Induced Emission with Chain-like Oligo-BODIPY Superstructures

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a "masked" fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.

Synthesis, Photophysical and Electrochemical Properties of Bis-Squaraine Dyes Fused on Isomeric Benzodipyrrole Central Units

Exciton interactions are not only observed in assembled molecules but also in compounds with multiple chromophores referred to as superchromophores. We have developed isomeric bis-squaraine dyes as superchromophores in which two squaraine chromophores are fused onto the isomeric benzodipyrrole skeleton so as to regulate conformations and to reduce distances between two chromophores. The dyes with benzo[1,2-b:3,4-b']dipyrrole and benzo[1,2-b:5,4-b']dipyrrole moieties exhibited split electronic absorption originated from the intramolecular exciton interaction. The intensity of the split absorption bands varies in correlation with the orientation of chromophores. The isomeric dye with benzo[1,2-b:4,5-b']dipyrrole moiety exhibited a near-infrared absorption associated with the resonance throughout two chromophores. Their electrochemical and spectroelectrochemical properties are distinct from those of monomeric dyes owing to electronic interactions between the two chromophores. Thus, the structural isomerism of the central skeleton significantly affects their optical properties as well as their electrochemical properties.

Chiral NADH Model: Design, Synthesis, Asymmetric Reduction Reaction and Fluorescence Characteristics

Abstract:

A new type of NADH model compound, which has six asymmetric carbon centers, has been synthesized by an efficient and convenient method. We preliminarily attempted that NADH models combined with enzyme were used in the asymmetric reduction. It is the first time that dehydrogenase with NADH models instead of inorganic catalysts such as MgCl2 were used in the asymmetric reduction. Furthermore, the experimental results showed that the new NADH model has the strongest fluorescence emission properties compared with previous reported models.

Rational designs of structurally similar TADF and HLCT emitters with benzo- or naphtho-carbazole units as electron donors

Structurally similar D–A type molecules with the combination of benzo- or naphtho-carbazole units as electron donors and tunable electron acceptors with different electron-withdrawing ability are designed to realize HLCT and TADF emissions.

NIR-emitting benzene-fused oligo-BODIPYs for bioimaging

Near-infrared (NIR) fluorophores are emerging tools for biophotonics because of their reduced scattering, increased tissue penetration and low phototoxicity. However, the library of NIR fluorophores is still limited. Here, we report the NIR fluorescence of two benzene-fused oligo-BODIPYs in their hexameric (H) and octameric (O) forms. These dyes emit bright NIR fluorescence (H: maxima 943/1075 nm, O: maxima 976/1115 nm) that can be excited in the NIR (H = 921 nm, O = 956 nm) or non-resonantly over a broad range in the visible region. The emission bands of H show a bathochromic shift and peak sharpening with increasing dye concentration. Furthermore, the emission maxima of both H and O shift up to 20 nm in solvents of different polarity. These dyes can be used as NIR ink and imaged remotely on the macroscopic level with a stand-off distance of 20 cm. We furthermore demonstrate their versatility for biophotonics by coating microscale beads and performing microrheology via NIR video particle tracking (NIR-VPT) in biopolymer (F-actin) networks. No photodamaging of the actin filaments takes place, which is typically observed for visible fluorophores and highlights the advantages of these NIR dyes.

Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm^-1) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

Extended Benzene-Fused Oligo-BODIPYs: In Three Steps to a Series of Large, Arc-Shaped, Near-Infrared Dyes

We present a straightforward, three-step synthesis engaging an oligomerization and subsequent one-pot oxidation step to form fully conjugated, benzene-fused oligo-BODIPYs from simple BODIPY precursors. FeCl3 serves as an efficient, bifunctional oxidant for a (multiple) cyclization/desaturation process, applied to ethylene-bridged dimeric, trimeric and oligomeric species to transform linking ethano units into stiff benzene fusions between unsubstituted β-positions of each BODIPY unit. The structural integrity was verified by X-ray crystallography, and all target compounds were studied in detail by photophysical, electrochemical and computational means. The main S1 excited state gradually converges to a structure-specific excitation limit, displaying a strong shift of the absorption event from about 500 nm (BODIPY monomer) to 955 nm (octamer) with attenuation coefficients up to ca. 500 000 M-1  cm-1 .

New Design Strategy Toward NIR I Xanthene-Based Dyes

An effective design strategy with an efficient synthetic route to xanthene-based far-red to near-infrared dyes is reported. The dyes were prepared by the Suzuki cross-coupling of the electron-poor fluorescein ditriflate with the electron-rich boronic acid/ester-functionalized pyrrole (2C/3C) and indole (2D/3D) moieties. Upon treatment with trifluoroacetic acid, the closed nonfluorescent forms of the dyes (2C and 2D) ring-opened to their fluorescent forms (3C and 3D). The absorption maxima were 665 and 704 nm, while the emission maxima were 717 and 719 nm for 3C and 3D, respectively. The closed forms of the dyes were soluble in chloroform and acetonitrile. To test the efficacy of the dyes as probes, a turn-off fluoride ion probe was prepared from 3C, which consisted of a silyl ester receptor. The probe responded strongly to low concentrations of fluoride, carbonate, and acetate ions, weakly to phosphate ions, but not to the other halogens. Moreover, the probe can detect the minimum concentration of F^- in water.

