Naoki Haruta

Chemical Reactivity in Nucleophilic Cycloaddition to C70: Vibronic Coupling Density and Vibronic Coupling Constants as Reactivity Indices

The chemical reactivity in nucleophilic cycloaddition to C70 is investigated on the basis of vibronic (electron-vibration) coupling density and vibronic coupling constants. Because the e1″ LUMOs of C70 are doubly degenerate and delocalized throughout the molecule, it is difficult to predict the regioselectivity by frontier orbital theory. It is found that vibronic coupling density analysis for the effective mode as a reaction mode illustrates the idea of a functional group embedded in the reactive sites. Furthermore, the vibronic coupling constants for localized stretching vibrational modes enable us to estimate the quantitative reactivity. These calculated results agree well with the experimental findings. The principle of chemical reactivity proposed by Parr and Yang is modified as follows: the preferred direction is the one for which the initial vibronic coupling density for a reaction mode of the isolated reactant is a minimum.

Vibronic coupling density and related concepts

Vibronic coupling density is derived from a general point of view as a one-electron property density. Related concepts as well as their applications are presented. Linear and nonlinear vibronic coupling density and related concepts, orbital vibronic coupling density, reduced vibronic coupling density, atomic vibronic coupling constant, and effective vibronic coupling density, illustrate the origin of vibronic couplings and enable us to design novel functional molecules or to elucidate chemical reactions. Transition dipole moment density is defined as an example of the one-electron property density. Vibronic coupling density and transition dipole moment density open a way to design light-emitting molecules with high efficiency.

Fluorescence via Reverse Intersystem Crossing from Higher Triplet States in a Bisanthracene Derivative

To elucidate the high external quantum efficiency observed for organic light-emitting diodes using a bisanthracene derivative (BD1), non-radiative transition processes as well as radiative ones are discussed employing time-dependent density functional theory. It has been previously reported that the observed high external quantum efficiency of BD1 cannot be explained by the conventional thermally activated delayed fluorescence involving T1 exciton nor triplet-triplet annihilation. The calculated off-diagonal vibronic coupling constants of BD1, which govern the non-radiative transition rates, suggest a fluorescence via higher triplets (FvHT) mechanism, which entails the conversion of a high triplet exciton generated during electrical excitation into a fluorescent singlet exciton. This mechanism is valid as long as the relaxation of high triplet states to lower states is suppressed. In the case of BD1, its pseudo-degenerate electronic structure helps the suppression. A general condition is also discussed for the suppression of transitions in molecules with pseudo-degenerate electronic structures.

Reactivity of endohedral metallofullerene la2@c80 in nucleophilic and electrophilic attacks: vibronic coupling density approach.

The regioselectivities of La2@C80 in thermal nucleophilic and electrophilic attacks were theoretically investigated using vibronic coupling density (VCD) analysis. Nucleophilic and electrophilic cycloadditions to La2@C80 were experimentally reported to yield [6,6] and [6,5] adducts, respectively, as major products. VCD analysis provided a clear explanation for these experimental results. For nucleophilic reactions, it was found that the reactive [6,6] bonds did not have a large lowest unoccupied molecular orbital (LUMO) density and Fukui function but a large potential derivative with respect to a reaction mode. The VCD illustrates the origin of the interaction between the electronic and vibrational states. On the other hand, conventional reactivity indices such as frontier orbital density take only the electronic state into account. The result suggested that the stabilization due to vibronic couplings plays an important role in the regioselectivity of nucleophilic cycloadditions. The VCD with respect to the effective mode could provide a picture of the functional groups, which are the double bonds of ethylene moieties. VCD analysis with respect to hypothetical localized modes enabled the quantitative prediction of regioselectivities.

Vibronic couplings in cycloadditions to fullerenes

It is known to be difficult to predict the regioselectivity of cycloadditions to fullerenes based on the frontier orbital theory. To discuss the chemical reactivity of C60, vibronic coupling constants for localized stretching vibrational modes of C60 anion which are the hypothetical reaction modes of cycloadditions are calculated. Furthermore, the product ratio of C60 bisadduct (C60X2) is evaluated in the same way by considering the electronic state of C60 monoadduct anion (C60X anion). It is found that there is a good correlation between vibronic coupling constants for the localized modes and experimental regioselectivity.