Charge Separation and Intersystem Crossing in Homo- and Hetero-Compact Naphthalimide Dimers

Abstract

Naphthalimide (NI) homo- and hetero-dimers adopting orthogonal geometry were prepared to study photo-induced symmetry-breaking charge transfer (SBCT) and charge recombination (CR)-induced intersystem crossing (ISC). The two moieties in the dimer are connected either at the 3-C or 4-C position of the NI unit. The photophysical properties of the dimers were studied with steady-state and transient absorption spectroscopic methods. Significant CT only occurs for the hetero-dimer, in which one NI unit has a 4-amino substituent and the other NI unit is without it. The CR-induced ISC is most efficient for this dimer (singlet oxygen quantum yield ΦΔ = 50.3%). For the homo-dimer, in which both NI units did not present amino substitution, SBCT was not observed. Based on the electrochemical studies, we propose that the absence of SBCT for the homo-dimer is attributed to its high oxidation potential and low reduction potential. Femtosecond transient absorption (fs TA) spectra show that there is no charge separation (CS) for the homo-dimer. Nanosecond transient absorption spectroscopy indicate the formation of a triplet state with electron delocalization for the homo dimer, with a lifetime of 72.0 μs, while for the hetero dimer a triplet state with an intrinsic lifetime of 206.4 μs is observed. CS (11.6 ps) and slow CR-induced ISC (>1.5 ns) were observed for the hetero-dimer. Time-resolved electron paramagnetic resonance spectra give the zero-field splitting parameters (|D| = 1894 MHz and |E| = 111 MHz) and electron spin polarization patterns (e, e, e, a, a, a) for the triplet state of the hetero-dimer, inferring that the triplet state of the hetero-dimer is confined on the amino-substituted NI moiety.

Tuning the photophysical property of naphthalimide-phenothiazine dyads by variation of the electron donating and accepting capability: synthesis and study of the intersystem crossing and thermally-activated delayed fluorescence

“…Moreover, note the CS emission wavelength of NI-PTZ-F-O and NI-PTZ-Ph-O is different from the 4-diphenylamine NI [58]. These results indicate the 1 CS state of these two dyads involves the oxidized PTZ unit, as a whole of electron donor, not only the N atom of the PTZ unit [59,60]. Note, the NI-PTZ-F-O and NI-PTZ-Ph-O adopt orthogonal geometry, and the N atom in PTZ unit is not in -conjugation with NI moiety, due to conformation restriction [55].…”

Probing the excited state dynamics in perinone molecules for photovoltaic applications using transient absorption spectroscopy

“…Several groups have probed the triplet generation mechanism in such molecules by transient absorption spectroscopy. 8–11 The photophysical characteristics of perinone are similar to those of perylene and naphthalene diimide derivatives which are widely used as n-type semiconductors in photovoltaics, light-emitting diodes, and organic field-effect transistors. 12 Erten et al fabricated dye-sensitized nanocrystalline TiO 2 solar cells based on perinone dye.…”

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor‐Acceptor Systems

“…a CS state is formed for NI-dimer-2, with the NI unit as electron acceptor and the ANI unit as the electron donor. [46] The emissive states of NIdimer-2 and NI-2 show different k r and k nr values, indicates that the emissive CT states of the two compounds are different. Interestingly, NI-dimer-1 shows solvent polarityindependent fluorescence, the fluorescence emissive wavelength (ca.…”

“…We studied the possible SBCT in two NI dimers (Figure 5). [46] Similar to 9,9′‐bianthryl, [47] NI‐dimer‐1 is strictly a dimer that able to show SBCT, because the two units are literally the same. Concerning this aspect, NI‐dimer‐2 is different from NI‐dimer‐1 , as well as bis ‐ANI , in that there is an ANI unit and a different NI unit in the dimer.…”

“…9.8 ps in DCM or 2.5 ps in ACN. No CS state was observed for NI‐dimer‐1 [46] . For NI‐dimer‐2 , significant singlet oxygen ( 1 O 2 ) production was observed in toluene (TOL) (quantum yield, Φ Δ =46.5 %) and DCM ( Φ Δ =26.8 %), although the Φ Δ values are negligible in HEX and ACN.…”