RESUMEN
We used a systematic approach to shed light on the inherent differences in perylenes, namely monoimides versus diimides, including coplanarity and dipole moment, and their impact on singlet fission (SF) by designing, synthesizing, and probing a full fledged series of phenylene- and naphthalene-linked dimers. Next to changing the functionality of the perylene core, we probed the effect of the spacers and their varying degrees of rotational freedom, molecular electrostatic potentials, and intramolecular interactions on the SF-mechanism and -efficiencies. An arsenal of spectroscopic techniques revealed that for perylene-monoimides, a strong charge-transfer mixing with the singlet and triplet excited states restricts SF and yields low triplet quantum yields. This is accompanied by an up-conversion channel that includes geminate triplet-triplet recombination. Using perylene-diimides alters the SF-mechanism by populating a charge-separated-state intermediate, which either favors or shuts-down SF. Napthylene-spacers bring about higher triplet quantum yields and overall better SF-performance for all perylene-monoimides and perylene-diimides. The key to better SF-performance is rotational freedom because it facilitates the overall excited-state polarization and amplifies intramolecular interactions between chromophores.
RESUMEN
Strong push-pull interactions between electron donor, diaminoazobenzene (azo), and an electron acceptor, perylenediimide (PDI), entities in the newly synthesized A-D-A type triads (A=electron acceptor and D=electron donor) and the corresponding A-D dyads are shown to reveal wide-band absorption covering the entire visible spectrum. Electrochemical studies revealed the facile reduction of PDI and relatively easier oxidation of diaminoazobenzene in the dyads and triads. Charge transfer reversal using fluorescence-spectroelectrochemistry wherein the PDI fluorescence recovery upon one-electron oxidation, deterring the charge-transfer interactions, was possible to accomplish. The charge transfer state density difference and the frontier orbitals from the DFT calculations established the electron-deficient PDI to be an electron acceptor and diaminoazobenzene to be an electron donor resulting in energetically closely positioned PDIδ- -Azoδ+ -PDIδ- quadrupolar charge-transfer states in the case of triads and Azoδ+ -PDIδ- dipolar charge-transfer states in the case of dyads. Subsequent femtosecond transient absorption spectral studies unequivocally proved the occurrence of excited-state charge transfer in these dyads and triads in benzonitrile wherein the calculated forward charge transfer rate constants, kf , were limited to instrument response factor, meaning >1012 â s-1 revealing the occurrence of ultrafast photo-events. The charge recombination rate constant, kr , was found to depend on the type of donor-acceptor conjugates, that is, it was possible to establish faster kr in the case of triads (â¼1011 â s-1 ) compared to dyads (â¼1010 â s-1 ). Modulating both ground and excited-state properties of PDI with the help of strong quadrupolar and dipolar charge transfer and witnessing ultrafast charge transfer events in the studied triads and dyads is borne out from the present study.
RESUMEN
Photoinduced charge transfer and separation events in a newly synthesized azobenzene-bridged perylenediimide-dimer (PDI-dimer) are demonstrated. Trans-to-cis conversion (â¼50 % efficiency) from the initial trans PDI-dimer by 355â nm pulsed laser light, and its reversal, cis-to-trans, process by 435â nm laser light irradiation has been possible to accomplish. Efficient fluorescence quenching in the PDI-dimer, more so for the cis isomer was witnessed, and such quenching increased with increasing solvent polarity. DFT-calculated geometry and electronic structures helped in visualizing the charge transfer in the PDI-dimer in both isomeric forms, and also revealed certain degree of participation of the azobenzene entity in the charge transfer events. Femtosecond transient absorption spectral studies confirmed occurrence of both charge transfer followed by charge separation in the studied PDI-dimer in both trans and cis forms in polar solvents, and the evaluated time constants from Global target analysis revealed accelerated events in the cis PDI-dimer due to proximity effects. The present study offers key insights on the role of the azobenzene bridge, and the dimer geometry in governing the excited state charge transfer and separation in symmetrically linked PDI dimer.
RESUMEN
Triplet-excited-state energies of perylene-monoimides (PMIs) lie in the range 1.12 eV ± 2 meV when compared to singlet-excited-state energies of about 2.39 eV ± 2 meV; therefore, the corresponding naphthalene-linked PMI-Dimer was investigated as a novel singlet-fission (SF) material. Ultrafast transient absorption measurements demonstrated the (S1S0)-to-1(T1T1) transformation and the involvement of a mediating step in the overall 1(T1T1) formation. The intermediate is a charge-transfer state that links the initial (S1S0) with the final 1(T1T1), and imposes charge-transfer character on both, which are thus denoted (S1S0)CT and 1(T1T1)CT. At room temperature, the decorrelation and stability of 1(T1T1)CT is affected by the geminate triplet-triplet recombination (G-TTR) of the two triplets. Independent confirmation for G-TTR to afford up-converted (S1S0)UC in fsTA and nsTA measurements with PMI-Dimer, came from probing PMI-Monomer (T1)s in triplet-triplet annihilation up-conversion (TTA-UC). The G-TTR channel, active in the PMI-Dimer at room temperature, is suppressed by working at either low temperatures (â¼140 K) or in polar solvents (benzonitrile): Both scenarios assist in stabilizing (T1T1)CT. As a consequence, the triplet quantum yields are 4.2% and 14.9% at room temperature and 140 K, respectively, in 2-methyltetrahydrofuran.
