Phenothiazine−Pyrene Dyads:  Photoinduced Charge Separation and Structural Relaxation in the CT State

Abstract

Electron donor-acceptor systems with phenothiazine linked directly to pyrene exhibit a dual emission in moderately and very polar solvents. Steady state and time-resolved fluorescence spectroscopy provide evidence that the "blue" and "red" emission bands originate from different species. Fluorescence excitation spectra show a similar appearance when the emission is monitored either in the red or blue spectral range, but they are slightly shifted against each other. This suggests that different isomers exist with a distinctly different photophysical behavior. Semiempirical (AM1/CI) molecular orbital calculations with a continuum solvent treatment have been used to establish the geometry of the two nearly isoenergetic stereoisomers and to calculate the properties of their excited Franck-Condon states. Geometry optimization of various excited states provides evidence for different internal relaxation coordinates for the phenothiazine-localized S 1 state and the charge transfer state S 6 , on one hand, and the pyrene-localized S 2 (S 3 ) state, on the other hand. The relaxed geometries in the excited states of both isomers represent mirror images with identical properties. The different photophysical behavior of the two isomers is most likely caused by the different potential energy curves, or more precisely speaking, by different location and/or heights of the barrier along the reaction coordinates from the locally excited to the geometry-relaxed CT states.

A π‐Stacked Porphyrin–Fullerene Electron Donor–Acceptor Conjugate That Features a Surprising Frozen Geometry

“…[6] Note that "harpoon" folded charge-transfer conformations can also be formed from more extended groundstate geometries on excitation. [7] Importantly, such folded conformations allow through-space interactions between the donor and acceptor moieties. Such through-space communications result in efficient and rapid deactivation of the porphyrin singlet excited state to give either fullerene excited states by energy transfer or the charge-separated state by electron transfer.…”

“…This has been attributed to strong van der Waals interactions between the two chromophores that are further augmented by Coulomb interactions in charge‐transfer states 6. Note that “harpoon” folded charge‐transfer conformations can also be formed from more extended ground‐state geometries on excitation 7. Importantly, such folded conformations allow through‐space interactions between the donor and acceptor moieties.…”

Photo-induced dynamics in a pyrene–isoalloxazine(flavin)–phenothiazine triad

“…1 for structural formula) [41,[64][65][66][67][68] and on the three subchromophores (BMF, PYM, PPT) of PYFPT [40,41,[61][62][63] allow an assignment on the energetic ordering of the frontier molecular orbitals (FMOs), namely highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), as follows: E LUMO (phenothiazine) > E LUMO (pyrene) > E LUMO (flavin) and E HOMO (phenothiazine) > E HOMO (pyrene) > E HOMO (flavin). A qualitative energetic evaluation of the inter-component pathways via photo-induced electron transfer is delineated on the basis of the HOMO/LUMO schemata given in Figs.…”

“…The photo-dynamics of pyrene-phenothiazine with photo-induced charge-separation and conformational relaxation was studied in [64][65][66][67][68]. The photo-dynamics of a pyrene-flavine dyad and of a phenothiazine-flavin dyad were analysed in [63,69] and [62,69], respectively.…”

A DFT and TDDFT investigation of interactions between 1-hydroxypyrene and aromatic amino acids

“…Many aromatic hydrocarbons may form charge transfer complexes in the excited states with amino acids which resemble aromatic amines upon excitation of their p-electronic systems [14][15][16][17][18][19]. Therefore, it is very important to know their interactions with biologically important molecules.…”