Donor-Acceptor-Donor NIR II Emissive Rhodindolizine Dye Synthesized by C-H Bond Functionalization

A NIR II emissive dye was synthesized by the C-H bond functionalization of 1-methyl-2-phenylindolizine with 3,6-dibromoxanthene. The rhodindolizine (RhIndz) spirolactone product was nonfluorescent; however, upon opening of the lactone ring by the formation of the ethyl ester derivative, the fluorophore absorbs at 920 nm and emits at 1092 nm, which are both in the NIR II region. In addition, 4-cyanophenyl- (CNRhIndz) and 4-methoxyphenyl-substituted rhodindolizine (MeORhIndz) could also be prepared by the C-H activation reaction.

Bis(merocyanine) Hetero-Folda-Dimers: Evaluation of Exciton Coupling between Different Types of π-Stacked Chromphores

Exciton coupling between different types of chromophores has been rarely investigated. Herein, a systematic study on the exciton coupling between merocyanine chromophores of different conjugation length with varying excited state energies is presented. In this work well-defined hetero-dimer stacks were obtained upon folding of bis(merocyanine) dyes in nonpolar solvents. They show distinctly different absorption properties in comparison with the spectra of the single chromophores, revealing a significant coupling between the different chromophores. The simulated absorption spectra obtained from time-dependent density functional theory (TD-DFT) calculations are in good agreement with the experimental spectra. Our theoretical analysis based on an extension of Kasha's exciton theory discloses strong coupling between the dyes' transition dipole moments despite of an excited-state energy difference of 0.60 eV between the chromophores.

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.

Strategic tuning of excited-state properties of electroluminescent materials with enhanced hot exciton mixing

Deep blue emitters with excellent stability, high quantum yield and multifunctionality are the major issues for full-color displays. In line with this, new multifunctional, thermally stable blue emitters viz., N-(4-(10-(1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)anthracen-9-yl)phenyl)-N-phenylbenzenamine (DPIAPPB) and 2-(10-(9H-carbazol-9-yl)anthracen-9-yl)-1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-1H-phenanthro[9,10-d]imidazole (CADPPI) with hybridized local charge transfer state (HLCT) and hot exciton properties have been synthesized. These molecules show high photoluminescence quantum yield (Φ_s/f): (DPIAPPB – 0.82/0.70 and CADPPI – 0.91/0.83). The CADPPI based device (EL – 467 nm) shows high efficiencies [η_c – 9.85 cd A⁻¹; η_p – 10.84 lm W⁻¹; η_ex – 4.78% at 2.8 V; CIE (0.15, 0.10)] compared to the DPIAPPB device (EL − 472 nm) [η_c – 6.56 cd A⁻¹; η_p – 6.16 lm W⁻¹; η_ex – 4.15% at 2.8 V with CIE (0.15, 0.12)]. The green device with CADPPI:Ir(ppy)₃ exhibits a maximum L – 59 012 cd m⁻²; η_ex – 16.8%; η_c – 37.3 cd A⁻¹; η_p – 39.8 lm W⁻¹ with CIE (0.30, 0.60) and the red device with CADPPI:Ir(MDQ)₂(acac) shows a maximum L – 43 456 cd m⁻²; η_ex – 21.9%; η_c – 36.0 cd A⁻¹; η_p – 39.6 lm W⁻¹ with CIE (0.64, 0.35).

The CADPPI:Ir(ppy)₃ device exhibits L – 90 12 cd m⁻²; η_ex – 18.8%; η_c − 27.3 cd A⁻¹; η_p – 29.8 lm W⁻¹; CIE (0.30, 0.60).

Near-Infrared-Absorbing Metal-Free Organic, Porphyrin, and Phthalocyanine Sensitizers for Panchromatic Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near-infrared (NIR) spectral region. Metal-free organic sensitizers frequently employ strong electron-donating or -withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge-transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal-free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

Alternative fluorophores designed for advanced molecular imaging

Fluorescent molecular imaging has advanced drastically over the past decade. With the development of high-resolution microscopy techniques and the ability to visualize intracellular molecular events, there is a growing need for new fluorophores to accompany these fast-developing techniques. Therefore, there has been substantial development of alternative fluorophores for single-molecule detection and molecular imaging. These rationally designed fluorophores have infinite possibilities and novel fluorophores are constantly being produced for different applications. This review focuses on the recent developments in novel fluorophores designed for molecular imaging and single-molecule detection. Here, single-molecule imaging, smart fluorescent probes, two-photon microscopy, Förster resonance energy transfer (FRET) and super-resolution microscopy are discussed in detail.

