Promoting Intersystem Crossing of a Fluorescent Molecule via Single Functional Group Modification

Abstract

Pure light-atoms organic phosphorescent molecules have been under scientific scrutiny because they are inexpensive, flexible, and environment friendly.The development of such materials, however, faces a bottleneck problem of intrinsically small spin-orbit couplings (SOC), which can be addressed by seeking a proper balance between intersystem crossing (ISC) and fluorescence rates. Using Nsubstituted naphthalimides (NNI) as the prototype molecule, we applied chemical modifications with several electrophilic and nucleophilic functional groups, to approach the goal. The selected electron donating groups actively restrain the fluorescence, enabling an efficient ISC to the triplet manifold. Electron withdrawing groups do not change the luminescent properties of the parent species. The changes in ISC and fluorescence rates are related to the nature of the lowest singlet state, which changes from localized excitation into charge-transfer excitation. This finding opens an alternative strategy for designing pure light-atoms organic phosphorescent molecules for emerging luminescent materials applications.It has been reported that small ST E  led to efficient ISC and thereby strong phosphorescence in dibenzothiophene-S,S-dioxide and 2-biphenyl-4,6-bis(12phenylindolo[2,3-a]carbazole-11-yl)-1,3,5-triazine complexes. [26][27] In suitable conditions, the metal-ligand compounds may emit phosphorescence thanks to the ligand-localized nature ( 3 IL * or 3 π-π * ) of the lowest triplet state and the relaxation of 3 LMCT/ 3 MLCT to 3 πLC * . 28 However, the risk of toxicity and instability of fluoresce as long as conventional conditions are met: nonradiative rates are small, and the chromophore is in an oxygen-depleted atmosphere. Restrained-fluorescence ISC turns out to be so efficient that it can deplete the singlet population even in systems with spin-orbit couplings as small as those in El-Sayed forbidden transitions.This study cast new insight into the design of pure light-atoms organic phosphorescent molecules via chemical modifications of functional groups. By exploring more organic molecules using the same strategy and employing more comprehensive theoretical framework that can compute phosphorescent radiative decay rate, [44][45] we are on the way of rational design of new organic phosphorescent molecules.The computational details, transition energies, transition related molecular orbitals and corresponding electron-hole distribution of the lowest singlet excited state, the energy level diagrams of low-lying excited states, the Cartesian coordinates of all optimized molecules.

Excited state mechanisms in crystalline carbazole: the role of aggregation and isomeric defects

“…The reorganisation energy was approximated as λ ≈ E T m (R min S n ) − E T m (R min T m ), as it has been done in previous works. 36 The spin-orbit couplings (SOCs) were evaluated at the TD-B3LYP/6-311++G(d,p) level of theory adopting the Breit-Pauli spin-orbit Hamiltonian with effective charge approximation as implemented in the PySOC package. 37 The ET rate coefficients were also calculated using the Marcus-Levich-Jortner (MLJ) equation as implemented in references 38 and 39:…”

Enhancing Intersystem Crossing to Achieve Thermally Activated Delayed Fluorescence in a Water-Soluble Fluorescein Derivative with a Flexible Propenyl Group

“…We try to find out the main factors for enhancing ISC efficiency. According to the semiclassical Marcus theory , (eq S2), the main factors affecting the ISC rate constant are the vertical free energy gap as Δ G ST ≈ Δ E ST = E Tm ( R minS1 ) – E S1 ( R minS1 ) and the spin–orbit coupling constant ( V SOC ) . Briefly, the formula of the ISC rate can be qualitatively expressed as follows: It should be noted that the smaller energy gap Δ E ST is, the more likely ISC is to occur.…”

“…According to the semiclassical Marcus theory 40,41 (eq S2), the main factors affecting the ISC rate constant are the vertical free energy gap as ΔG ST ≈ ΔE ST = E Tm (R minS1 ) − E S1 (R minS1 ) and the spin− orbit coupling constant (V SOC ). 42 Briefly, the formula of the ISC rate can be qualitatively expressed as follows:…”

Organic room-temperature phosphorescence from halogen-bonded organic frameworks: hidden electronic effects in rigidified chromophores

“…10 2 to 10 3 cm À1 for organometallic complexes), the introduction of a carbonyl functionality to aromatic rings oen opens up a 1 (n-p*) / 3 (p-p*) (or 1 (p-p*) / 3 (n-p*)) channel with SOC $100 cm À1 . [29][30][31][32][33] Such a small increase is sufficient to allow efficient ISC and populate the triplet of, for instance, benzophenone or benzaldehyde with a near-unitary quantum efficiency. 34,35 The structure of the asgenerated triplet states can be rigidied in the solid state with the aid of non-covalent interactions (e.g.…”