Energy transfer and spectroscopic characterization of a perylenetetracarboxylic diimide (PDI) hexamer

Abstract

We report a comprehensive study on a newly synthesized perylenetetracarboxylic diimide (PDI) hexamer together with its corresponding monomer and dimer by means of steady-state absorption and fluorescence as well as femtosecond broadband transient absorption measurements. The structure of the PDI hexamer is nearly arranged in a 3-fold symmetry by three identical and separated dimers. This unique structure makes the excited state energy relaxation processes more complex due to the existence of two different intramolecular interactions: a strong interaction between face-to-face PDIs in dimers and a relatively weak interaction between the three separated PDI dimers. The steady-state spectra and the ground state structural optimization show that the steric effect plays a dominant role in keeping the formation of the face-to-face stacked PDI-dimer within the PDI-hexamer, indicating that some level of a pre-associated excimer had formed already in the ground state for the dimer in the hexamer. Femtosecond transient absorption experiments on the PDI hexamer reveal a fast (∼200 fs) localization process and a sequential relaxation to a pre-associated excimer trap state from the delocalized exciton state with about 1.2 ps after the initially delocalized excitation. Meanwhile, excitation energy transfer among the three separated dimers within the PDI-hexamer is also revealed by the anisotropic femtosecond pump-probe transient experiments, where the hopping time is about 2.8 ps. A relaxed excimer state is further formed in 7.9 ps after energy hopping and conformational relaxation.

Excited‐State Deactivation of Branched Phthalocyanine Compounds

“…In our results, the spectrum does not show an apparent blueshift, but shows a significant change of the intensity ratio between the vibrational bands, which is in accordance with an extension of the exciton theory. In some studies a change of the relative intensity have been reported, similar to that occurring in the PDI chromophore dimers . Ideally, the up‐exciton state has a 100 % probability and the absorption spectra are blue‐shifted.…”

“…This would lead to the excitation energy being almost fully localized on one of the monomeric subunits. In other words, the pre‐associated face‐to‐face stacking in the ground state is known to promote excimer formation through a greatly enhanced interaction of the initially localized excited state with one of the ground‐state molecule in bis‐Pc . Finally, the excimer state appears after energy redistribution and structural rearrangements.…”

“…Alternatively, the structural rearrangements may also result in slow excimer formation. In some studies a two‐step excimer formation has been reported. The first step is the initial hot excimer formation followed by vibrational cooling with structural relaxation to form the stable excimer.…”

Irreversible One‐Way [2+2] Photocycloaddition in Bis‐Styrylbenzo[f]quinoline Dyad: Photoactive and Photoinert Excimers in the Same System

“…The dyad 1 has two identical chromophores, so forward and backward excitation hopping between the SBQ units is expected . However, this cannot hinder PCA, since quenching of one SBQ photochrome is accompanied by excitation of the other identical photochrome with the same possibility of the cyclobutane formation.…”

Conjugated polymers based on 1,8‐naphthalene monoimide with high electron mobility

“…Polymeric acceptors with NDIs as acceptor parts have shown high performance in organic thin‐film transistors (OTFTs) and organic solar cells . Imide groups have been widely used to construct highly efficient acceptors, such as NDI, phthalimide, and perylene diimides derivatives . 1,8‐Naphthalene monoimide (NMI) with the similar structure and energy level as that of NDI, has been used to construct small functional molecules which were used in fluorescent probes, OTFTs and organic solar cells .…”