Spin–Orbit Charge-Transfer Intersystem Crossing of Compact Naphthalenediimide-Carbazole Electron-Donor–Acceptor Triads

Abstract

Compact electron donor−acceptor triads based on carbazole (Cz) and naphthalenediimide (NDI) were prepared to study the spin−orbit charge-transfer intersystem crossing (SOCT-ISC). By variation of the molecular conformation and electron-donating ability of the carbazole moieties, the electronic coupling between the two units was tuned, and as a result charge-transfer (CT) absorption bands with different magnitudes were observed (ε = 4000−18 000 M −1 cm −1 ). Interestingly, the triads with NDI attached at the 3-C position or with a phenyl spacer at the N position of the Cz moiety, thermally activated delayed fluorescence (TADF) was observed. Femtosecond transient absorption (fs-TA) spectroscopy indicated fast electron transfer (0.8−1.5 ps) from the Cz to NDI unit, followed by population of the triplet state (150−600 ps). Long-lived triplet states (up to τ T = 45−50 μs) were observed for the triads. The solvent-polarity-dependent singlet-oxygen quantum yield (Φ Δ ) is 0−26%. Timeresolved electron paramagnetic resonance (TREPR) spectral study of TADF molecules indicated the presence of the 3 CT state for NDI-Cz-Ph (zero-field-splitting parameter D = 21 G) and an 3 LE state for NDI-Ph-Cz (D = 586 G). The triads were used as triplet photosensitizers in triplet−triplet annihilation upconversion by excitation into the CT absorption band; the upconversion quantum yield was Φ UC = 8.2%, and there was a large anti-Stokes shift of 0.55 eV. Spatially confined photoexcitation is achieved with the upconversion using focusing laser beam excitation, and not the normally used collimated laser beam, i.e., the upconversion was only observed at the focal point of the laser beam. Photo-driven intermolecular electron transfer was demonstrated with reversible formation of the NDI −• radical anion in the presence of the sacrificial electron donor triethanolamine.

Solvent‐Free Synthesis of Core‐Functionalised Naphthalene Diimides by Using a Vibratory Ball Mill: Suzuki, Sonogashira and Buchwald–Hartwig Reactions

“…The scope of the reaction was assessed (Scheme 3) using a range of diaryl and aryl/alkyl secondary amines to yield 4 a – 4 d in the solid state (Figure 5). These reactions proceeded in similar yields to the those reported for comparable solution‐state reactions ( 4 a (22 %), vs. 20–50 % in solution [28a,c] ) and carbazole ( 4 b (36 %) vs. 42–43 % in solution [28a,c] ). Electron rich groups were readily tolerated, for example para ‐methoxy substituted diphenyl amine gave 4 c in 44 % yield (cf.…”

“…These conditions gave an excellent isolated yield for the phenyl substituted c‐NDI after 60 min reaction time ( 2 b , 83 %). This may be compared to reported solution‐state synthetic routes to phenyl substituted c‐NDIs (10 %; Suzuki coupling, 14 h, 100 °C, under nitrogen) [28c,33] and 80 % (CH arylation, 72 h, benzene under reflux) [34] …”

“…Aminated c‐NDIs have been demonstrated to behave as electron donor triads which exhibit spin‐orbit charge‐transfer intersystem crossing [28c] . Compounds that have these properties are actively being studied for use in photo‐dynamic therapy (PDT), [43] triplet‐triplet annihilation (TTA) upconversion, [44] photocatalysis [45] and photovoltaic applications [46]…”

Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

“…[3][4][5][6]59,64 Any compounds with closely-lying 1 CT, 3 CT and 3 LE states may exhibit TADF properties (or more generally, close-lying S 1 /T 1 states). 65,66 Recently, we prepared a compact, orthogonal electron donor-acceptor dyad, with naphthalimide (NI) as the electron acceptor and phenothiazine (PTZ) as the electron donor (Fig. 6).…”

Intersystem Crossing and Electron Spin Dynamics of Photoexcited Bodipy Dimers

“…We have previously studied intramolecular triplet energy transfer in Bodipy dimers exhibiting SOCT-ISC , and intermediate triplet states in a thermally activated delayed fluorescence (TADF) material. , It has been shown that reversible intramolecular processes, including energy transfer between the two subunits in the dimer, change the electron spin polarization (ESP) of the observed states and the shape of the spectrum and increase the anisotropic spin–lattice relaxation. , These properties enable us to describe the processes that occur during photoexcitation of dimers and to establish the presence of dynamic equilibrium between the excited states of the two monomers. To further study these phenomena, we have prepared two new dimers (Scheme ).…”