A Comparative Study on the Photochemistry of Two Bipyridyl Derivatives: [2,2′‐Bipyridyl]‐3,3′‐diamine and [2,2′‐Bipyridyl]‐3,3′‐diol

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. Herein, density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations are used to study the double-proton-transfer processes in the ground and first singlet pi-->pi* excited state. The quantum-chemistry calculations indicate 1) the existence of only one energy minimum in the ground electronic state corresponding to reactants (thus avoiding the possibility of a fast fluorescent relaxation process from the photoproducts region), 2) an endoergic process of the complete double proton transfer, and 3) the presence of a conical intersection in the excited intermediate region of [2,2'-bipyridyl]-3,3'-diamine. These facts explain the very low fluorescence quantum yield in [2,2'-bipyridyl]-3,3'-diamine compared to [2,2'-bipyridyl]-3,3'-diol.

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.…”

“…2 in a previous work. [35] In that work the stationary states of the ground (S 0 ) and first singlet-excited (S 1 ) electronic states, the latter resulting from an allowed p!p* excitation, were carefully located. The results are summarized in Figures 2 and 3.…”

“…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.…”

“…[28] In recent years we have studied a series of isoA C H T U N G T R E N N U N G electronic molecules with [2,2'-bipyridyl]-3,3'-diol (hereafter referred www.chemeurj.org to as BP(OH) 2 ), one of the simplest molecules that is experimentally known to suffer a double proton transfer in the excited state. [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.…”