Ethylene-Bridged Oligo-BODIPYs: Access to Intramolecular J-Aggregates and Superfluorophores

A versatile and rapid access to various chain lengths of ethylene-bridged BODIPY motifs was discovered. Corresponding oligomers comprising up to eight monomeric units were studied with respect to their microstructures by photophysical, X-ray crystallographic, and computational means. The investigation of three different dipyrrin cores revealed a crucial dependence on the substitution pattern of the core, whereas the nature of the meso-periphery is less critical. The impact of substituent effects on the conformational space was investigated by Monte Carlo simulations and a set of DFT methods (B3LYP, PBEh-3c, TPSS/PWPB95), including dispersion effects. Cryptopyrrole-derived oligo-BODIPYs are characterized by a tight intramolecular arrangement triggering a dominant J-type excitonic coupling with red-shifts up to 45 nm, exceptionally small line widths of the absorption and emission event (up to 286 cm^-1), outstandingly high attenuation coefficients (up to 1 042 000 M^-1 cm^-1), and quantum yields of up to unity.

Asymmetric Dipyrrin and F-BODIPYs Conjugated to Terminal Alkynes and Alkenes

An asymmetric meso-H dipyrrin featuring a conjugated terminal alkyne substituent was converted to its corresponding difluoro boron complex, and the extent of π-conjugation was extended using Sonogashira cross-coupling. Treatment of the alkyne-substituted dipyrrin with BF₃·OEt₂ and NEt₃ revealed the reactivity of the conjugated terminal alkyne toward Lewis-activated electrophilic substitution and led to the isolation of F-BODIPYs bearing terminal bromovinyl and enol substituents.

J-Type Heteroexciton Coupling Effect on an Asymmetric Donor-Acceptor-Donor-Type Fluorophore

The novel donor-acceptor-donor (D-A-D)-type fluorophore with an asymmetric structure is reported. The twisted-induced charge transfer (TICT) luminescence was observed. The degree of charge transfer and radiative rate constant in the luminescence increased simultaneously with increase in orientational polarizability of solvents. In contrast to the numerous CT fluorophore researches, this behavior has never been previously observed. This characteristic behavior reveals the existence of an effective exciton coupling between the CT states in the donor-acceptor-donor-type fluorophore for the first time.

Cucurbit[8]uril Regulated Activatable Supramolecular Photosensitizer for Targeted Cancer Imaging and Photodynamic Therapy

Activatable photosensitizers (aPSs) have emerged as promising photodynamic therapy (PDT) agents for simultaneous imaging and selective ablation of cancer. However, traditional synthetic aPSs are limited by complex design and tedious synthesis. Here, aPS regulated by cucurbit[8]uril (CB[8]) for targeted cancer imaging and PDT is reported. This system is based on the host-guest interaction between biotinylated toluidine blue (TB-B) and CB[8] to form 2TB-B@CB[8]. Moreover, a facile strategy to turn off/on the fluorescence and photodynamic activity of TB-B is developed through the reversible assembly/disassembly of 2TB-B@CB[8]. This established system can achieve selective accumulation in tumor, light-up cancer imaging, and enhanced anticancer behavior. Therefore, this work provides a novel and promising strategy for the aPS build via simple and facile regulation of supramolecular chemistry.

Decoupling Charge Transport and Electroluminescence in a High Mobility Polymer Semiconductor

Fluorescence enhancement of a high-mobility polymer semiconductor is achieved via energy transfer to a higher fluorescence quantum yield squaraine dye molecule on 50 ps timescales. In organic light-emitting diodes, an order of magnitude enhancement of the external quantum efficiency is observed without reduction in the charge-carrier mobility resulting in radiances of up to 5 W str(-1) m(-2) at 800 nm.

Hybridization and de-hybridization between the locally-excited (LE) state and the charge-transfer (CT) state: a combined experimental and theoretical study

Gradual evolution of hybridization process between the LE and CT states is investigated in a series of TPA–PI (H_xV_y) derivatives by TD-DFT method.

Localised and delocalised excitons in star-like squaraine homo- and heterotrimers

Steady state and time resolved spectroscopy shows localisation and delocalisation of excitons in star-like squaraine trimers which are formed by the combination of two different squaraines.

Cooperative enhancement versus additivity of two-photon-absorption cross sections in linear and branched squaraine superchromophores

Our investigation of the nonlinear optical properties of a series of oligomeric squaraine dyes showed enhanced cross sections for linear oligomers but only additivity for branched systems.

On the relation of energy and electron transfer in multidimensional chromophores based on polychlorinated triphenylmethyl radicals and triarylamines

Chromophores with many donors and acceptors show electron transfer which is identical to energy transfer.