Light-Harvesting and Photoisomerization in Benzophenone and Norbornadiene-Labeled Poly(aryl ether) Dendrimers via Intramolecular Triplet Energy Transfer

Abstract

A series of benzophenone (BP) and norbornadiene (NBD)-labeled poly(aryl ether) dendrimers (Gn-NBD), generations 1-4, were synthesized, and their photophysical and photochemical properties were examined. The phosphorescence of the peripheral BP (donor) chromophore was efficiently quenched by the NBD (acceptor) group attached to the focal point. Time-resolved spectroscopic measurements indicated that the lifetime of the triplet state of the BP chromophore was shortened due to the proximity of the NBD group. Selective excitation of the BP chromophore resulted in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that an intramolecular triplet energy transfer occurs in Gn-NBD molecules. The light-harvesting ability of these molecules increases with generation due to an increase in the number of peripheral chromophores. The energy transfer efficiencies are ca. 0.97, 0.54, 0.45, and 0.37 for generations 1-4, respectively, and the rate constant of the triplet-triplet energy transfer is ca. 10(6)-10(7) s(-1), which decreases inconspicuously with increasing generation. The intramolecular triplet energy transfer is proposed to proceed mainly via a through-space mechanism involving the closest donor (folding back conformation) and acceptor groups.

A Study of the Dynamics of the Branch Ends of a Series of Pyrene-Labeled Dendrimers Based on Pyrene Excimer Formation

“…One advantage of analyzing the monomer and excimer decays globally is the possibility to assign the molar fractions of each pyrene species present in solution using the procedure outlined in the Theory section (see eqs [17][18][19][20]. Those fractions are listed in Table 1.…”

Internal Dynamics of Dendritic Molecules Probed by Pyrene Excimer Formation

“…Fluorescence resonance energy transfer (FRET), energy hopping or migration, and electron transfer between a donor and an acceptor are all photophysical processes whose efficiency depends strongly on the distance between the donor and acceptor [135][136][137]. The design of dendrimers used for this type of applications consists in placing many donors at the dendrimer periphery and a single acceptor molecule at the dendrimer focal point [3,36,[39][40][41][42][43][44][120][121][122][123]125]. As the dendrimer generation increases, the efficiency of the photophysical process of interest decreases according to the increased distance separating the donor from the acceptor [135][136][137].…”

“…From a theoretical standpoint however, the continuous increase in dendrimer density with generation number leads to the prediction that a crossover generation exists where the available free volume inside the dendrimer vanishes. Considering the importance of the end groups for applications, this decrease in free volume with increasing G led numerous theoreticians and experimentalists [4,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] to question what became of the end groups in higher generation dendrimers as they might be trapped in the congested dendrimer interior. The first theory dealing with this issue was proposed by de Gennes and Hervet (DGH) [22].…”

“…A computational study carried out by Lescanec and Muthukumar [23] 12 years later contradicted the DGH prediction by finding that many terminal ends of a dendrimer do not remain at the dendrimer periphery but rather fold back into the dendrimer interior. Since then, numerous theoretical [12,13, and experimental [4,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] studies have been conducted which led to the current consensus [55] that the end groups of dendrimers made of a flexible backbone are distributed throughout the dendrimer interior in a core-dense manner. Only in a few rare studies were the terminal groups found to lay at the dendrimer periphery, when the ends were subject to some repulsive electrostatic forces [35,56,57] or attractive hydrogen-bond interactions [58] or the dendrimer was made of stiff repeating units [50].…”

Dendrimers: A Mimic Natural Light‐Harvesting System

“…Our group created a dendritic system in which the energy transfer between the antenna chromophores and the core is suitable for Dexter triplet state energy transfer. [33] Benzophenone (BP) units were attached to the outer space of poly(benzyl ether) dendrimers, while norbornadiene (NBD) was located at the core to probe the energy transfer occurrence (Scheme 1). NBD was chosen as it is a photochemically active group that is capable of storing solar energy through isomerization to quadricyclane (QC).…”