Manipulating triplet states: tuning energies, absorption, lifetimes, and annihilation rates in anthanthrene derivatives

Three distinct conductance states in polycyclic aromatic hydrocarbon derivatives

Tight-binding model (TBM) and density functional theory (DFT) calculations were employed. Both simulations have demonstrated that the electrical conductance for eight polycyclic aromatic hydrocarbons (PAHs) can be modulated by varying the number of aromatic rings (NAR) within the aromatic derivatives. TBM simulations reveal three distinct conductance states: low, medium and high for the studied PAH derivatives. The three distinct conductance states suggested by TBM are supported by DFT transmission curves, where the low conductance evidenced by T(E) = 0, for benzene, naphthalene, pyrene and anthracene. While azulene and anthanthrene exhibit a medium conductance as T(E) = 1, and tetracene and dibenzocoronene possess a high conductance with T(E) = 2. Low, medium and high values were elucidated according to the energy gap E g and E g gaps are strongly dependent on the NAR in the PAH derivatives. This study also suggests that any PAH molecules are a conductor if E g < 0.20 eV. A linear relationship between the conductance and NAR (G ∝ NAR) was found and conductance follows the order G (benzene, 1 NAR) < G (anthanthrene, 4 NAR) < G (dibenzocoronene, 9 NAR). The proposed study suggests a relevant step towards the practical application of molecular electronics and future device application.

Energy gap and aromatic molecular rings

The manuscript combines rational density functional theory simulations and experimental data to investigate the electrical properties of eight polycyclic aromatic hydrocarbons (PAHs). The optimized geometries reveal a preference for one-row, two-row and three-row ring distributions. Band structure plots demonstrate an inverse correlation between the number of aromatic rings and band gap size, with a specific order observed across the PAHs. Gas phase simulations support these findings, though differences in values are noted compared to the literature. Introducing a two-row ring distribution concept resolves discrepancies, particularly in azulene. The B3LYP function successfully bridges theoretical and experimental gaps, particularly in large PAHs. The manuscript highlights the potential for designing electronic devices based on different-sized PAHs, emphasizing a multi-ring distribution approach and opening new avenues for practical applications.

Adjusting the photovoltaic performance of big fused ring-based small molecules by tailoring with different modifications

Three novel A-D-A type small-molecule donor materials, namely AAN-DPP2, AAN(T-DPP)2 and AANT(T-DPP)2, with anthanthrene (AAN) as the electron-donating core, diketopyrrolopyrrole (DPP) as the electron-accepting moiety, and thiophene as π-bridge units, have been designed and synthesized for application in bulk-heterojunction (BHJ) organic solar cells (OSCs). Compared to AAN-DPP2, devices based on AAN(T-DPP)2 and AANT(T-DPP)2 show better photovoltaic performance due to broader absorption and better planarity of the molecular backbone. A maximum power conversion efficiency (PCE) of 2.33% with a short-circuit current density (J sc) of 6.82 mA cm-2 and a fill factor (FF) of 39.80 was obtained in the AAN(T-DPP)2/PC71BM-based solar cells. This is resulting from the suitable thickness of the active layer, improving the ability of catching light and decreasing the twist angle of the backbone by inserting a thiophene spacer. The results indicate that strategic substitution of π-bridges and side-chains in A-D-A type SMs is an efficient strategy to improve photovoltaic performance.

Zn Coordination and the Identity of the Halide Ancillary Ligand Dramatically Influence the Excited-State Dynamics and Bimolecular Reactions of 2,3-Di(pyridin-2-yl)benzo[g]quinoxaline

The optical properties of coordination complexes with ligands containing nitrogen heterocycles have been extensively studied for decades. One subclass of these materials, metal complexes utilizing substituted pyrazines and quinoxalines as ligands, has been employed in a variety of photochemical applications ranging from photodynamic therapy to organic light-emitting diodes. A vast majority of this work focuses on characterization of the metal-to-ligand charge-transfer states in these metal complexes; however, literature reports rarely investigate the photophysics of the parent pyrazine or quinoxaline ligand or perform control experiments utilizing metal complexes that lack low-lying charge-transfer (CT) states in order to determine how metal-atom coordination influences the photophysical properties of the ligand. With this in mind, we examined the steady-state and time-resolved photophysics of 2,3-di(pyridin-2-yl)benzo[g]quinoxaline (dpb) and explored how the coordination of ZnX₂ (X = Cl^-, Br^-, I^-) affects the photophysical properties of dpb. In dpb, we find that the dominant mode of deactivation from the singlet excited state is intersystem crossing (ISC). Coordination of ZnX₂ perturbs the relative energies of the ππ* and nπ* excited states of dpb, leading to drastically different rates of ISC as well as radiative and nonradiative decay in the [Zn(dpb)X₂] complexes compared to dpb. These differences in the rates change the dominant singlet-excited-state decay pathway from ISC in dpb to a mixture of ISC and fluorescence in [Zn(dpb)Cl₂] and [Zn(dpb)Br₂] and to nonradiative decay in [Zn(dpb)I₂]. Coordination of ZnX₂ and the choice of the halide ligand also have profound effects on the rate constants for excited-state bimolecular reactions, including triplet-triplet annihilation and oxygen quenching. These results demonstrate that metal coordination, even in complexes lacking low-lying CT states, and the choice of the ancillary ligand can dramatically alter the photophysical properties of chromophores containing nitrogen heterocycles.

