A combination of theory and experiment is used to identify a novel variable excitonic coupling in a series of building blocks for small phenylacetylene dendrons. Systematic changes in the experimental emission spectra, radiative lifetimes, and polarization anisotropies as the number of meta-conjugated branches increases provide evidence for a qualitative change in the electronic structure in the relaxed excited state. The excited state electronic structure is investigated theoretically using ab initio CASSCF and CASPT2 calculations, which indicate the presence of large electronic coupling in the emitting geometry that is not seen for the absorbing geometry of the same molecules. The changes in electronic structure that occur upon excited-state relaxation can be understood in terms of a variable excitonic coupling between the phenylactylene branches, which takes these molecules from the weak coupling to the strong coupling regime as they relax on the excited state. The origin of this geometry-dependent coupling is investigated through the interpretation of ab initio calculations in terms of Fo ¨rster, Dexter, and through-bond charge-transfer interactions. We find that the change in the coupling arises primarily from an increase in the through-bond or charge-transfer component of the coupling, despite the absence of large changes in charge distribution. A theoretical comparison of metaversus para-substituted phenylacetylenes clarifies why this effect is so pronounced in the meta-substituted molecules.
skeletal structures from relatively simple acyclic isoprenoid precursors.[2] However, while the cyclization of triterpenes and many diterpenes are mechanistically similar in that they are initiated by protonation of a C=C double bond, identification and analysis of the genes for the relevant enzymes has revealed that there is no corresponding phylogenetic relationship. In particular, rather than being related to triterpene cyclases, these diterpene cyclases were found to be homologous to "lower" terpene synthases (TPS), which typically initiate catalysis by ionization of the allylic diphosphate ester bond in acyclic isoprenoid substrates, such as the universal diterpenoid precursor (E,E,E)-geranylgeranyl diphosphate (GGPP, 1). Accordingly, TPSs can be divided into two mechanistically distinct groups, the prevalent class I enzymes, which catalyze diphosphate ionization-initiated reactions, and atypical class II enzymes, which catalyze protonation-initiated cyclization reactions. [2,3] Structural and mechanistic studies of the common class I TPS enzymes have demonstrated that these synthases contain a characteristic aspartate-rich DDXXD motif that binds divalent metal ions required for catalysis of diphosphate ionization. [2,3] The first class II TPS identified, ent-copalyl/labdadienyl diphosphate (ent-CPP, 2) synthase (CPS) from Arabidopsis thaliana (AtCPS), was found to lack this DDXXD motif, and instead contained a separately placed DXDD motif.[4] Despite the lack of any other homology, it was suggested that the AtCPS DXDD motif is important for class II catalysis based on the occurrence of a similar motif in previously identified squalene-hopene (triterpene) cyclases (SHC).[5] Structural and mechanistic studies have demonstrated that the DXDD motif in SHC, and the corresponding VXDC motif in oxidosqualene (sterol) cyclases (OSC), initiate cyclization with the "middle" aspartate, which is presumed to act as the catalytic acid. The other conserved residues are thought to "activate" this aspartate for protonation of the terminal C=C double bond of squalene (SHC) or corresponding epoxide ring of oxidosqualene (SHC or OSC). [5] Class II TPS also characteristically contain a DXDD motif, [2] and mutational analysis has demonstrated that the DXDD motif in the bifunctional (i.e., class I and II) TPS abietadiene synthase (AS) is required for class II activity, [6,7] which is consistent with the suggestion that the DXDD motif in class II TPS also plays a role in initiating cyclization by protonating the terminal C=C double bond of GGPP. However, these studies also reported two important differences between the class II activity of AS and SHC/OSC. First, the class II activity of AS requires divalent metal ions (preferably Mg 2 + ), [7] whereas SHC and OSC do not.[8] Second, aza analogues that mimic the initial carbocations formed by protonation are very effective inhibitors of the class II activity of AS, but not SHC/OSC. Specifically, 14,15-dihydro-15-azaGGPP (15-azaGGPP, 3) exhibits approximately nanomolar affinity for t...
Traditional pictures of optical properties in phenylacetylene dendrimers view the molecule as a collection of independent chromophores, linked by meta-substitution at the central phenyl ring. While this picture is reasonable for explaining the observed absorption trends, it breaks down in describing the emission behavior. We utilize a combination of ab initio theory and experiment to demonstrate that differences in the absorbing and emitting states can be described using an exciton model with very weak chromophore coupling for the absorption geometry and strong coupling for the emission geometry. This result may have significant implications for the design of energy-funneling dendrimeric molecules.
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