Conformational Flexibility Determines Electronic Coupling and Charge Transfer Character in Single Propeller-Shaped Perylene Diimide Tetramer Arrays

Abstract

We investigate the effect of molecular geometry and conformational flexibility on electronic coupling and charge transfer interactions within propeller-shaped perylene diimide (PDI) tetramer arrays differing by the number of covalent linkages to a central spirobifluorene core. Electronic spectra of tetramers with one (“floppy”) or two (“rigid”) bay covalent linkages display evidence of charge transfer character in either ground or excited states. Floppy tetramers exhibit marked red-shifted and broadened absorption features that we assign as overlapping inter-PDI charge transfer and PDI-centered π–π* transitions, whereas rigid tetramers retain features similar to single PDI molecules, albeit with broader line widths. Interestingly, both tetramers exhibit charge transfer character in their fluorescence emission, but this is most prominent in the rigid tetramer, which displays dominant long-lived excimer behavior in addition to a minority component resembling single PDI-like emission. We then use single-molecule spectroscopy and imaging to understand how conformational-dependent charge transfer properties influence tetramer photophysics. Over 90% of single rigid tetramers display telegraphic (i.e., two-level) blinking behavior with relatively short “on” times compared to ∼60% of single floppy tetramer transients, which tend to exhibit emission from multiple levels. Electronic structure simulations were next performed to aid in the assignment of electronic transitions and photophysical behavior. Floppy tetramer canonical and natural transition orbitals reveal remarkable similarities with significant charge transfer character in the lowest energy excited states involving transverse PDI units and appreciable spirobifluorene contributions in the ground electronic state. Rigid tetramers exhibit greater electronic delocalization, and calculated absorption transition energies show good agreement with experiment, although excited-state interactions are less straightforward to discern from simulations. Raman spectroscopy and polarization-dependent single-molecule spectroscopy were also performed, supporting assignments based on theoretical predictions and electronic spectroscopy results. Overall, we demonstrate the importance of molecular geometry and conformational flexibility of multichromophore arrays in determining the nature of electronic interactions in ground and excited states, which can eventually be harnessed to improve performance attributes at the materials level.

Excited-State Symmetry Breaking in a Multiple Multipolar Chromophore Probed by Single-Molecule Fluorescence Imaging and Spectroscopy

“…As exemplified in Figure a, the DB6 single molecules in Zeonex primarily showed on–off blinking and multistep intensity jumps followed by photobleaching. Such stepwise photobleaching behavior in single-molecule measurements is similar to the emission behavior of multichromophoric systems. − Importantly, the corresponding photon coincidence histogram showed a clear photon antibunching (see the inset in Figure a), confirming that the intensity time trajectory was obtained from a single molecule. Other examples of fluorescence intensity time traces are shown in Figure S7.…”

“…The presence of three D−π–D portions (or six π–D sites) in the DB6 molecule suggests that this chromophore could behave as a multichromophore with plural emission sites because the substructure chromophores of DB6 , i.e., π–D ( DB1 ) and D−π–D ( DB2 ), are highly fluorescent (Chart S1). For the investigation on complex photophysical processes in multichromophoric systems, single-molecule fluorescence imaging and spectroscopy has emerged as an effective method. − As listed in Table S1, DB6 has a large molar absorption coefficient (ε max > 10 5 M –1 cm –1 ) and a high fluorescence quantum yield in nonpolar solvents, which render it suitable for single-molecule fluorescence detection. Herein, to thoroughly elucidate the ESB of DB6 , we have performed its ensemble spectroscopic measurements in solution, single-molecule fluorescence imaging and spectroscopy in a nonpolar polymer film, and theoretical calculations for both the ground state (S 0 ) and excited singlet states (S n ).…”

Fully sp2-carbon connected polymeric frameworks with rotatable conformation-enhanced lithium-storage performance

“…Flexible TPT-CMP exhibited red-shifted and broadened absorption features, whereas rigid BFT-CMP retained absorption features slightly blue-shifted showing the effect of conformational flexibility on electronic coupling and charge transfer interactions. 89 These phenomena suggested that such kinds of porous polymers exhibited excellent semiconducting properties, also manifesting their π-extended skeletons, thus being favorable for electron transfer over the 3D networks.…”

Facile Synthesis and Evaluation of Electron Transport and Photophysical Properties of Photoluminescent PDI Derivatives

“…There are only a few reports on the study of PDI/donor mixtures for charge separation dynamics. 28 Recently, Tamai and co-workers reported the time evolution of electroabsorption caused by the electric field generated within electron–hole pairs using transient absorption spectroscopy (TAS) in a polymer solar cell with a perylene diimide (PDI) dimer acceptor. 29 From the study, they found that the charge carriers were generated very efficiently, i.e., ∼90% of quantum yield, and separated within <1 ps.…”

“…There is still lack of complete characterization and analysis of substituents’ effect on electrical and photophysical properties in neat as well donor/PDI blends. There are only a few reports on the study of PDI/donor mixtures for charge separation dynamics . Recently, Tamai and co-workers reported the time evolution of electroabsorption caused by the electric field generated within electron–hole pairs using transient absorption spectroscopy (TAS) in a polymer solar cell with a perylene diimide (PDI) dimer acceptor .…”