Moving Beyond Boron-Based Substituents To Achieve Phosphorescence in Tellurophenes

Heavy Atom at Bay of Perylene Significantly Improves Intersystem Crossing

We studied the photophysical properties of substituted perylenes using time-dependent density functional theory (TDDFT) with Tamm-Dancoff Approximation (TDA). The TDA-TDDFT method allowed us to examine how luminescence activity alters by substituting halogens at different positions (bay, ortho, and peri) of perylenes. Substituting larger halogens like chlorine and bromine at the bay position significantly affects the planarity of the π-system in perylenes. Interestingly, bay-bromoperylene (P-bBr) showed pronounced spin-orbit coupling (SOC) between singlet and triplet excited states. The heavy atom effect (HAE) functioned efficiently with a distorted π-system and substantially enhanced the SOC in P-bBr. Therefore, a rapid intersystem crossing (ISC) is responsible for turning off the fluorescence of P-bBr. In contrast, bromine substitution other than the bay position (i.e., ortho- and peri-bromoperylenes (P-oBr and P-pBr), which maintained planarity), or substituting lighter elements like a methyl group (similar in size to Br) at the bay position of perylene did not substantially improve the SOC. Thus, the ISC is insufficient to quench the fluorescence in these systems. Additionally, substituting multiple bromines in perylene with at least one in the bay position (i.e., P-boBr2, P-bpBr2, and P-bopBr3) further improved the SOC, leading to much faster ISC (1011 s-1) in P-bopBr3. While multiple bromine substitutions other than the bay position (i.e., P-opBr2) exhibited low ISC due to the planar π-system. So, the heavy bromine at the bay position of perylene causes significant enhancement of the ISC.

Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence

Photoluminescent molecules exploiting the sizable spin-orbit coupling constants of main group metals and metalloids to access long-lived triplet excited states are relatively rare compared to phosphorescent transition metal complexes. Here we report the synthesis of three air- and moisture-stable group 14 compounds E(^MePDP^Ph)₂, where E = Si, Ge, or Sn and [^MePDP^Ph]^2- is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine. In solution, all three molecules exhibit exceptionally long-lived triplet excited states with lifetimes in the millisecond range and show highly efficient photoluminescence (Φ ≤ 0.49) due to competing prompt fluorescence and thermally activated delayed fluorescence at and around room temperature. Temperature-dependent steady-state emission spectra and photoluminescent lifetime measurements provided conclusive evidence for the two distinct emission pathways. Picosecond transient absorption spectroscopy allowed further analysis of the intersystem crossing (ISC) between singlet and triplet manifolds (τ_ISC = 0.25-3.1 ns) and confirmed the expected trend of increased ISC rates for the heavier elements in otherwise isostructural compounds.

New Light on an Old Story: Breaking Kasha's Rule in Phosphorescence Mechanism of Organic Boron Compounds and Molecule Design

In this work, the phosphorescence mechanism of (E)-3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate-BF2 compound (S-BF2) is investigated theoretically. The phosphorescence of S-BF2 has been reassigned to the second triplet state (T2) by the density matrix renormalization group (DMRG) method combined with the multi-configurational pair density functional theory (MCPDFT) to approach the limit of theoretical accuracy. The calculated radiative and non-radiative rate constants support the breakdown of Kasha's rule further. Our conclusion contradicts previous reports that phosphorescence comes from the first triplet state (T1). Based on the revised phosphorescence mechanism, we have purposefully designed some novel compounds in theory to enhance the phosphorescence efficiency from T2 by replacing substitute groups in S-BF2. Overall, both S-BF2 and newly designed high-efficiency molecules exhibit anti-Kasha T2 phosphorescence instead of the conventional T1 emission. This work provides a useful guidance for future design of high-efficiency green-emitting phosphors.

Synthesis and reactivity of alkynyl boron compounds

Over the last century, there have been considerable developments in organoboron chemistry due to the stability, non-toxicity, and easy commercial availability of various boronic esters. Several organoboron reagents have emerged and play an increasingly important role in everyday organic synthesis. Among them, alkynyl boron compounds have attracted significant attention due to their easy synthesis and diverse reactivity. In this review, we summarize the advancement of research on alkynyl boron compounds, highlighting their importance in the synthesis of valuable compounds.

Tellura(benzo)bithiophenes: Synthesis, Oligomerization, and Phosphorescence

A series of planar π-extended Te-containing heteroacenes, termed tellura(benzo)bithiophenes, were synthesized. This new structural class of heterocycle features a tellurophene ring fused to a benzobithiophene unit with aromatic side groups (either -C₆H₄ⁱPr or -C₆H₄OCH₃) positioned at the 2- and 5-positions of the tellurophene moiety. Although attempts to enhance molecular rigidity and extend ring-framework π-delocalization in a cumenyl (-C₆H₄ⁱPr)-capped tellura(benzo)bithiophene led to oxidation (and Te-C bond scission) to form a diene-one, the formation of an oligomeric tellura(benzo)bithiophene was possible via Kumada catalyst-transfer polycondensation (KCTP). Furthermore, one tellura(benzo)bithiophene derivative exhibits orange-red phosphorescence at room temperature in air when incorporated into a poly(methyl methacrylate) host; accompanying TD-DFT computations provided insight into a potential mechanism for the observed phosphorescence.

