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PubMed2005

Triplet-triplet energy transfer controlled by the donor-acceptor distance in rigidly held palladium-containing cofacial bisporphyrins.

Eleven new complexes, including mono-, heterobi-, and homobimetallic cofacial bisporphyrins, (Pd)H2DPS, (M)H2DPX, (M)H2DPB, (PdZn)DPS, (PdZn)DPX, (Pt)2DPX, (M)2DPB (M = Pd, Pt), and (Pt)P (DPS4- = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene tetraanion, DPX(4-) = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene tetraanion, DPB4- = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene tetraanion, P2- = 5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin dianion) have been synthesized and characterized. The photophysical properties of the donor (M)P (M=Pd or Pt, P=porphyrin chromophore) and the acceptor (free base H(2)P or (Zn)P) depend on the C(meso)-C(meso) distance and the presence of a heavy atom such as Pd(II) or Pt(II). The data were compared with those for the known compounds (Pd)2DPS, (Pd)2DPX, H4DPS, H4DPX, H4DPB, (Pd)P, (Zn)P, and H(2)P. The rate constants for triplet-triplet energy transfer (k(ET)) were measured for the heterobimetallic (PdZn) and monometallic [(M)H2] derivatives (M=Pd, Pt). The fluorescence lifetimes (Deltatau(F)) of the acceptors decrease as a result of the heavy-atom effect, and vary as follows: (Pd)H2DPS<<(Pd)H2DPX approximately (Pd)H2DPB. The k(ET) values calculated according to the equation k(ET)=(1/tau(emi)-1/tau(emi) (0)), where tau(emi) (0) is the emission lifetime of the homobimetallic bisporphyrins (no ET occurs), are equal to 0, 247+/-57 and 133+/-52 s(-1) for DPS, DPX, and DPB, respectively, in the (Pd)H(2) series. These measurements allowed the range of distance over which the Dexter mechanism for T(1)-T(1) energy transfer ceases to operate to be determined. This distance is somewhere between 4.3 and 6.3 A, in agreement with our recent findings on singlet-singlet energy transfer. During the course of this study, the X-ray crystal structure for (Pd)H2DPX was obtained; triclinic (P1), a = 11.1016(1), b = 14.9868(2), c = 20.6786(3) A, alpha = 102.091(1), beta = 100.587(1), gamma = 101.817(1) degrees , V = 3199.19(7) A(3), Z = 2.

Crystallography, X-Ray↗
PubMed2005

Higher triplet excited states of benzophenones and bimolecular triplet energy transfer measured by using nanosecond-picosecond two-color/two-laser flash photolysis.

The lifetimes of benzophenone in the higher triplet excited state (BP(T(n))) and several BP derivatives in the T(n) states were measured directly to be tau(T(n))=37+/-7 ps and 20-33 ps, respectively, by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method. Based on the direct measurements of tau(T(n)) of BP(T(n)), the triplet energy transfer (TET) from BP(T(n)) to quenchers (Q), such as carbon tetrachloride (CCl4), benzene (Bz), and p-dichlorbenzene (DCB), was investigated. The fast TET from BP(T(n)) to Q can be attributed to the lifetime-dependent quenching process, according to the Ware theoretical model of the bimolecular energy transfer reaction. The contribution of the lifetime-dependent term on k(TET) was 27, 60, and 86% for CCl4, Bz, and DCB as the Q of BP(T(n)), respectively, indicating that the TET from BP(T(n)) to Q is influenced not only by tau(T(n)), but also by the size of Q.

Absorption↗
PubMed2005

Metal-enhanced up-conversion fluorescence: effective triplet-triplet annihilation near silver surface.

Up-conversion phenomena are traditionally related to two- or multiphoton processes occurring under relatively high excitation intensities. Here we present the first results of ultralow excitation intensity (in order of Wcm(-2)) continuous-wave (CW) excited up-conversion fluorescence in Kretschmann surface plasmon geometry. The active system is a blue-emitting polymer matrix blended with metalated porphyrine macrocycles. The up-conversion fluorescence is a consequence of a two-particle triplet-triplet annihilation process (TTA).

Dose-Response Relationship, Radiation↗
PubMed2006

Triplet-triplet energy-transfer coupling: theory and calculation.

Triplet-triplet (TT) energy transfer requires two molecular fragments to exchange electrons that carry different spin and energy. In this paper, we analyze and report values of the electronic coupling strengths for TT energy transfer. Two different methods were proposed and tested: (1) Directly calculating the off-diagonal Hamiltonian matrix element. This direct coupling scheme was generalized from the one used for electron transfer coupling, where two spin-localized unrestricted Hartree-Fock wave functions are used as the zero-order reactant and product states, and the off-diagonal Hamiltonian matrix elements are calculated directly. (2) From energy gaps derived from configuration-interaction-singles (CIS) scheme. Both methods yielded very similar results for the systems tested. For TT coupling between a pair of face-to-face ethylene molecules, the exponential attenuation factor is 2.59 A(-1)(CIS6-311+G(**)), which is about twice as large as typical values for electron transfer. With a series of fully stacked polyene pairs, we found that the TT coupling magnitudes and attenuation rates are very similar irrespective of their molecular size. If the polyenes were partially stacked, TT couplings were much reduced, and they decay more rapidly with distance than those of full-stacked systems. Our results showed that the TT coupling arises mainly from the region of close contact between the donor and acceptor frontier orbitals, and the exponential decay of the coupling with separation depends on the details of the molecular contacts. With our calculated results, nanosecond or picosecond time scales for TT energy-transfer rates are possible.

