Intersystem Crossing in Naphthalenediimide–Oxoverdazyl Dyads: Synthesis and Study of the Photophysical Properties

Abstract

Oxoverdazyl( Vz) radicalu nits were covalently linked to the naphthalenediimide (NDI) chromophore to study the effect of the radicalo nt he photophysical properties, especially the radicale nhanced intersystem crossing (REISC), which is ap romisinga pproach to develop heavyatom-free triplet photosensitizers. Rigid phenylo re thynylphenyll inkers betweent he two moieties were used, thus REISC andf ormation of doublet (D 1 ,t otal spin quantum number S = 1/2) and quartet states (Q 1 , S = 3/2) are anticipated. The photophysical properties of the dyadsw ere studied with steady-statea nd femtosecond/nanosecond transient absorption (TA) spectroscopiesa nd DFT computations. Femtosecond transienta bsorption spectras how af ast electron transfer (< 150 fs), and ISC (ca. 1.4-1.85ps) is induced by charger ecombination (CR, in toluene). Nanosecond transient absorption spectra demonstrated ab iexponential decay of the triplet state of the NDI moiety.T he fast component (lifetime:5 0ns; population ratio:8 0%)i sa ssigned to the D 1 ! D 0 decay,a nd the slow decay component (2.0 ms; 20 %) to the Q 1 !D 0 ISC. DFT computations indicated ferromagnetic interactions between the radicala nd chromophore( J = 0.07-0.13 eV). Reversible formation of the radical aniono ft he NDI moiety by photoreduction of the radical-NDI dyadsi nt he presence of sacrificial electron donor triethanolamine (TEOA) is achieved. This work is useful for design of new triplet photosensitizers based on the REISC effect.

Radical‐Enhanced Intersystem Crossing in a Bay‐Substituted Perylene Bisimide−TEMPO Dyad and the Electron Spin Polarization Dynamics upon Photoexcitation**

“…However, both D 1 and Q states are with the PBI moiety in the triplet state, therefore, the D 1 and the Q states are indistinguishable in optical transient absorption spectra [47] . We didn't observe biexponential decay of the triplet state, which is different from the recently reported NDI‐verdazyl radical dyads [82] …”

“…Interestingly, PBI−NH shows a triplet state lifetime of 15.0 μ s (intermolecular photosensitizing method, see Figures S21 in Supporting Information). These results infer that there is an electron spin‐spin exchange interaction between the radical and the PBI moiety in PBI−TEMPO , by which the triplet state of the PBI may be shortened [56,82] …”

Radical‐Enhanced Intersystem Crossing in Perylene‐Oxoverdazyl Radical Dyads

“…The data were analyzed with single value decomposition (SVD) and global fitting, employing a linear unidirectional decay scheme, obtaining the time constants and the Evolution Associated Difference Spectra (EADS). In previously reported radical‐containing molecules, for instance, NDI labeled with TEMPO, the ISC occurs within 338 ps, [7] whereas for the NDI‐veradazyl dyad a charge recombination‐induced ISC was observed (time constant: 1.40 ps), instead of the expected electron spin‐spin exchange interaction‐induced ISC [41] . Furthermore, in the compact perylene‐NO⋅ dyad, the REISC takes place within 1 ps [26] .…”

“…The fluorescence spectra of the dyads were also measured in solvents of different polarities, the results indicated that the emission is quenched in all solvents compared with the pristine perylene (Figure S24 and S25 in the Supporting Information). The strong quenching of the perylene fluorescence in the dyads can be ascribed to different reasons, including Förster Resonance Energy Transfer (FRET) towards the verdazyl moiety, photoinduced electron transfer (PET), spin‐spin exchange interactions to form high spin states or radical enhanced ISC [7,41,44–46] …”

Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

“…Nowadays, mode-locked Ti:sapphire lasers can routinely provide highly stable pulses, even as short as about 4 fs. The resulting fundamental 800 nm laser mode can be efficiently tuned in the UV and visible spectral region in order to provide suitable pump wavelengths [7,[21][22][23]. The wide tunability is accomplished by means of OPA (Optical Parametric Amplification), OPG (Optical Parametric Generation) or by Second Harmonic (SHG) and Third Harmonic (THG) Generation.…”