π -π stacking vs. C–H/π interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9′-dimethylfluorene

Abstract

Studies of exciton and hole stabilization in multichromophoric systems underpin our understanding of electron transfer and transport in materials and biomolecules. The simplest model systems are dimeric, and recently we compared the gas-phase spectroscopy and dynamics of van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9-dimethylfluorene (F1) to assess how sterically controlled facial encumbrance modulates the dynamics of excimer formation and charge resonance stabilization (CRS). Dimers of fluorene and MF show only excimer emission upon electronic excitation, and significant CRS as evidenced in a reduced ionization potential for the dimer relative the monomer. By contrast, the dimer of F1 shows no excimeric emission, rather structured emission from the locally excited state of a tilted (non π-stacked) dimer, evidencing the importance of C-H/π interactions and increased steric constraints that restrict a cofacial approach. In this work, we report our full results on van der Waals clusters of F1, using a combination of theory and experiments that include laser-induced fluorescence, mass-selected two-color resonant two-photon ionization spectroscopy, and two-color appearance potential measurements. We use the latter to derive the binding energies of the F1 dimer in ground, excited, and cation radical states. Our results are compared with van der Waals and covalently linked clusters of fluorene to assess both the relative strength of π-stacking and C-H/π interactions in polyaromatic assemblies and the role of π-stacking in excimer formation and CRS.

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models

“…Some of the subsequent TD-DFT based studies on excimer and exciplex systems either do not explore the dispersion problem of DFT 85,86 or erroneously justied the use of Minnesota functionals 87 for consideration of dispersion effects, 22,88 despite ground-state based studies that disprove claims they are able to treat such interactions. 25,27,37,38,89 Some other studies recognised the dispersion problem, but for a lack of a better choice, followed Huenerbein and Grimme's recommendation to use a global-hybrid combined with DFT-D2, 6,[90][91][92][93][94][95] while fewer applied the newer, ground-state based DFT-D3 method in its zero-damping 96 [DFT-D3(0)] or in its Becke-Johnson-damping [97][98][99] [DFT-D3(BJ) 40 ] form. [100][101][102][103][104][105][106] To our knowledge, only three studies have considered adjusting dispersion corrections for excited states.…”

Enhancement of Singlet Fission Yield by Hindering Excimer Formation in Perylene Film

“…The optimal stabilization of an excimer needs a perfect sandwichlike geometry with maximal overlap. Under these conditions, the rate of excimer generation is usually very fast . For Per and PDI dimer, the coupling between dimers decreases and the excimer is weaker and is formed slower. , Therefore in the Per film, the cofacial stacking with a π–π distance of 3.24 Å offers a more favorable condition to the excimer.…”

Intra- vs Intermolecular Aurophilic Contacts in Dinuclear Gold(I) Compounds: Impact on the Population of the Triplet Excited State

“…1 H NMR (400 MHz, CDCl 3 ): δ 7.76−7.72 (m, 4H D,9 ), 7.63−7.55 (m, 6H 10,A,B ), 7.53−7.53−7.27 (m, 20H Ph ), 7.15−7.09 (m, 2H 6 ), 7.04−6.99 (m, 2H 13 ), 6.93 (tt, J = 7.6, 1.4 Hz, 2H C ), 6.78 (ddd, J = 8.2, 7.4, 2.3 Hz, 4H 7,8 ), 6.24 (ddd, J = 7.7, 4.5, 1.2 Hz, 2H 11 ), 1.57 (s, 6H CHd 3 ), 1.45 (s, 6H CHd 3 ). 31 P{ 1 H} NMR (161.9 MHz, CDCl 3 , ppm): δ 37.97.…”