New Two-Photon Absorbing Fluorene Derivatives: Synthesis and Nonlinear Optical Characterization

Degenerate and non-degenerate two-photon absorption of coumarin dyes

Two-photon absorption (2PA) spectroscopy is a robust bioimaging tool that depends on the determined cross-sections (σ^2PA). The absorption of both photons occurs simultaneously with equivalent (degenerate) or different (non-degenerate) photon energies, D-2PA and ND-2PA, respectively. The former has been investigated experimentally and computationally for many systems, while the latter remains relatively unexplored computationally and limited experimentally. In this study, response theory using time-dependent density functional theory (TD-DFT) and the 2-state model (2SM) have been utilized to investigate σ^D-2PA and σ^ND-2PA for the excitation to the lowest energy singlet state (S₁) of coumarin, coumarin 6, coumarin 120, coumarin 307, and coumarin 343. Solvents involved were methanol (MeOH), chloroform (ClForm), and dimethylsulfoxide (DMSO), where the latter leads to the largest σ^2PA. Values of σ^2PA are largest for coumarin 6 and lowest for coumarin, which illustrates the effect of substituents. The 2SM clarifies how the largest cross-sections correspond to molecules with the largest transition dipole moments, μ₀₁. In general, σ^D-2SM computations agree with σ^D-2PA. Moreover, σ^ND-2SM are in qualitative agreement with σ^ND-2PA with comparable enhancement relative to σ^D-2PA. Overall, σ^ND-2PA are larger than σ^D-2PA where the increase is in the range of 22% to 49%, depending on the coumarin as well as the relative energies of the two photons. This work aids in future investigations into various fluorophores to understand their photophysical properties for ND-2PA.

Two-Photon Absorption Cooperative Effects within Multi-Dipolar Ruthenium Complexes: The Decisive Influence of Charge Transfers

One- and two-photon characterizations of a series of hetero- and homoleptic [RuL_3-n(bpy)_n]²⁺ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D−π−A−A−π−D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.

Directed ortho and Remote Metalation-Suzuki-Miyaura Cross Coupling Route to Azafluorenol Core Liquid Crystals

Two new smectic C* mesogens containing a hexyloxy side chain and an azafluorenone (3a) or azafluorenol (3b) core were synthesized using a combined directed ortho metalation-directed remote metalation-Suzuki-Miyaura cross-coupling strategy. 3b was formed in 10 steps and 25% overall yield based on starting benzamide 1a. 3a forms a nematic phase, while 3b forms a smectic A phase. The large temperature range of the smectic phase for the azafluorenol 3b is indicative of mesophase stabilization by intermolecular hydrogen bonding between the hydroxyl group and pyridine nitrogen of neighboring 3b molecules.

Near-infrared dyes for two-photon absorption in the short-wavelength infrared: strategies towards optical power limiting

The recent advances in the field of two-photon absorbing chromophores in the short-wavelength infrared spectral range (SWIR 1100–2500 nm) are summarized, highlighting the development of optical power limiting devices in this spectral range.

Static- and frequency-dependent NLO properties of dithienothiophene and thienothiophene bridges — A computational investigation

We have explored detailed linear and nonlinear optical properties of push-pull systems bearing thienothiophene and dithienothiophene spacers. By using density functional theory (DFT), frequency-dependent strategies were applied to examine the polarizability ([Formula: see text] and hyperpolarizability ([Formula: see text] and [Formula: see text]. The set of global and range-separated hybrid functionals with different Hartree–Fock (HF) exchange percentage at two basis sets cc-pVDZ and cc-pVTZ were used to evaluate the nonlinear optical (NLO) properties. The observed trends in the absorption maxima supported by perturbation potential analysis; as the absorption maxima increases, the respective amplitude potential decreases. For the investigated compounds, [Formula: see text]-conjugation along with the type of substituted acceptor plays a crucial role in the enhancement of NLO properties. The presence of acceptor group and length of conjugation increase between the D and A group; the first- and second-order intrinsic hyperpolarizability increases, leads to enhanced first- as well as second-order hyperpolarizability. Bond length alternation (BLA)/bond order alteration (BOA) exploration suggested that compounds attain cyanine limit. The trends in NLO properties for investigated compounds are supported by chemical reactivity descriptors, hardness and hyperhardness analysis. The polarizability is linearly correlated with the hyperpolarizability parameters ([Formula: see text] and [Formula: see text] and shows a good regression coefficient by figures of merit analysis.