Photoinduced degradation of indigo carmine: insights from a computational investigation

In this work, we present a computational investigation on the photoexcitation of indigo carmine (IC). Physical insights regarding IC photoexcitation and photolysis were obtained from a fundamental perspective through quantum chemistry computations. Density functional theory (DFT) was used to investigate the ground state while its time-dependent formalism (TD-DFT) was used for probing excited state properties, such as vertical excitation energies, generalized oscillator strengths (GOS), and structures. All the computations were undertaken using two different approaches: M06-2X/6-311+G(d,p) and CAM-B3LYP/6-311+G(d,p), in water. Results determined using both methods are in systematic agreement. For instance, the first singlet excited state was found at 2.28 eV (with GOS = 0.4730) and 2.19 eV (GOS = 0.4695) at the TD-DFT/CAM-B3LYP/6-311+G(d,p) and TD-DFT/M06-2X/6-311+G(d,p) levels of theory, respectively. Excellent agreement was observed between the computed and the corresponding experimental UV-Vis spectra. Moreover, results suggest IC undergoes photodecomposition through excited state chemical reaction rather than via a direct photolysis path. To the best of our knowledge, this work is the first to tackle the photoexcitation, and its potential connections to photodegradation, of IC from a fundamental chemical perspective, being presented with expectations to motivate further studies.

Examining the degradation of environmentally-daunting per- and poly-fluoroalkyl substances from a fundamental chemical perspective

In this work, ground and excited-state properties were used as descriptors for probing mechanisms as well as to assess potential alternatives for tackling the elimination of per- and poly-fluoroalkyl substances (PFAS).

Metallacycle Transfer and its Link to Light-Emitting Materials and Conjugated Polymers

Major advances in optoelectronic technologies (e. g., solar cells, organic light-emitting diodes, etc…) are prefaced by the discovery of new synthetic methodologies. In this review, the key role of the Fagan-Nugent reaction in enabling our team (and others) to gain access to new building blocks for luminescent materials and conjugated polymers bearing p-block elements will be described. The Fagan-Nugent reaction is extremely powerful as a synthetic tool since the efficient zirconium-element atom exchange involved affords a wide range of unsaturated inorganic heterocycles of controllable composition and function.

Photoactivity and optical applications of organic materials containing selenium and tellurium

Sulfur-containing compounds, particularly derivatives of thiophene, are well studied for organic optoelectronic applications. Incorporating selenium or tellurium in place of sulfur imparts different physical properties due to the fundamental differences of these atoms relative to their lighter analogues. This has a profound influence on the properties of molecules and materials that incorporate chalcogens that may ultimately lead to new opportunities and applications. This mini-review will focus on the quantitative and qualitative photophysical characteristics of organic materials containing selenium and tellurium as well as their emerging applications as molecular photoactive species, including light-emitting sensors, triplet sensitizers, and beyond.

The Synthesis and Optoelectronic Applications for Tellurophene-Based Small Molecules and Polymers

Tellurophene-based small molecules and polymers have received great attentions owing to their applications in thin-film transistors, solar cells, and sensors. This article reviews the current progress of the synthesis and applications of tellurophene-based small molecules and polymers. The physicochemical properties and optoelectronic applications of tellurophene-based materials are summarized and discussed. In the end, the challenges and outlook of tellurophene-based materials are presented.

Synthesis of Tellurium-Containing π-Extended Aromatics with Room-Temperature Phosphorescence

A synthesis of tellurium-embedded π-extended aromatics from tellurium powder and readily available cyclic diaryliodonium salts has been developed. The versatility of this method has been demonstrated by the synthesis of various functionalized dibenzotellurophenes (DBTe's), a ladder-type π-system, and a heterosumanene. These compounds demonstrated good air/moisture stability and high thermal stability. Remarkably, many DBTe's exhibited interesting tunable room-temperature phosphorescence (RTP) in the solid state.

A Modular Approach to Phosphorescent π-Extended Heteroacenes

A modular route to previously inaccessible classes of ring-fused π-extended heteroacenes bearing the heavy inorganic element tellurium (Te) is presented. These new materials can be viewed as n-doped analogs of molecular graphene subunits that exhibit color tunable visible light phosphorescence in the solid state and in the presence of air. The general mechanism of phosphorescence in these systems was probed experimentally and computationally via time-dependent density functional theory (TD-DFT). The incorporation of Te into π-extended oligoacene frameworks was achieved by an efficient Zr/Te transmetalation protocol; related zirconium-element exchange reactions have been used to prepare both electron-rich and electron-deficient heterocycles containing different elements from throughout the p-block. Therefore, the current study provides a clear path to incorporate inorganic elements into heteroacenes of greater complexity and side group selectivity compared to existing synthetic routes.

