Role of Halobenzene Guest Molecules in Modulating Room Temperature Phosphorescence of Benzophenone-Naphthalene Diimide Inclusion Crystals

Materials exhibiting room temperature phosphorescence (RTP) have recently emerged as a subject of significant interest. In this study, we successfully created inclusion crystals by introducing halobenzenes as guests into a host molecule combining benzophenone with naphthalene diimide. This approach led to the creation of fascinating fluorescence and RTP properties dependent on the guest molecules. Notably, crystals containing chlorobenzene showed cyan fluorescence, while those with iodobenzene displayed red RTP. This difference highlights the impact of the guest molecule on the luminescent properties, with the significant external heavy-atom effect of iodobenzene playing a key role in promoting efficient intersystem crossing between the excited singlet and triplet states. Crystals with bromobenzene exhibited a unique blend of fluorescence and RTP, both from benzophenone and naphthalene diimide, highlighting the moderate heavy-atom effect. These findings reveal composite materials with remarkably diverse and interesting optical characteristics.

Red aqueous room-temperature phosphorescence modulated by anion–π and intermolecular electronic coupling interactions

Aqueous room temperature phosphorescence (aRTP) from purely organic materials has been intriguing but challenging. In this article, we demonstrated that the red aRTP emission of 2Br–NDI, a water-soluble 4,9-dibromonaphthalene diimide derivative as a chloride salt, could be modulated by anion–π and intermolecular electronic coupling interactions in water. Specifically, the rarely reported stabilization of anion–π interactions in water between Cl⁻ and the 2Br–NDI core was experimentally evidenced by an anion–π induced long-lived emission (λ_Anion–π) of 2Br–NDI, acting as a competitive decay pathway against the intrinsic red aRTP emission (λ_Phos) of 2Br–NDI. In the initial expectation of enhancing the aRTP of 2Br–NDI by inclusion complexation with macrocyclic cucurbit[n]urils (CB[n]s, n = 7, 8, 10), we surprisingly found that the exclusion complexation between CB[8] and 2Br–NDI unconventionally endowed the complex with the strongest and longest-lived aRTP due to the strong intermolecular electronic coupling between the nπ* orbit on the carbonyl rims of CB[8] and the ππ* orbit on 2Br–NDI in water. It is anticipated that these intriguing findings may inspire and expand the exploration of aqueous anion–π recognition and CB[n]-based aRTP materials.

The aqueous room temperature phosphorescence of 2Br–NDI is modulated by long-lived-emitting anion–π interactions and tremendously enhanced by intermolecular electronic coupling interactions with the ISC-boosting carbonyl rims of CB[8] host.

Room-Temperature Phosphorescence Emitters Exhibiting Red to Near-Infrared Emission Derived from Intermolecular Charge-Transfer Triplet States of Naphthalenediimide-Halobenzoate Triad Molecules

Room-temperature phosphorescence (RTP) emitters have attracted significant attention. However, purely organic RTP emitters in red to near-infrared region have not been properly investigated. In this study, a series of naphthalenediimide-halobenzoate-linked molecules are synthesized, one of which exhibits efficient RTP properties, showing red to near-infrared emission in solid and aqueous dispersion. Spectroscopic studies and single-crystal X-ray diffraction analysis have shown that the difference in the stacking modes of compounds affects the optical properties, and the formation of intermolecular charge-transfer complexes of naphthalenediimide-halobenzoate moiety results in a bathochromic shift of absorption and RTP properties. The time-dependent density functional theory calculations showed that the formation of charge-transfer triplet states and the external heavy atom effect of the halogen atom enhance the intersystem crossing between excited singlet and triplet states.

Red-Emitting Delayed Fluorescence and Room Temperature Phosphorescence from Core-Substituted Naphthalene Diimides

Unprecedented ambient triplet-mediated emission in core-substituted naphthalene diimide (cNDI) derivatives is unveiled via delayed fluorescence and room temperature phosphorescence. Carbazole core-substituted cNDIs, with a donor-acceptor design, showed deep-red triplet emission in solution processable films with high quantum yield. This study, with detailed theoretical calculations and time-resolved emission experiments, enables new design insights into the triplet harvesting of cNDIs; an important family of molecules which has been, otherwise, extensively been investigated for its n-type electronic character and tunable singlet fluorescence.