Donor-Pi-Acceptor Fluorene Conjugates, Based on Chalcone and Pyrimidine Derivatives: an Insight into Structure-Property Relationship, Photophysical and Electrochemical Properties

A small set of four new fluorenyl chromophores (5-5a-c) was accomplished by stepwise nucleophilic substitution, Friedel-Crafts acylation, Ullman coupling, aldol condensation and cyclization reactions. The fluorene moiety was substituted at 2,7,9 and 9' positions with diverse groups. The synthesized derivatives were characterized by FTIR, ¹H-NMR and ¹³C-NMR spectroscopic techniques. The optical properties were evaluated by by UV-VIS absorption and Fluorescence studies. HOMO and LUMO energy levels were evaluated by electrochemical studies and were found at -5.37-5.83 eV and - 2.47-2.94 eV respectively with band gap energy values 2.88 to 2.91 eV. The band gap energy values suggested that these synthesized molecules can be manipulated in the designing of blue and green OLEDS. Graphical Abstract.

Two-Photon Absorption and Cell Imaging of Fluorene-Functionalized Epicocconone Analogues

Epicocconone 1 is a natural chromophore isolated from the fungus Epicoccum nigrum that has shown applications in proteomics and fluorescent microscopy thanks to its unique pro-fluorescence properties. The modification of the skeleton of the natural product by replacing the triene side chain by a fluorenyl scaffold can noticeably increase the fluorophore's absorption coefficient. The synthesis of the analogues of the natural product has been made possible by the use of a palladium-catalyzed carbonylation reaction, allowing the construction of the β-keto-dioxinone key intermediate. Two-photon absorption cross-section measurements of the fluorenyl epicocconone analogues show a structure dependency with values ranging from 60 to 280 GM and live cell imaging show intense staining of intracellular vesicle-like structures around the nucleus.

Measurement of the relative non-degenerate two-photon absorption cross-section for fluorescence microscopy

In non-degenerate two-photon microscopy (ND-TPM), the required energy for fluorescence excitation occurs via absorption of two photons of different energies derived from two synchronized pulsed laser beams. ND-TPM is a promising imaging technology offering flexibility in the choice of the photon energy for each beam. However, a formalism to quantify the efficiency of two-photon absorption (TPA) under non-degenerate excitation, relative to the resonant degenerate excitation, is missing. Here, we derive this formalism and experimentally validate our prediction for a common fluorophore, fluorescein. An accurate quantification of non-degenerate TPA is important to optimize the choice of photon energies for each fluorophore.

State of the Art of Boron and Tin Complexes in Second- and Third-Order Nonlinear Optics §

Boron and tin complexes have been a versatile and very interesting scaffold for the design of nonlinear optical (NLO) chromophores. In this paper we present a wide range of reports since the 1990s to date, which include second-order (e.g., second harmonic generation) and third-order (e.g., two-photon absorption) NLO properties. After a short introduction on the origin of the NLO response in molecules, the different features associated with the introduction of these inorganic motifs in the organic-based NLO materials are discussed: Their effect on the accepting/donating capabilities of the substituents, on the efficiency of the π-conjugated linkage, and on the topology of the chromophores which can be tuned from the first generation of “push-pull” chromophores to more sophisticated two- or three-dimensional architectures.

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules

Excited-state decay rate constant is controlled by hydrogen bonding, which is switched from intramolecular to intermolecular by choice of solvent.

Nonlinear optical properties, upconversion and lasing in metal–organic frameworks

The building block modular approach that lies behind coordination polymers (CPs) and metal–organic frameworks (MOFs) results not only in a plethora of materials that can be obtained but also in a vast array of nonlinear optical properties that could be aimed at.

9,9-Diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-2-amine

In the title compound, C31H27N, the fluorene unit is approximately planar (r.m.s deviation = 0.0255 Å). The dihedral angles between the fluorene fused ring system and two phenyl rings are 88.37 (5) and 66.31 (6)°. Weak intermolecular C—H...π(ring) interactions help to stabilize the crystal structure.

Experimental and theoretical investigations on spectroscopic properties of the imidazole-fused phenanthroline and its derivatives

Two phenanthroline derivatives, 1H-imidazo[4,5-f][1,10]phenanthroline (imPhen) and 2-(9H-fluoren-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (Flu-imPhen), have been synthesized and characterized and the corresponding absorption and emission spectroscopic properties have been studied in CH2Cl2 solution. The imPhen exhibits the main two absorption bands at 282 nm and 229 nm and these bands are assigned as the typical π→π*(Phen) state. In addition, the weak absorption bands at 313 nm associated with a shoulder near 302 nm were assigned to the π→π*(Phen) state with partial charge transfer (CT) character. A similar absorption spectra are observed in the case of the Flu-imPhen in the region of 200-300 nm, while the region of 300-400 nm of the spectra are dominated by the characteristic π→π* transition of the fluorene moiety. imPhen shows the typical ligand-centered (1)π→π* emission, while Flu-imPhen emits from the mixed (1)π→π*/CT states. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of these ligands studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states.