Using boryl-substitution and improved Suzuki-Miyaura cross-coupling to access new phosphorescent tellurophenes

A new di(isopropoxy)boryl -B(OⁱPr)₂ tellurophene precursor is described, from which several previously inaccessible phosphorescent borylated tellurophenes are formed via exchange of the -OⁱPr groups. One such tellurophene Mes(ⁱPrO)B-Te-6-B(OⁱPr)Mes, bearing a sterically encumbered mesityl (Mes) substituent at each boron center, exhibits bright yellow-orange phosphorescence in the solid state at room temperature and in the presence of the known quencher O₂. Furthermore, Suzuki-Miyaura cross-coupling between the newly prepared borylated tellurophenes and the test substrate 2-bromothiophene was examined with the pre-catalyst Cl(XPhos)Pd(aminobiphenyl). While more electron deficient boryl groups such as catecholatoboryl (-Bcat) yield significant protodeboronation in place of productive C-C bond formation, efficient formation of the desired thiophene-capped tellurophene thienyl-Te-6-thienyl was noted from tellurophenes bearing the readily accessible pinacolatoboryl (-Bpin) and 1,8-naphthalenediaminatoboryl (-Bdan) functional groups. These findings open the door for the efficient synthesis of aryl tellurophenes and polytellurophenes via the ubiquitous Suzuki-Miyaura coupling of borylated tellurophenes, which was previously hampered by protodeboronation.

Rapid access to (cycloalkyl)tellurophene oligomer mixtures and the first poly(3-aryltellurophene)

New poly- and oligotellurophenes bearing cycloalkyl and 3-aryl substituents have been reported, with narrow band gaps approaching 1.3 eV observed.

Aerobic Solid State Red Phosphorescence from Benzobismole Monomers and Patternable Self-Assembled Block Copolymers

The synthesis of the first bismuth-containing macromolecules that exhibit phosphorescence in the solid state and in the presence of oxygen is reported. These red emissive high molecular weight polymers (>300 kDa) feature benzobismoles appended to a hydrocarbon scaffold, and were built via an efficient ring-opening metathesis (ROMP) protocol. Moreover, our general procedure readily allows for the formation of cross-linked networks and block copolymers. Attaining stable red phosphorescence with non-toxic elements remains a challenge and, thus, our new class of soluble (processable) polymeric phosphor is of great interest. Furthermore, the formation of bismuth-rich cores within organic-inorganic block copolymer spherical micelles is possible, leading to patterned arrays of bismuth in the film state.

Synthesis and self-assembly of thiol-modified tellurophenes

An asymmetric thiol-modified tellurophene was designed and synthesized, and the ability of the compound to form a monolayer on a gold electrode was confirmed. The surface-active tellurophene was synthesized using Cadiot–Chodkiewicz coupling followed by ring closing and thiol modification. The tellurophene compound forms a monolayer on gold surfaces from a concentrated solution within 24 h. The ability of the compound to conjugate to gold is confirmed by X-ray photoelectron spectroscopy (XPS). A surface blocking experiment was used to evaluate the extent of formation of a monolayer on a gold electrode.

Peraryl Arsoles: Practical Synthesis, Electronic Structures, and Solid-State Emission Behaviors

2,3,4,5-Tetraaryl-1-phenylarsoles were synthesized by utilizing safely generated diiodophenylarsine and zirconacyclopentadienes. The obtained peraryl arsoles showed aggregation-induced emission (AIE), where intense emission was observed in the solid states (quantum yields up to 0.61), whereas the corresponding solutions were very weakly emissive. The optical and electronic properties were examined by experimental and computational methods. It was elucidated that the aryl groups at the 2,5-positions affected the frontier orbitals and the aromaticity of the arsole core. On the other hand, those at the 1,3,5-positions were perpendicular to the luminophore and effective for a restriction of aggregation-caused quenching. Because the lone pair of the arsenic atom has a sufficient coordination ability due to the low aromaticity of the arsole moiety, a gold(I) chloride complex of 1,2,3,4,5-pentaphenylarsole was synthesized. The complex formation caused a blue shift of the emission from the bare ligand. Interestingly, the complex showed luminescent mechanochromism; grinding the crystals with a blue emission (λ_em =445 nm) gave amorphous samples with a greenish-blue emission (λ_em =496 nm).

Understanding the Origin of Phosphorescence in Bismoles: A Synthetic and Computational Study

A series of bismuth heterocycles, termed bismoles, were synthesized via the efficient metallacycle transfer (Bi/Zr exchange) involving readily accessible zirconacycles. The luminescence properties of three structurally distinct bismoles were explored in detail via time-integrated and time-resolved photoluminescence spectroscopy using ultrafast laser excitation. Moreover, time-dependent density functional theory computations were used to interpret the nature of fluorescence versus phosphorescence in these bismuth-containing heterocycles and to guide the future preparation of luminescent materials containing heavy inorganic elements. Specifically, orbital character at bismuth within excited states is an important factor for achieving enhanced spin-orbit coupling and to promote phosphorescence. The low aromaticity of the bismole rings was demonstrated by formation of a CuCl π-complex, and the nature of the alkene-CuCl interaction was probed by real-space bonding indicators derived from Atoms-In-Molecules, the Electron Localizability Indicator, and the Non-Covalent Interaction index; such tools are of great value in interpreting nonstandard bonding environments within inorganic compounds.