Algorithms↗
PubMed1999

The speed limit for protein folding measured by triplet-triplet energy transfer.

A direct measure of intramolecular chain diffusion is obtained by the determination of triplet-triplet energy-transfer rates between a donor and an acceptor chromophore attached at defined points on a polypeptide chain. Single exponential kinetics of contact formation are observed on the nanosecond time scale for polypeptides in which donor and acceptor are linked by repeating units of glycine and serine residues. The rates depend on the number of peptide bonds (N) separating donor and acceptor and show a maximum for the shortest peptides (N = 3) with a time constant (tau = 1/k) of 20 ns. This sets an upper limit for the speed of formation of the first side-chain contacts during protein folding.

Energy Transfer↗
PubMed2006

Novel Josephson effect in triplet-superconductor-ferromagnet-triplet-superconductor junctions.

We predict a novel type of Josephson effect to occur in triplet-superconductor-ferromagnet-triplet-superconductor Josephson junctions. We show that the Josephson current, IJ, exhibits a rich dependence on the relative orientation between the ferromagnetic moment and the d vectors of the superconductors. This dependence can be used to build several types of Josephson current switches. Moreover, we predict an unconventional sign change of IJ with increasing temperature.

Journal Article↗
PubMed1990

Formation of cyclobutane thymine dimers photosensitized by pyridopsoralens: a triplet-triplet energy transfer mechanism.

The 365 nm irradiation of thymine thin films in the presence of pyridopsoralens is shown to induce the formation of cyclobutane thymine dimers, in contrast to other compounds such as 8- and 5-methoxypsoralen. In order to elucidate the mechanism of such a photosensitized reaction, we have determined the energy of the lowest triplet state (T1) of these compounds, using phosphorescence spectroscopy and CNDO/S quantum chemistry calculations. The T1 energy values were found to be significantly higher for pyridopsoralens--up to 0.3 eV--than for 8- and 5-methoxypsoralen (approximately 2.8 eV), which are not able to photoinduce cyclobutane thymine dimers. The determination of the relative efficiency of cyclobutane thymine dimer formation was performed using chromatographic analysis. A good correlation was found between the energy of the T1 state of the psoralen derivatives and the related cyclobutane thymine dimer formation. Moreover, the photosensitized cyclobutane thymine dimer formation appeared to be temperature-dependent. Our results are consistent with a mechanism involving a triplet energy transfer from the pyridopsoralen to thymine.

Energy Transfer↗
PubMed2007

Electron binding capabilities of some silylenes having small singlet-triplet splittings or triplet ground states.

Several silyl and alkaline metal substituted silylenes have been investigated using the CAS-ACPF method in conjunction with the aug-cc-pVTZ basis sets. Silylsilylene and disilylsilylene are found to have singlet ground states with DeltaEST(-) values of 0.676 and 0.319 eV, respectively. The adiabatic ground state electron affinities are found to be 1.572 and 2.361 eV for HSiSiH(3) and Si(SiH(3))(2). respectively. Both silylenes possesses a stable 2A1 excited negative ion state, with respective adiabatic EA values of 0.037 and 1.000 eV. In contrast, all silylenes with at least one akaline metal substituent exhibit triplet neutral ground states. The metalated silylenes HSiLi, HSiNa, LiSiSiH(3), NaSiLi, SiLi(2), and SiNa(2) have adiabatic ground state EAs somewhat below 1 eV, but each of these negatively charged system possesses up to three bound excited negative ion states, some of which are dipole-bound states.

Journal Article↗
PubMed1999

Nonintegrable dynamics of the triplet-triplet spatiotemporal interaction.

In this paper we examine the coupling of two wave triplets sharing two common modes. The analysis is performed in the solitonic sector of the parameter space where uncoupled solutions departing from linearly unstable homogeneous initial conditions evolve into a collection of regularly interspersed, spatiotemporally localized spikes. The uncoupled system is integrable, but coupling destroys integrability. As coupling grows, energy transfer to smaller spatial scales does appear and becomes faster not only in linearly unstable, but also in linearly stable cases. Chaos in low-dimensional subsystems appears to be responsible for the transfer. We perform a series of numerical tests to verify this idea.

Journal Article↗
PubMed2001

Influence of Structural and Rotational Isomerism on the Triplet Blinking of Individual Dendrimer Molecules J.H. thanks the FWO for a post-doctoral fellowship. T.V. wishes to thank the I.W.T. for a doctoral fellowship. Financial support by the FWO, the DWTC (through IUAP-4-11), and the Flemish Ministry of Education (through GOA/1/2001) is gratefully acknowledged. The EC (through TMR Sisitomas), ESF (through Smarton), and the Volkswagen-Stiftung are also thanked for financial support. Dr. L. Latterini is thanked for the triplet absorption measurements.

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Journal Article↗