Broadband Two-Photon Absorption Characteristics of Highly Photostable Fluorenyl-Dicyanoethylenylated [60]Fullerene Dyads

We synthesized four C₆₀-(light-harvesting antenna) dyads C₆₀ (>CPAF-C_n) (n = 4, 9, 12, or 18) 1-C_n for the investigation of their broadband nonlinear absorption effect. Since we have previously demonstrated their high function as two-photon absorption (2PA) materials at 1000 nm, a different 2PA wavelength of 780 nm was applied in the study. The combined data taken at two different wavelength ranges substantiated the broadband characteristics of 1-C_n. We proposed that the observed broadband absorptions may be attributed by a partial π-conjugation between the C₆₀ > cage and CPAF-C_n moieties, via endinitrile tautomeric resonance, giving a resonance state with enhanced molecular conjugation. This transient state could increase its 2PA and excited-state absorption at 800 nm. In addition, a trend of concentration-dependent 2PA cross-section (σ₂ ) and excited-state absorption magnitude was detected showing a higher σ value at a lower concentration that was correlated to increasing molecular separation with less aggregation for dyads C₆₀(>CPAF-C₁₈) and C₆₀(>CPAF-C₉), as better 2PA and excited-state absorbers.

Steric hindrance inhibits excited-state relaxation and lowers the extent of intramolecular charge transfer in two-photon absorbing dyes

Steric hindrance inhibits excited-state relaxation and intramolecular charge-transfer, which significantly alters the photophysical properties.

Two-photon "caging" groups: effect of position isomery on the photorelease properties of aminoquinoline-derived photolabile protecting groups

High two-photon photolysis cross sections and water solubility of probes are important to avoid toxicity in biomedical applications of photolysis. Systematic variation of the position of a carboxyl electron-withdrawing group (EWG) on photolysis of 8-dimethylaminoquinoline protecting groups identified the C5-substituted isomer as a privileged dipole. The 5-benzoyl-8-DMAQ substitution yields a caging group with an enhanced two-photon uncaging cross section (δu = 2.0 GM) and good water solubility (c ≤ 50 mM, pH 7.4).

Synthesis and characterization of novel symmetrical two-photon chromophores derived from bis(triphenylaminotetrathienoacenyl) and fused-thiophene units

Four new donor–π–donor (D–π₁–π₂–π₁–D) fused-thiophene-based chromophores, end-functionalized with electron-donating triphenylamine (TPA) groups, were developed and characterized for a two-photon absorption study.

Strong solvent dependence of linear and non-linear optical properties of donor–acceptor type pyrrolo[3,2-b]pyrroles

The pyrrolo[3,2-b]pyrrole core was determined to be an efficient linker allowing the conjugation of peripheral benzene rings. The resulting dipolar compounds displayed strong solvatochromism of fluorescence.

Synthesis and nonlinear optical properties in the near-IR range of stilbazolium dyes

A series of stilbazolium salts based on donor-π-acceptor (D-π-A) structure have been synthesized and fully characterized. Photophysical properties including linear absorption, one-photon excited fluorescence (OPEF), two-photon absorption (2PA) properties were systematically investigated. The results suggest that increasing electron-releasing character of the terminal group leads to a more pronounced donor-to-acceptor intramolecular charge transfer (ICT). In addition, the dyes possess the largest 2PA cross sections in the near infrared region (NIR) and display maximum two-photon absorption cross sections within the narrow wavelength range from 950 to 970nm and BL3 exhibits a large nonlinear refractive index coefficient and possesses very large values of the real part of the cubic hyperpolarizability χ((3)) at 960nm. Furthermore, the initial density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations provide reasonable explanations for their absorption spectra, meanwhile we used the Lippert-Mataga equation to evaluate the dipole moment changes of the dyes with photoexcitation, the results are corresponding with linear and nonlinear optical properties of the dyes.

