Inter- and intramolecular excited-state interactions of surfactant-active rhenium(I) photosensitizers

Abstract

A new series of surfactant-active complexes of the form [(bpy)Re(CO),NC(CH,).CH,I+, n = 0-17, have been synthesized and characterized. These complexes exhibit a unique intramolecular perturbation of their excited-state manifold by the normally passive alkyl chain. Intramolecular fold back, a strong function of chain length, alters the solvent environment around the excited portion of the molecule with a concomitant change in the state energies and decay paths. Molecular models, cyclodextrin binding studies, absorption and emission spectroscopy, excited-state lifetime measurements, and oxygen-quenching studies support this model. The alkyl chain can function as a molecular switch and invert the lowest excited states in the molecules at low temperature. Energy level diagrams are developed that explain both room-temperature and 77 K results.Luminescent transition-metal complexes have been utilized as photosensitizers in such areas as solar energy conversion,' electron-transfer studies,2 chemi-and electroluminescent system^,^,^ binding dynamics of heterogeneous media,s and probes of macromolecular structure.6 R U " ( L )~~+ complexes (L = 2,2'-bipyridine, 1,lO-phenanthroline, or substituted derivatives) have been the most frequently used probes in these applications due to strong visible absorption, high photochemical stability, efficient luminescence, and relatively long-lived metal to ligand chargetransfer (MLCT) excited states7Further, the emitting-state energies and excited-state redox properties of the sensitizers can be very sensitive to the nature of the metal, coordinating ligands, and solvent environment. Solvent and substituents have been used to control the relative positions of MLCT excited states and effectively "tune" their photophysical and photochemical properties.8-'0 Many of these sensitizers exhibit a wide variety of energetically accessible charge-transfer (CT), ligand-field (LF), and intraligand (IL) excited states. These excited states are of differing orbital parentage and, therefore, can have quite different excited-state characteristics. Since the photophysical properties of the metal complexes are determined predominantly by the lowest energy excited states," it is important to understand thoroughly their excited-state energetics and dynamics in order to be able to design rationally new and more useful photosensitizers and probes.For those sensitizers having lowest lying MLCT states, models are based predominantly on the photophysics and photochemistry of the 'Ru(bpy)32+ (bpy = 2,2'-bipyridine) and related sensitizers, leading to a general picture of metal complex excited-state behavior. Recently, OS(II),'~ Ir(III),I3 Mo(0) and W(O),I4 and R~( I ) " s '~ complexes have been studied with increasing interest.In some cases, these complexes exhibit even more desirable excited-state properties than the Ru(I1) sensitizers. An additional level of richness in the form of multiple luminescences has been observed for a number of transition-metal compounds with a-diimine ligands. As we will show, this ...

A Long-Lived, Highly Luminescent Re(I) Metal–Ligand Complex as a Biomolecular Probe

“…4). These results are consistent with the temperature dependence of many other MLCT complexes, wherein the energy gap between the ground and excited states increases with decreasing temperature, resulting in a blue-shifted emission spectrum, increased lifetime, and higher quantum yield (22,23,28,31).…”

“…Importantly, metal-ligand tion at room temperature. This compound's unique complexes display luminescence decay times ranging spectral properties along with its conjugatability alfrom 100 ns to 100 ms (22,23). Consequently, these probes lowed us to utilize it as biomolecular probe in a variety extend the observable time scale of anisotropy decay meaof environments.…”

Quenching Dynamics of the Photoluminescence of [Ru(bpy)3]2+-Pendant PAMAM Dendrimers by Nitro Aromatics and Other Materials

“…That is, for the larger generations, the dendrimer branches function progressively more as the solvent. [27][28][29] The observed k q value of 3 M -1 ns -1 is typical for diffusioncontrolled quenching. 30 If the quenching were due to the formation of nonfluorescent ground state complexes (static quenching) then the lifetimes would not depend on the quencher concentration because the uncomplexed fraction would be unperturbed regardless of the number of complexes formed.…”

<title>DNA dynamics by using highly luminescent phosphine complex of osmium (II)</title>

“…The fractional intensity (f1) of each decay time component to the steady state emission can be obtained from (2) The anisotropy decay data were also analyzed with IBH, Inc., software, using the multicorrelation time model (3) where rj are the amplitudes associated with thejth correlation time (6).…”