Can the Excited State Energy of a Pyrenyl Unit Be Directly Transferred to a Perylene Bisimide Moiety?

A pyrenyl unit (Py) was chemically connected to a perylene bisimide (PBI) moiety through a long and flexible linker, 4,7,10-trioxa-1,13-tridecanediamine (TOA), resulting in a fluorescent dyad, PBI–TOA–Py. Ultraviolet–visible absorption and fluorescence studies revealed that the two fluorescent units of PBI–TOA–Py behave independently. However, efficient Förster resonance energy transfer (FRET) from the Py unit to the PBI moiety in solution state was also observed. Temperature and solvent effect studies demonstrated that the energy transfer efficiency is highly dependent upon solution temperature and solvent nature. Specifically, for the dimethylformamide (DMF) solution of PBI–TOA–Py, the FRET efficiency is close to 88% at temperatures below ∼40 °C, but the efficiency greatly decreases to nearly zero when the temperature exceeds ∼80 °C. Moreover, addition of HAc into the DMF solution at room temperature could reduce the energy transfer efficiency to nearly zero, suggesting that the excited state energy of Py cannot be directly transferred to the PBI structure even though they are properly and chemically bonded. On the basis of the observations and time-resolved studies, it is believed that the observed efficient FRET from the Py unit to the PBI moiety occurs mainly through Py excimer formation, which could be a result of intermolecular association of the compound. Thus, the applications of the fluorescent dyad in solvent discrimination and trace water determination in organic solvents were verified through example studies

Resonance-Enhanced Two-Photon Absorption and Optical Power Limiting Properties of Three-Dimensional Perylene Bisimide Derivatives

Push–pull organic structures characterized by an intramolecular charge transfer (ICT) process and π-electron delocalization are potentially interesting luminescent materials. A series of three-dimensional o-carborane-containing perylene bisimide derivatives (PBIs) were synthesized, and their optical properties were systematically investigated to illustrate the stereo effect, especially on the two-photon absorption (2PA) and optical power limiting (OPL) properties. Open-aperture Z-scan curves showed that all four PBIs displayed strong and broad two-photon absorptivities based on the resonance-enhanced phenomenon. The maximum degenerate two-photon absorption cross section (δ2PA) increased with the number of PBI substituents. The derivative CB-PBI possessed a δ2PA value of ∼2400 GM at 650 nm, a significant enhancement in comparison with that of the parent PBI (∼719 GM), ascribed to the present stereo effect. When the aromatic-donating units changed from naphthyl and pyrenyl to PBI, the generated multidimensional intramolecular charge transfer (ICT) from the aromatic units to the o-carborane cage contributed to the 2PA processes. All of the fluorophores exhibited excellent optical power limiting (OPL) performances as well as a minimum limiting threshold of ∼4.98 mJ/cm2 for CB-PBI. These significant results not only allow us to get deep insight into the nature of the fundamental stereo effect and nonlinear optical (NLO) response involved but also guide us toward the design of new multifunctional luminescent materials

Functionality-Oriented Derivatization of Naphthalene Diimide: A Molecular Gel Strategy-Based Fluorescent Film for Aniline Vapor Detection

Modification of naphthalene diimide (NDI) resulted in a photochemically stable, fluorescent 3,4,5-tris­(dodecyloxy)­benzamide derivative of NDI (TDBNDI), and introduction of the long alkyl chains endowed the compound with good compatibility with commonly found organic solvents and in particular superior self-assembly in the solution state. Further studies revealed that TDBNDI forms gels with nine of the 18 solvents tested at a concentration of 2.0% (w/v), and the critical gelation concentrations of five of the eight gels are lower than 1.0% (w/v), indicating the high efficiency of the compound as a low-molecular mass gelator (LMMG). Transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy studies revealed the networked fibrillar structure of the TDBNDI/methylcyclohexane (MCH) gel. On the basis of these findings, a fluorescent film was developed via simple spin-coating of the TDBNDI/MCH gel on a glass substrate surface. Fluorescence behavior and sensing performance studies demonstrated that this film is photochemically stable, and sensitive and selective to the presence of aniline vapor. Notably, the response is instantaneous, and the sensing process is fully and quickly reversible. This case study demonstrates that derivatization of photochemically stable fluorophores into LMMGs is a good strategy for developing high-performance fluorescent sensing films

Metal Substitution Effects on the Charge Transport and Spin Crossover Properties of [Fe_1–<i>x</i>Zn_<i>x</i>(Htrz)₂(trz)](BF₄) (trz = Triazole)