Aqueous Phase Phosphorescence: Ambient Triplet Harvesting of Purely Organic Phosphors via Supramolecular Scaffolding

Ambient solution and amorphous state room temperature phosphorescence (RTP) from purely organic chromophores is rarely achieved. Remarkable stabilization of triplet excitons is realized to obtain deep red phosphorescence in water and in amorphous film state under ambient conditions by a unique supramolecular hybrid assembly between inorganic laponite clay and heavy atom core substituted naphthalene diimide (NDI) phosphor. Structural rigidity and oxygen tolerance of the inorganic template along with controlled molecular organization via supramolecular scaffolding are envisaged to alleviate the unprecedented aqueous phase phosphorescence.

Theoretical insights into the excited-state properties of room-temperature phosphorescence-emitting N-substituted naphthalimides

The optical absorption, fluorescence, and phosphorescence properties along with the intersystem crossing (ISC) rate constants of a series of N-substituted naphthalimides (NNIs), which can be used as organic emitters, were calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated absorption, fluorescence, and phosphorescence energies as well as the fluorescence and ISC rate constants were found to be in good agreement with available experimental data. According to the calculations, the first excited singlet and first excited triplet states are π-π* in nature, and ¹π-π^* → ³n-π* is the dominant ISC channel for 1,8-naphthalimide (NI) and N-methyl-1,8-naphthalimide (Me-NI). Intermolecular charge-transfer (CT) states are observed in the energy region between the ¹π-π* and ³π-π* states for NNIs with an electron-donating group. The presence of these CT states leads to a reduction in the S₁-state and T₁-state energy splitting in NNIs with an electron-donating group compared to the energy splitting observed in NI and Me-NI. The singlet CT state (¹CT) has very weak emission oscillator strength, so it is nonemissive. For NNIs with an electron-donating group, the ¹CT → ³π-π* transition was found to be the dominant ISC channel. Our results indicate that for NNIs with an electron-donating group, the nonemissive S₁ state (¹CT) transits to the T₁ state (³π-π*) via ISC. The ¹CT → ³π-π* transition increases the population of the T₁ state and thus the phosphorescence quantum yield at room temperature, even in the absence of a heavy atom. Graphical Abstract The S₁ state (¹CT), which is nonemissive, transits to the T₁ state (³π-π*) via intersystem crossing and increases the population of the T₁ state in N-substituted naphthalimides with an electron-donating group.

Tuning Light Emission towards White Light from a Naphthalenediimide-Based Entangled Metal-Organic Framework by Mixing Aromatic Guest Molecules

Taking advantage of the outstanding properties of a naphthalenediimide-based entangled porous coordination polymer, a simple strategy for the achievement of white light emission is herein presented. The dynamic structural transformation of the [Zn₂(bdc)₂(dpNDI)]n metal-organic framework enhances the interactions with aryl-guests giving rise to different luminescence colors upon UV (ultraviolet) illumination. Thus, through the rational selection of those small aromatic guest molecules with different electron donor substituents at the appropriate proportion, the emission color was tuned by mixture ratio of guest molecules and even white light emission was achieved. Furthermore, domains in large crystals with a complementary response to linearly polarized light were noticed.

Organic Nanocrystals with Bright Red Persistent Room-Temperature Phosphorescence for Biological Applications

Persistent room-temperature phosphorescence (RTP) in pure organic materials has attracted great attention because of their unique optical properties. The design of organic materials with bright red persistent RTP remains challenging. Herein, we report a new design strategy for realizing high brightness and long lifetime of red-emissive RTP molecules, which is based on introducing an alkoxy spacer between the hybrid units in the molecule. The spacer offers easy Br-H bond formation during crystallization, which also facilitates intermolecular electron coupling to favor persistent RTP. As the majority of RTP compounds have to be confined in a rigid environment to quench nonradiative relaxation pathways for bright phosphorescence emission, nanocrystallization is used to not only rigidify the molecules but also offer the desirable size and water-dispersity for biomedical applications.