Photoblinking dynamics in single calix[4]arene-linked perylene bisimide dimers

Symmetry-Breaking Charge Separation in a Chiral Bis(perylenediimide) Probed at Ensemble and Single-Molecule Levels

Chiral molecular assemblies exhibiting symmetry-breaking charge separation (SB-CS) are potential candidates for the development of chiral organic semiconductors. Herein, we explore the excited-state dynamics of a helically chiral perylenediimide bichromophore (Cy-PDI2) exhibiting SB-CS at the ensemble and single-molecule levels. Solvent polarity-tunable interchromophoric excitonic coupling in chiral Cy-PDI2 facilitates the interplay of SB-CS and excimer formation in the ensemble domain. Analogous to the excited-state dynamics of Cy-PDI2 at the ensemble level, single-molecule fluorescence lifetime traces of Cy-PDI2 depicted long-lived off-states characteristic of the radical ion pair-mediated dark states. The discrete electron transfer and charge separation dynamics in Cy-PDI2 at the single-molecule level are governed by the distinct influence of the local environment. The present study aims at understanding the fundamental excited-state dynamics in chiral organic bichromophores for designing efficient chiral organic semiconductors and applications toward charge transport materials.

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

Excited-state symmetry breaking (ESB) has attracted much attention because it is often observed in symmetric multipolar chromophores designed as two-photon absorption/emission materials. Herein, we report an ensemble and single-molecule fluorescence imaging and spectroscopy investigation of ESB in hexakis[4-(p-dioctylaminostyryl)phenylethynyl]benzene(DB6), a two-photon absorber possessing a C₆-symmetric π-D₆ structure (π = hexaethynylbenzene, D = (p-dioctylaminostyryl)phenyl group) consisting of three equivalent D-π-D moieties. Ensemble and single-molecule measurements and theoretical calculations revealed that DB6 undergoes a photoabsorption process with two orthogonal transition dipole moments, whereas it fluoresces with a single transition dipole moment after one- or two-step ESB upon photoexcitation, depending on the environmental polarity. In nonpolar solvents and polymer films, one of the three D-π-D sites becomes planar, and the excited state is localized on this moiety: a [D^δ+-π^δ--D^δ+]* quadrupolar state is formed. In polar solvents, the symmetry is further broken within the planarized D-π-D moiety, and the excited state is localized on one of the two D-π sites; i.e., a D-[π^δ--D^δ+]* dipolar state is generated. Hence, DB6 can behave like a multichromophore with multiple emission sites in the molecule, which was demonstrated by stepwise photobleaching under photon antibunching conditions.

Stepwise decrease of fluorescence versus sequential photobleaching in a single multichromophoric system

For individual molecules from the newly synthesized calix[4]arene tethered perylene bisimide (PBI) trimer, we studied the emitted fluorescence intensity as a function of time. Owing to the zigzag arrangement of PBI dyes in these trimers, the polarization state of the emission provides directly information about the emitting subunit within the trimer. Interestingly, we observed emission from all neutral subunits within a trimer rather than exclusively from the subunit with the lowest site energy. This can be understood in terms of thermally activated uphill energy transfer that repopulates the higher energetic chromophores. Together with the simultaneously recorded polarization-resolved emission spectra, this reveals that the emission from a multichromophoric system is governed by a complex interplay between the temporal variations of the photophysical parameters of the subunits, bidirectional hopping processes within the trimer, and unavoidable photobleaching. Moreover, it is demonstrated that the typically observed stepwise decrease of the signal from a multichromophoric system does not necessarily reflect sequential bleaching of the individual chromophores within the macromolecule.