Exploring the Limits of the Electrostatically Induced Conformational Folding Process in Charge-Separated Excited States: Retarding Effect of Long Alkyl Tails Attached to the Chromophores

Abstract

Six new donor‐bridge‐acceptor compounds have been synthesized which contain a long n‐tetradecyl chain attached to the donor or acceptor moiety, or to both of them. Systems 1, 2, and 3 are analogs of the fluorescent probe molecule Fluoroprobe (4). They contain a rigidly extended 4‐methylenepiperidine bridge and show relatively strong charge transfer fluorescence in solvents of low and medium polarity. Systems 1a, 2a, and 3a contain a semiflexible 4‐methylpiperidine bridge, obtained after hydrogenation of the exocyclic double bond of 1, 2 and 3, respectively. These systems undergo a conformational change following photoinduced charge separation (“harpooning”) in nonpolar solvents and probably also in solvents of medium polarity. Both the steady state fluorescence spectra and the fluorescence decay times of the extended charge transfer (ECT) species show that the photoinduced folding process is effectively slowed down by the introduction of the long alkyl tails. This is most pronounced for 1a which has an n‐tetradecyl group attached to both donor and acceptor. In solution a small difference in the rate of folding is observed between 2a and 3a, which have a single n‐tetradecyl chain attached to the acceptor only and to the donor only, respectively.

Conformational Dynamics of Semiflexibly Bridged Donor−Acceptor Systems Studied with a Streak Camera and Spectrotemporal Parametrization of Fluorescence

“…Recently, we have reported the synthesis of the new “harpooning” systems 1 − 3 (Chart ) . As in compound 4 , which we studied earlier, exciplex-like compact charge-transfer (CCT) species with the bridging piperidine ring in a (twist) boat conformation are formed in these compounds from extended charge-transfer (ECT) species in which the donor and acceptor are well separated in space.…”

Conformational Dynamics of Charge-Transfer States in Donor−Bridge−Acceptor Systems

“…30 D. [9,10] Piperazine-bridged systems such as 3, on the other hand, can be regarded as (D 2 ϪD 1 ϪA) triads, the trialkyl-substituted nitrogen atom acting as an electron donor group D 1 . [11] Trapping of the positive charge on D 1 to yield a D 2 ϪD 1 ϩ ϪA Ϫ species prevents the formation of an extended CT (ECT) state (D 2 ϩ ϪD 1 ϪA Ϫ ) and thereby also precludes the electrostatically driven folding from ECT to CCT.…”

“…Although the conformational flexibility of the piperazine ring is similar to that of the piperidine ring, the charge distribution in the initially formed charge-transfer state is very different for the two systems: In donorϪacceptor compounds 1 and 2 the positive charge resides on the (methoxy)aniline group, yielding an ECT species with a dipole moment of ca. 30 D. [9,10] Piperazine-bridged systems such as 3, on the other hand, can be regarded as (D 2 ϪD 1 ϪA) triads, the trialkyl-substituted nitrogen atom acting as an electron donor group D 1 . [11] Trapping of the positive charge on D 1 to yield a D 2 ϪD 1 ϩ ϪA Ϫ species prevents the formation of an extended CT (ECT) state (D 2 ϩ ϪD 1 ϪA Ϫ ) and thereby also precludes the electrostatically driven folding from ECT to CCT.…”

Influence of solvent viscosity and permittivity on the dynamics of large amplitude motions in semiflexibly bridged electron donor–acceptor systemsDedicated to Professor F. Dörr on the occasion of his 80th birthday.

“…It must, therefore, be checked how sensitive the value of exponent a is on the choice of the other parameters entering eqn. (23). Since the parameters relevant for the slope of vs. *G EX t are not known precisely, we have adopted slightly di †er-1/e r ent values of and repeated the evaluation.…”

“…, expression [eqn. (23)]. Therefore one must also be cautious with the interpretation of the kinetic parameters for WDA summarized in Table 2.…”