Excitation Energy Transfer between bodipy Dyes in a Symmetric Molecular Excitonic Seesaw

Abstract

We examine the redistribution of energy between electronic and vibrational degrees of freedom that takes place between a π-conjugated oligomer, a phenylene–butadiynylene, and two identical boron–dipyrromethene (bodipy) end-caps using femtosecond transient absorption spectroscopy, single-molecule spectroscopy, and nonadiabatic excited-state molecular dynamics (NEXMD) modeling techniques. The molecular structure represents an excitonic seesaw in that the excitation energy on the oligomer backbone can migrate to either one end-cap or the other, but not to both. The NEXMD simulations closely reproduce the characteristic time scale for redistribution of electronic and vibrational energy of 2.2 ps and uncover the vibrational modes contributing to the intramolecular relaxation. The calculations indicate that the dihedral angle between the bodipy dye and the oligomer change upon excitation of the oligomer. Single-molecule experiments reveal a difference in photoluminescence lifetime of the bodipy dyes depending on whether they are excited by direct absorption or by redistribution of energy from the backbone. This difference in lifetime may be attributed to the difference in dihedral angle. The simulations also suggest that a strong coupling can occur between the two end-caps, giving rise to a reversible shuttling of excitation energy between them. Strong coupling should lead to a pronounced loss in polarization memory of the fluorescence since the oligomer backbone tends to be slightly distorted and the two bodipy transition dipoles have different orientations. A sensitive single-molecule technique is presented to test for such coupling. However, although redistribution of electronic and vibrational energy between the end-caps can occur, it appears to be unidirectional and irreversible, suggesting that an additional localization mechanism is at play which is, as yet, not fully accounted for in the simulations.

NEXMD v2.0 Software Package for Nonadiabatic Excited State Molecular Dynamics Simulations

“…112 The package also incorporates practical analyses of electronic excitations in terms of orbital distribution of transition density and its timedependent flux 67,123 as well as real-space representation of transition density matrices. 96,124,125 These features are discussed in detail elsewhere. 50 NEXMD v2.0 incorporates additional new features besides the implementation of EHR and AIMC methods (Table I).…”

“…Additional NEXMD features implemented in the original release of the software package and retained in the updated NEXMD v2.0 release include useful tools for molecular dynamics simulations such as the Langevin thermostat, geometry optimization routine, continuum solvation models, , trivial unavoided crossings, , and decoherence corrections . The package also incorporates practical analyses of electronic excitations in terms of orbital distribution of transition density and its time-dependent flux , as well as real-space representation of transition density matrices. ,, These features are discussed in detail elsewhere . NEXMD v2.0 incorporates additional new features besides the implementation of EHR and AIMC methods (Table ).…”

An Azobenzene-Clamped Bichromophore

“…On the one hand, such quenching could also Template for Organic Materials Thieme conceivably be caused by the flexibility and mobility of the clamp pieces, as has already been observed for molecular rotors such as BODIPY dye molecules. 30,31 On the other hand, it is also possible that energy or electron transfer occurs to the azobenzene groups, for which low fluorescence quantum yields have already been reported. 32 A possibility to test whether efficient energy transfer can occur within the clamped bichromophore structure 2 is to measure the fluorescence photon statistics of single molecules as discussed in Figure S10 of the SI.…”