In this study we analyze the metal substitution effects on the structural, morphological, charge transport, and spin transition properties of the [Fe_1–<i>x</i>Zn_<i>x</i>(Htrz)₂(trz)]­(BF₄) (trz = triazole, <i>x</i> = 0, 0.26, or 0.43) compound using electron microscopy, powder X-ray diffraction, optical reflectivity, Raman, FTIR, ⁵⁷Fe Mössbauer, and broadband (10^–2–10⁶ Hz) dielectric spectroscopies. The crystal structure and the morphology of the objects remain nearly unaffected, whereas the thermal spin transition shifts from 362 to 316 K and the thermal hysteresis width decreases from 45 to 8 K for increasing values of <i>x</i>. For each compound the electrical conductivity drops when the iron­(II) electronic configuration is switched from the low-spin to the high-spin state. A strong overall decrease in conductivity with increasing Zn concentration is also observed in both spin states. These results, together with the analysis of the charge carrier dynamics, suggest that the ferrous ions participate directly in the charge transport mechanism, explaining the strong spin-state dependence of the electrical properties in this compound

Unlocking Multicolor Emissions in the Crystalline State through Dimerization and Configurational Transformation of a Single Fluorophore

Multicolor luminescent materials with tunable properties hold great promise for a wide range of applications in materials science. Unfortunately, the conventional approach to achieving multicolor emissions by blending multiple types of fluorophores is hindered by limitations, notably, spectral instability, aggregation-caused quenching, and energy transfer. The pursuit of multicolor emissions from a single type of fluorophore in the solid state has, until now, remained a formidable challenge. In this study, we have introduced N,N′-diphenyl dihydrodibenzo[a,c]-phenazines (DPAC), augmented with two o-carboranyl units, to create a novel fluorophore CbDPAC. The CbDPAC crystal exhibits three distinct emission bands peaking at 405, 470, and 620 nm, respectively, arising from a rich intermolecular interaction network that generates novel emission centers, such as conformational isomers and excimers. This work inspires the rational molecular engineering of smart fluorophores with tailorable properties and inaugurates diverse possibilities for stimuli-responsive luminescent technologies

Polymorphism-Dependent Spin-Crossover: Hysteretic Two-Step Spin Transition with an Ordered [HS–HS–LS] Intermediate Phase

A mononuclear iron­(II) complex has been isolated in two polymorphs. Polymorph <b>1b</b> remains high-spin over all temperatures, whereas polymorph <b>1a</b> undergoes a cooperative two-step spin crossover accompanied by symmetry breaking, showing an ordered 2:1 high-spin–low-spin intermediate phase

Rigid Bay-Conjugated Perylene Bisimide Rotors: Solvent-Induced Excited-State Symmetry Breaking and Resonance-Enhanced Two-Photon Absorption

Intramolecular charge transfer and excited-state symmetry breaking have a significant effect on the nonlinear optical properties of multipolar chromophores. Rigid and nonplanar perylene bisimide derivatives (PBIs) functionalized at bay positions were comparatively and comprehensively investigated. In apolar solvents, two quadrupolar molecular rotors showed an obvious decrease of the A0‑0/A0‑1 ratios, suggesting strong exciton coupling with the adjacent PBI units initiated by the π–π stacking. The vanishment of the preferable dimer emission in polar solvents supported the plausible phenomena of excited-state symmetry breaking, thanks to the facile rotation around the rigid linkers. Comparative femtosecond transition absorption studies confirmed their notable differences in relaxation dynamics and the generation of radical anions (PBI•–) and cations (PBI•+). The maxima two-photon absorption (2PA) wavelengths obtained for the molecular rotors were slightly red-shifted to 670 nm with intrinsic resonance-enhanced characteristics, reflecting the synergistic effect of functional positions and molecular architectures. Meanwhile, the obvious increase of significant 2PA cross-section values in polar solvents illustrated the stabilization of the symmetry-broken dipolar states. Further femtosecond Z-scan also manifested the contribution of excited-state dynamics on the nonlinear optical properties of multipolar chromophores

Unlocking Multicolor Emissions in the Crystalline State through Dimerization and Configurational Transformation of a Single Fluorophore

Multicolor luminescent materials with tunable properties hold great promise for a wide range of applications in materials science. Unfortunately, the conventional approach to achieving multicolor emissions by blending multiple types of fluorophores is hindered by limitations, notably, spectral instability, aggregation-caused quenching, and energy transfer. The pursuit of multicolor emissions from a single type of fluorophore in the solid state has, until now, remained a formidable challenge. In this study, we have introduced N,N′-diphenyl dihydrodibenzo[a,c]-phenazines (DPAC), augmented with two o-carboranyl units, to create a novel fluorophore CbDPAC. The CbDPAC crystal exhibits three distinct emission bands peaking at 405, 470, and 620 nm, respectively, arising from a rich intermolecular interaction network that generates novel emission centers, such as conformational isomers and excimers. This work inspires the rational molecular engineering of smart fluorophores with tailorable properties and inaugurates diverse possibilities for stimuli-responsive luminescent technologies