Comparison of 1,4-distyrylfluorene and 1,4-distyrylbenzene analogues: synthesis, structure, electrochemistry and photophysics

A series of nine 1,4-distyrylfluorene derivatives (2) functionalized with substituents of variable electrondonating or -accepting capabilities was synthesised. The photophysical properties of the molecules were investigated, including UV/vis absorption, photoluminescence emission, and fluorescence quantum yields. Photophysical properties of chromophores 2 were found to exhibit significant solvatochromic effects, especially in the Stokes shift and photoluminescence maxima. The electrochemical properties of series 2 were also assessed by cyclic voltammetry and differential pulse voltammetry. Results of photophysical and electrochemical analyses were further supported by DFT calculations (B3LYP/6-31G*) and single crystal X-ray diffraction on select molecules. The contributions of intermolecular π-stacking and hydrogen bonding to crystal packing are discussed. A series of nine 1,4-distyrylphenylene derivatives (3) were also synthesised and similarly characterized for comparison to photophysical and solvatochromic effects observed in series 2. Properties of similarly-substituted molecules in series 2 and 3 were compared to one another in order to assess the influence of the 1,4-fluorenylene unit.

Multi-photon absorption effect and intra-molecular charge transfer of donor-π-acceptor chromophore ethyl p-amino benzoate

Fourier transform (FT)-Raman and infrared (IR) spectra of the nonlinear optical (NLO) material ethyl p-amino benzoate (EPAB) have been recorded and analyzed. The geometry and harmonic vibrational wavenumbers are calculated with the help of B3LYP density functional theory method. The detailed interpretation of the vibrational spectra has been carried out with the aid of normal coordinate analysis following the scaled quantum mechanical force field methodology. Stability of the molecule arising from hyperconjugative interactions leading to its NLO activity and charge delocalization have been analyzed using natural bond orbital (NBO) analysis. Employing the open-aperture z-scan technique, NLO absorption of the sample has been studied in two excitation regimes, using 100 fs and 5 ns laser pulses respectively. It is found that EPAB is a three-photon absorber for 100 fs pulses at the excitation wavelength of 800 nm. For ns pulses at 532 nm it exhibits strong optical limiting, indicating possible photonics applications.

Synthesis and self-organization of fluorene-conjugated bisimidazolylporphyrin and its optical properties

A conjugated-bisimidazolylporphyrin bridged by bis(ethynylfluorene) was synthesized and organized into linear polymer through self-coordination having mean molecular weights, M_w and M_n, of ~2.1 × 10⁵ Da and ~1.6 × 10⁵ Da, respectively. A large two-photon absorption cross section value of 3.4 × 10⁵ GM (per dimer unit) was observed. This value was comparable to that of the previously reported self-assembled linear polymer consisting of butadiyne-bridged imidazolylporphyrins. The two-photon absorption properties could be controlled by tuning the wavelength and absorption intensity of the one-photon absorption.

THEORETICAL STUDIES OF ONE- AND TWO-PHOTON ABSORPTION PROPERTIES FOR SYMMETRIC MOLECULES BASED ON BIS(STILBENE)DIETHYLENE

In order to investigate the substituting effects on the two-photon absorption (TPA) cross-section and the position of the TPA peak, the one- and two-photon absorption properties of a series of symmetric molecules with bis(stilbene)diethylene π-conjugated bridge have been studied using ZINDO-SOS method. The transition moments from the ground state to the lowest excited state (M0k) and from the lowest excited state to the TPA final state (Mkn), together with the detuning energy ΔE = E0k - Ekn/2, were calculated using the INDO1 method included in the ZINDO program. The results indicate that bis-donor and bis-acceptor substitutions mainly affect the transition dipole moments M0k and Mkn, and that donor or acceptor side groups have a strong influence on the detuning energy. Meanwhile, it was found that the molecules with the bis-donor structure possess enhanced TPA cross sections, and the molecules with cyano or methoxy side groups have a large red-shift with respect to molecules without side groups.

Synthesis, characterization, crystal structure and DFT studies on 1',3'-dihydrospiro[fluorene-9,2'-perimidine]

The title compound, 1',3'-dihydrospiro[fluorene-9,2'-perimidine] has been synthesized and characterized by NMR, ESI-MS, IR, elemental analysis, UV-vis and fluorescence spectroscopy. The crystal structures of the title compound and its co-crystal with 9-fluorenone have also been determined by X-ray single crystal diffraction. Density functional theory (DFT) calculations and vibrational frequencies have been performed at B3LYP/6-31G* level. The comparisons between the experimental vibrational frequencies and the predicted data show that B3LYP/6-31G* method can simulate the IR of the title compound on the whole. The theoretical electronic absorption spectra have been calculated by using TD-DFT method and compared with the experimental result. The solid-fluorescence determination of the title compound reveals two emission bans at 430 and 590 nm while its co-crystal reveals only one emission band at 590 nm.