Jacob J. Piet

Substituent effects on the excited states of phenyl-capped phenylene vinylene tetramers

The singlet and triplet excited states of phenyl-capped tetramers of phenylene vinylene with different alkyl, alkoxy or cyano substituents, were investigated in benzene solution. The lowest singlet states were studied by laser flash-photolysis with time-resolved microwave conductivity and fluorescence. The triplets were generated by pulse radiolysis, via energy transfer from the solvent excited singlet followed by inter-system crossing or by energy transfer from the naphthalene triplet. Their absorption spectra and rates of quenching by oxygen were determined.

Time-resolved microwave measurements of the polarizability of photoexcitations on conjugated polymer chains

Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.

Photo-induced charge separation on conjugated porphyrin chains

Abstract Photoexcitation of isolated conjugated chains of butadiyne-linked porphyrins ([−P-≡−≡−]n with n ≈ 10) in dilute solution in benzene is followed by intrachain charge separation. The product of the quantum-yield and the effective one-dimensional mobility of the ion-pair state formed, φΣμ1D, is 6 × 10−8 m2/V s and the first half-life is ca. 100 ns.

The polarizability of triplet excitons on oligothiophene chains

Abstract The excess polarizability volume, ΔV p , associated with the formation of the T 1 states of oligothiophenes, OTn with n from 2 to 12, has been determined by measuring the change in the imaginary component of the complex microwave conductivity resulting on flash-photolysis of dilute solution. ΔV p gives a direct measure of the additional electronic delocalization associated with the halffilled HOMO and LUMO orbitals of these states. ΔV p increases with increasing length of the oligomer chain from 5 A 3 for n=2 to 59 A 3 for the dodecamer. The increased delocalization in T 1 is discussed in terms of coupling with higher states in the triplet manifold.

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

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.

Charge separation in a triosmium cluster zwitterion revealed by time-resolved microwave conductivity: first application of TRMC in organometallic chemistry

The light-induced transformation of an Os3(CO)10(α-diimine) cluster into an intramolecularly stabilized zwitterion has been established with time-resolved microwave conductivity, this is the first application of this technique in organometallic chemistry.