Photoinduced Processes within Compact Dyads Based on Triphenylpyridinium‐Functionalized Bipyridyl Complexes of Ruthenium(II)

Abstract

As an alternative to conventional charge-separation functional molecular models based on long-range ET within redox cascades, a "compact approach" has been examined. To this end, spacer elements usually inserted between main redox-active units within polyad systems have been removed, allowing extended rigidity but at the expense of enhanced intercomponent electronic communication. The molecular assemblies investigated here are of the P-(theta (1))-A type, where the theta (1) twist angle is related to the degree of conjugation between the photosensitizer (P, of {Ru(bpy)(3)}(2+) type) and the electron-acceptor (A). 4-N- and 4-N-,4'-N-(2,4,6-triphenylpyridinio)-2,2'-bipyridine ligands (A(1)-bpy and A(2)-bpy, respectively) have been synthesized to give complexes with Ru(II), 1-bpy and 2-bpy, respectively. Combined solid-state analysis (X-ray crystallography), solution studies ((1)H NMR, cyclic voltammetry) and computational structural optimization allowed verifying that theta (1) angle approaches 90 degrees within 1-bpy and 2-bpy in solution. Also, anticipated existence of strong intercomponent electronic coupling has been confirmed by investigating electronic absorption properties and electrochemical behavior of the compounds. The capability of 1-bpy and 2-bpy to undergo PET process was evaluated by carrying out their photophysical study (steady state emission and time-resolved spectroscopy at both 293 and 77 K). The conformational dependence of photoinduced processes within P-(theta (1))-A systems has been established by comparing the photophysical properties of 1-bpy (and 2-bpy) with those of an affiliated species reported in the literature, 1-phen. A complementary theoretical analysis (DFT) of the change of spin density distribution within model [1-bpy(theta (1))](-) mono-reduced species as a function of theta (1) has been undertaken and the possibility of conformationally switching emission properties of P was derived.

Conformationally gated photoinduced processes within photosensitizer–acceptor dyads based on ruthenium(II) and osmium(II) polypyridyl complexes with an appended pyridinium group

“…The same issues also hold for the electronic perturbation of the P0 bpy /Ru chromophore by the N pyridinio atom of TPH 3 + , even though a splitting of the 1 MLCT absorption band is observed [45] instead of the equivalent hyperchromic effect previously noticed [111]. The point is that P0 bpy /Ru is a significantly better electron-donating agent in the excited state (E(III/II*)) than P0 tpy /Ru: −0.92 V versus −0.76 V versus SCE, respectively.…”

“…7) and the P0 L core within 1-tpy and 1-bpy. As a result of the presence of bulky phenyl substituents at either side of the N pyridinio atom, the  0A value approaches 90 • in both cases [45,111,114,115,147,148]. This structural feature is usually referred to as geometrical decoupling [111,114,115].…”

“…In doing so, the aim is to delineate the role of the intramolecular conformation (Â 0A ) in this determination (P0 L -(Â 0A )-A versus P L 1 A ). , 1-bpy [45] and 1-phen [45,142] compact assemblies: P0-(Â 0A )-A/Ru supermolecules (see Section 1.2.3) or P0 A /Ru (that is, P1/Ru also denoted P0(Â 0A ) A /Ru) large molecules? Room-temperature photophysical features (recorded in deaerated MeCN solution) of selected Ru-based compounds together with related bare P0L reference photosensitizers [143].…”

“…Therefore, P0/Ru → A0 energy transfer cannot explain the quenching of the luminescence of P0 bpy within 1-bpy. Neither can an energy equilibration process be proposed to account for the increased emission lifetime of both P0 tpy within 1-tpy and P0 phen within 1-phen as compared with relevant reference isolated photosensitizers [45].…”

“…2) of photoinduced events actually vanishes for a given intramolecular context. This point is of importance since (1) conformational changes can occur in the excited state upon charge redistribution and (2), achieving long-lived CS state formation (normally proper to supermolecules) within closely coupled assemblies has recently emerged as a challenge in the field of artificial photosynthesis [41][42][43][44][45]. The very nature of photoinduced phenomena taking place within electronically closely coupled and spatially compact molecules is also of basic interest, including for the related field of molecular electronics and also for the switching effect at the molecular scale.…”