Study of the Photochemical Properties and Conical Intersections of [2,2′‐Bipyridyl]‐3‐amine‐3′‐ol

Abstract

The two isoelectronic bipyridyl derivatives [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol are experimentally known to undergo very different excited-state double-proton-transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. In a previous study, these differences were explained from a theoretical point of view, because of topographical features in the potential energy surface and the presence of conical intersections (CIs). Here, we analyze the photochemical properties of a new molecule, [2,2'-bipyridyl]-3-amine-3'-ol [BP(OH)(NH(2))], which is, in fact, a hybrid of the former two. Our density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations indicate that the double-proton-transfer process in the ground and first singlet pi-->pi* excited state in BP(OH)(NH(2)) presents features that are between those of their "parents". The presence of two CIs and the role they may play in the actual photochemistry of BP(OH)(NH(2)) and other bipyridyl derivatives are also discussed.

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

“…Finally, we will analyze the hybrid BP(OH)A C H T U N G T R E N N U N G (NH 2 ), which has been synthesized but studied no further, but is very interesting in relation to our work here due to the asymmetry of the molecule that allows for a larger number of reaction paths. As there is no way of discussing our new results (the dark states) without also considering the "bright" states that were previously identified (using the same level of calculation) and analyzed by us, [34][35][36] Figures 2-9 show the structures in two different formats: bold letters, numbers, and characters refer to the dark states (new results) whereas normal letters, numbers, and characters represent the bright (previously obtained) states of the different studied systems. [38] This differentiation also allows for a clearer discussion.…”

“…[34][35][36] In this paper we reconsider these three systems. We shall begin the study with BPA C H T U N G T R E N N U N G (NH 2 ) 2 , the molecule with the lowest quantum fluorescence yield as this points to a more likely access of the conical intersection and, consequently, to the dark states.…”

“…Figures 6 and 7 show the results for the N channel and Figures 8 and 9 refer to the O channel. For the bright states we are using the same nomenclature as in our previous theoretical work: [36] The initial form is the enol amine (EA) and the product of the double proton transfer is the keto imine (KI) structure. A single hydrogen transfer leads from EA to the enol imine (EI) or the keto amine (KA) for the N and O channels, respectively.…”

“…[29][30][31][32][33] In addition to studying the parent molecule, [34] we have also investigated the isoelectronic deriva- [35] and [2,2'-bipyridyl]-3-amin-3'ol (BP(OH)A C H T U N G T R E N N U N G (NH 2 )). [36] In spite of the similarities between the three molecular systems (the only change being the substitution of one or two hydroxy groups by amino ones), they show quite different photochemical behavior. One of the most striking differences arising from time-resolved femtochemistry experiments is the fluorescence quantum yield, which differs by more than four orders of magnitude between BP(OH) 2 and BPA C H T U N G T R E N N U N G (NH 2 ) 2 .…”

“…These CIs appear in regions of the potential energy surface that correspond to a single proton transfer upon rotation of the two pyridyl rings and it appears that they are energetically accessible from the Franck-Condon initial excitation area in all the systems considered. In our previous work, [34][35][36] the different photochemical behavior of the three systems was discussed in light of the different energies and geometries of the stationary points in S 0 and S 1 and the CIs. However, in these analyses the fate of the molecule beyond the CI was not considered.…”