An Ultra-Long-Lived Triplet Excited State in Water at Room Temperature: Insights on the Molecular Design of Tridecafullerenes

Suitably engineered molecular systems exhibiting triplet excited states with very long lifetimes are important for high-end applications in nonlinear optics, photocatalysis, or biomedicine. We report the finding of an ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, measured for a globular tridecafullerene with a highly compact glycodendrimeric structure. A series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units have been synthesized and characterized. UV/Vis spectra and DLS experiments confirm their aggregation in water. Steady-state and time-resolved fluorescence experiments suggest a different degree of inner solvation of the multifullerenes depending on their molecular design. Efficient quenching of the triplet states by O2 but not by waterborne azide anions has been observed. Molecular modelling reveals dissimilar access of the aqueous phase to the internal structure of the tridecafullerenes, differently shielded by the glycodendrimeric shell.

Influence of Structural Isomerism on the Photophysical Properties of a Series of Donor-Acceptor 1-Naphthalenecarbonitrile Derivatives Possessing Amine Substituents

In an effort to probe the influence of structural isomerism on the excited-state properties of a naphthalene-based donor-acceptor (D-A) system, four 1-naphthalenecarbonitrile compounds with amine substituents in the 2-, 3-, and 4-positions were synthesized and their photophysical properties were examined. Specifically, the molecules 2-dimethylamino-1-naphthalenecarbonitrile (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P), 3-dimethylamino-1-naphthalenecarbonitrile (3DA), and 4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were studied. The substitution position of the amine donor has a significant impact on both the ground-state absorption and excited-state properties of the complexes in toluene solution. The energy, band shape, and extinction coefficient of the ground-state absorption spectra are highly dependent on the substitution position of the amine donor. All of the derivatives exhibit fluorescence at room temperature. The fluorescence observed from 2DA, 2P, and 3DA demonstrates a vibronic structure with all three molecules possessing Stokes shifts on the order of 40 nm, whereas the fluorescence observed from 4P is broad and has a Stokes shift 2 times greater than the other derivatives. The fluorescence lifetimes, fluorescence quantum yields, and intersystem crossing quantum yields vary greatly with the substitution position of the amine donor. 2DA and 2P display intermediate fluorescence lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing the greatest intersystem quantum yield (0.80). 3DA has a much greater fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82) at the expense of the intersystem crossing quantum yield (0.12). 4P has the shortest lifetime (0.53 ns), with the lowest fluorescence and intersystem crossing quantum yields (<0.05). The singlet-triplet energy gaps are nearly identical for 2DA, 2P, and 3DA with values on the order of 0.70 eV. This singlet-triplet gap is larger in 4P, with a calculated value of 0.94 eV. The triplet-triplet absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in the UV and visible regions with maxima around 330 and 500 nm characterize all three spectra. The triplet excited-state extinction coefficient values for 3DA were found to be 1.5 times larger than those in 2DA and 2P. The triplet-triplet absorption spectrum of 4P is markedly different from the triplet-triplet absorption spectra of the other derivatives. The spectrum is broad, with the four local maxima observed at 374, 445, 624, and 774 nm. All four molecules display delayed fluorescence and laser-power-dependent triplet excited-state decay kinetics, indicating the involvement of triplet-triplet annihilation in the deactivation of the triplet excited states. Both the intrinsic triplet lifetimes and triplet-triplet annihilation rate constants were determined. These values are similar for all of the derivatives with triplet lifetimes on the order of 100 μs and diffusion-controlled rates of triplet-triplet annihilation.

Amphiphilic anthanthrene trimers that exfoliate graphite and individualize single wall carbon nanotubes

A phosphodiester-linked dialkynyl substituted anthanthrene trimer (1) has been designed and synthesized. Its graphene ribbon like structure is expected to facilitate interactions with nanographene (NG) and single wall carbon nanotubes (SWCNT) to yield novel and stable carbon-based nanomaterials. Interactions with trimer 1 lead to exfoliation of NG and to the individualization of SWCNTs. Phosphate groups, in general, and their negative charges, in particular, render the resulting nanomaterials soluble in ethanol, which is ecologically favourable over DMF required for the processing of pristine NG or SWCNTs. The newly formed nanomaterials were probed by complementary spectroscopic and microscopic techniques. Of particular importance were transient absorption and fluorescence excitation measurements, which revealed an efficient energy transfer within the carbon-based nanomaterials.

Competition between Singlet Fission and Spin-Orbit-Induced Intersystem Crossing in Anthanthrene and Anthanthrone Derivatives

Singlet and triplet excited-state dynamics of anthanthrene and anthanthrone derivatives in solution are studied. Triisopropylsilyl- (TIPS) or H-terminated ethynyl groups are used to tune the singlet and triplet energies to enable their potential applications in singlet fission and triplet fusion processes. Time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies are used to obtain a mechanistic understanding of triplet formation. The anthanthrene derivatives form triplet states efficiently at a rate (ca. 10⁷  s^-1 ) comparable to radiative singlet fluorescence processes with approximately 30 % triplet yields, despite their large S₁ -T₁ energy gap (>1 eV) and the lack of carbonyl groups. In contrast, anthanthrone has a higher triplet yield (50±10 %) with a faster intersystem crossing rate (2.7 × 10⁸  s^-1 ) because of the n-π* character of the S₁ ←S₀ transition. Analysis of time-resolved spin-polarized EPR spectra of these compounds reveals that the triplet states are primarily generated by the spin-orbit-induced intersystem crossing mechanism. However, at high concentrations, the EPR spectrum of the 4,6,10,14-tetrakis(TIPS-ethynyl)anthanthrene triplet state shows a significant contribution from a non-Boltzmann population of the m_s =0 spin sublevel, which is characteristic of triplet formation by singlet fission.