Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad.

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long-lived triplet charge-transfer (3 CT) state, based on the electron spin control using spin-orbit charge transfer intersystem crossing (SOCT-ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT-ISC takes 8 ns to produce the 3 NI* state. Then the slow secondary CS (125 ns) gives the long-lived 3 CT state (0.94 µs in deaerated n-hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as 1 NI*→1 CT→3 NI*→3 CT. With time-resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron-spin polarization pattern was observed for the naphthalimide-localized triplet state. Our spiro compact dyad structure and the electron spin-control approach is different to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory.

Two competing acceptors: Electronic structure of PNDITBT probed by time-resolved electron paramagnetic resonance spectroscopy.

Balanced charge transport is particularly important for transistors. Hence, ambipolar organic semiconductors with comparable transport capabilities for both positive and negative charges are highly sought-after. Here, we report detailed insights into the electronic structure of PNDITBT, which is an alternating copolymer of naphthalene diimide (NDI), thiophene, benzothiodiazole (B), and thiophene (T) units, as gained by time-resolved electron paramagnetic resonance (TREPR) spectroscopy combined with quantum-chemical calculations. The results are compared to those obtained for PNDIT2 and PCDTBT, which are derivatives without B and NDI acceptor units, respectively. These two polymers show dominant n- and p-channel behavior in organic field-effect transistors. The TBT moiety clearly dominates the electronic structure of PNDITBT, although less so than in PCDTBT. Furthermore, the triplet exciton most probably delocalizes along the backbone, exhibits a highly homogeneous environment, and planarizes the polymer backbone. Obtaining the zero-field splitting tensors of these triplet states by means of quantum-chemical calculations reveals the triplet energy sublevel associated with the molecular axis parallel to the backbone to be preferentially populated, while the one perpendicular to the aromatic plane is not populated at all, consistent with the spin-density distribution. PNDITBT consisting of two acceptors (NDI and B) has a complex electronic structure, as evident from the two charge-transfer bands in its absorption spectrum. TREPR spectroscopy provides a detailed insight on a molecular level not available by and complementing other methods.

Impact of Side Chains of Conjugated Polymers on Electronic Structure: A Case Study.

Processing from solution is a crucial aspect of organic semiconductors, as it is at the heart of the promise of easy and inexpensive manufacturing of devices. Introducing alkyl side chains is an approach often used to increase solubility and enhance miscibility in blends. The influence of these side chains on the electronic structure, although highly important for a detailed understanding of the structure-function relationship of these materials, is still barely understood. Here, we use time-resolved electron paramagnetic resonance spectroscopy with its molecular resolution to investigate the role of alkyl side chains on the polymer PCDTBT and a series of its building blocks with increasing length. Comparing our results to the non-hexylated compounds allows us to distinguish four different factors determining exciton delocalization. Detailed quantum-chemical calculations (DFT) allows us to further interpret our spectroscopic data and to relate our findings to the molecular geometry. Alkylation generally leads to more localized excitons, most prominent only for the polymer. Furthermore, singlet excitons are more delocalized than the corresponding triplet excitons, despite the larger dihedral angles within the backbone found for the singlet-state geometries. Our results show TREPR spectroscopy of triplet excitons to be well suited for investigating crucial aspects of the structure-function relationship of conjugated polymers used as organic semiconductors on a molecular basis.