Energy Transfer to the Low-Energy Triplet States of 1,3-Dicarbonylazomethine Dyes:  The Role of Unique Geometries and Nonadiabatic Behavior

Abstract

The observed kinetics of energy transfer to PhCO-(CdNAr)-CONHPh (1), PhCO-(CdNAr)-COPh (2), and PhCONH-(CdNAr)-CONHPh (3) (Ar ) p-NMe 2 C 6 H 4 ) is consistent with formation of two triplet states for each dye: T 1 and T 2 . A Balzani analysis of the data observed for 1 affords triplet energies of 23.4 ( 1.6 and 38.5 ( 1.7 kcal/mol for T 1 and T 2 , respectively, and requires that T 1 be formed in reactions having substantial nonadiabatic character. A similar result is found for 2. For 3, T 1 and T 2 are estimated at 25.0 and 38.5 kcal/mol, respectively, but formation of 3-T 1 is found to be largely adiabatic. To rationalize these observations, the dyes have been studied by DFT methods (B3LYP/6-31G*). In the ground state (1-S 0 ) 1 is computed to have a geometry in which the azomethine double bond is coplanar with the anilide fragment but is perpendicular to the carbonyl of the ketone. Similarly, 2-S 0 is calculated to have a twisted 1,3-dicarbonyl arrangement. 3-S 0 , on the other hand, exhibits an intramolecular H-bond between the proton of one anilide and the carbonyl of another, and has a large degree of planarity in the 1,3-dicarbonyl moiety. For all compounds, the lowest triplet state (T 1 ) is calculated to have a geometry in which the azomethine bond is twisted by 70°-80°relative to the ground state, and to have a near-planar arrangement between the azomethine and the two carbonyl groups. The calculated lowest triplet energies are (in kcal/mol): 23.0 and 27.5 for two isomers of 1, 25.2 for 2, and 23.4 for 3. The large geometrical differences between the ground and the lowest triplet states are proposed to provide an efficient mechanism for triplet state stabilization, and to stand at the origin of the observed nonadiabatic energy transfer in 1 and 2. The presence of planarized equilibrium concentrations of 3 due to intramolecular H-bonding can explain the greater rates of energy transfer to 3 compared to 1 and 2.

Role of Structural Changes in the Triplet−Triplet Energy Transfer Process to Oxime Derivatives

“…The relaxed triplet energies were computed as the energy difference between the relaxed triplet and ground states. DFT methods have been shown to give reliable values in the case of polyolefin triplet-state calculations. , In the present case, the values obtained (Table ) are slightly lower than the experimental ones (from ∼34 kJ/mol), a fact already noticed for polyenes …”

“…DFT methods have been shown to give reliable values in the case of polyolefin triplet-state calculations. 29,32 In the present case, the values obtained (Table 3) are slightly lower than the experimental ones (from ∼34 kJ/mol), a fact already noticed for polyenes. 32 From these results, it appears that two main structural changes occur between the relaxed ground and triplet states: a decrease of β, which leads to a more planar carbon skeleton, and a twist along the C3N bond.…”

“…Therefore, in our case, the quenching rate constants were fitted with the classical Agmon-Levine-Balzani (ALB) model, which is known to give reliable results. 9,[28][29][30] Within this framework and assuming the mechanism described in Scheme 1, the quenching rate constant, k q , can be written as where k en is expressed according to the classical model of the absolute rate theory by k en 0 and ∆G # are the preexponential factor and the standard free activation energy, respectively. The key point in this treatment is the relationship between ∆G # and ∆G, which is given by where ∆G # (0) is the reorganizational intrinsic barrier related to the changes in nuclear positions that have to occur prior the energy transfer.…”

“…Furthermore, the idea that electron transfer (ET) and energy transfer (TT) are two conceptually related processes governed by the same theory of radiationless transition has led other groups to use different models based on Marcus ET theory. , Although this latter model was proved to account very well for the experimental data of trans - and cis -stilbene, as well as for azobenzene, the large number of spectroscopic parameters required and the assumptions that have to be made on the relative contribution of the relevant coordinates prevent any extensive use on most different systems. Therefore, in our case, the quenching rate constants were fitted with the classical Agmon−Levine−Balzani (ALB) model, which is known to give reliable results. ,− Within this framework and assuming the mechanism described in Scheme 1, the quenching rate constant, k q , can be written as where k en is expressed according to the classical model of the absolute rate theory by and Δ G # are the preexponential factor and the standard free activation energy, respectively. The key point in this treatment is the relationship between Δ G # and Δ G , which is given by where Δ G # (0) is the reorganizational intrinsic barrier related to the changes in nuclear positions that have to occur prior the energy transfer.…”

A theory ofnonverticaltriplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π* triplet donors to 1,3,5,7-cyclooctatetraene

“…In summary, the expression for the rate constant, as derived above, for a triplet-triplet transfer reaction between a series of rigid triplet donors and a flexible acceptor contains ͑i͒ a preexponential factor k 0 e associated with the donoracceptor orbital overlap through H i f 2 e H i f /k B T and other factors, which are not expected to depend on the triplet energy difference, as noted before, 12,16,35 and ͑ii͒ an exponential term with a quadratic dependence on the energy difference between the donor and the acceptor triplet states.…”