Bipyridyl Derivatives as Photomemory Devices: A Comparative Electronic‐Structure Study

Abstract

The two isoelectronic bipyridyl derivatives [2,2'-bipyridyl]-3,3'-diamine (BP(NH(2))(2)) and [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)) are experimentally known to undergo very different excited-state double proton transfer processes that result in fluorescence quantum yields that differ by four orders of magnitude. Such differences have been theoretically explained in terms of topographical features in the potential energy surface and the likely presence of conical intersections. The hypothetical hybrid compound [2,2'-bipyridyl]-3-amin-3'-ol (BP(OH)(NH(2))) presents intermediate photochemical features of its "ancestors". In this report we analyze the photochemical properties of a whole family of "dark" (not fluorescent) states that can be accessed from each bipyridyl derivative upon irradiation of light of a given wavelength and their potential application as photomemory devices. In the light of our density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations, BP(NH(2))(2) is the more likely candidate to become a photomemory device.

Modulating the Photochemistry of Bipyridylic Compounds by Symmetric Substitutions

“…This same scenario was previously observed for the non-substituted bipyridyls. [34][35][36] Globally, the pattern of the different relevant structures found in both electronic states mimics the results previously found for the parent BP(NH 2 ) 2 molecule. [35,36] However, now the more stable form in the excited state is the original DA*.…”

“…[34][35][36] Globally, the pattern of the different relevant structures found in both electronic states mimics the results previously found for the parent BP(NH 2 ) 2 molecule. [35,36] However, now the more stable form in the excited state is the original DA*. In fact the energies of both MI* and DI* minima are higher than the energy of the Franck-Condon geometry so that even if the CI has an energy below the Franck-Condon one, it is energetically inaccessible.…”

“…[33] We have recently performed a series of electronic calculations on both bipyridyl molecules which provides a theoretical basis for the experimentally observed differences between both compounds. [34][35][36] Our results indicate that the different quantum yields stem from subtle energy differences in the potential energy landscapes of the ground and first singlet excited electronic states. Later on we also reported on the "hybrid" (2,2'-bipyridyl)-3-amine-3'-ol molecule [BP(OH)(NH 2 )] whose actual photochemistry has remained unexplored.…”

“…As the deviation from the planarity and the corresponding energy stabilization with respect to the planar structure is small, we have not considered these structures in the photochemical cycles but for every case studied herein we report the energies (and the torsional angles) of the true nonplanar minima. For comparative purposes, the energies of the previously studied unsubstituted BP(NH 2 ) 2 and BP(OH) 2 parent molecules [35,36] are also shown in the last rows of Tables 1, 2 respectively. At first sight, it is clear that fluoride has little effect on the relative energies of the excited state.…”

“…[37] One of the more interesting results of our theoretical study of the bipyridyl compounds was the discovery that, upon irradiation of light at a given wavelength, a whole set of dark states (states that cannot be directly accessed from the ground state) can be obtained from each bipyridyl derivative. [36] The gate to these states is the conical intersection (CI) that is also responsible for the decrease in the fluorescence quantum yield of BP(NH 2 ) 2 with respect to BP(OH) 2 . The photochemical path that leads to the dark states makes BP(NH 2 ) 2 a likely candidate to be an efficient photomemory device, as reverting the whole process needs irradiation of a quite different wavelength.…”