Hydrogen bonds vs. π-stacking interactions in the p-aminophenol⋯p-cresol dimer: an experimental and theoretical study

Abstract

The gas phase structure and excited state lifetime of the p-aminophenolp-cresol heterodimer have been investigated by REMPI and LIF spectroscopy with nanosecond laser pulses and pump-probe experiments with picosecond laser pulses as a model system to study the competition between π-π and H-bonding interactions in aromatic dimers. The excitation is a broad and unstructured band. The excited state of the heterodimer is long lived (2.5 ± 0.5) ns with a very broad fluorescence spectrum red-shifted by 4000 cm with respect to the excitation spectrum. Calculations at the MP2/RI-CC2 and DFT-ωB97X-D levels indicate that hydrogen-bonded (HB) and π-stacked isomers are almost isoenergetic in the ground state while in the excited state only the π-stacked isomer exists. This suggests that the HB isomer cannot be excited due to negligible Franck-Condon factors and therefore the excitation spectrum is associated with the π-stacked isomer that reaches vibrationally excited states in the S state upon vertical excitation. The excited state structure is an exciplex responsible for the fluorescence of the complex. Finally, a comparison was performed between the π-stacked structure observed for the p-aminophenolp-cresol heterodimer and the HB structure reported for the (p-cresol) homodimer indicating that the differences are due to different optical properties (oscillator strengths and Franck-Condon factors) of the isomers of both dimers and not to the interactions involved in the ground state.

Unexpected solvent effects on the UV/Vis absorption spectra of o -cresol in toluene and benzene: in contrast with non-aromatic solvents

“…Very recently, aromatic stacking was experimentally demonstrated to play the key role in controlling solvent-dependent nucleation in crystal growth [17]. The search for a database of X-ray crystal structures revealed a high occurrence of parallel displaced (PD) and T-shaped (T) stacking styles of aromatic rings [9,[18][19][20][21][22][23][24][25][26], rationalized by electronic structure calculations [21,22,[27][28][29][30][31][32][33][34][35][36][37][38][39].…”

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 justified 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, groundstate 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.…”

“…Most of the aforementioned exciplex studies used older globalhybrid DFAs, while some 85,86,90,91,102,105,106,116 used rangeseparated hybrids. Of the two studies that investigated DHD-FAs 94,102 both utilised TDA-DFT, instead of the full TD-DFT scheme, but only Krueger and Blanquart 102 considered a dispersion correction.…”

“…Interestingly, most of the methods previously used to study exciplex systems are not those recommended by some of the latest single-molecule excitation studies, 52,53,57,62,64,66 which by itself is reason enough to investigate the latter in the exciplex/excimer context. The majority of excimer and exciplex studies investigated these methods via applications, 22,86,88,[91][92][93][94]100,101,104,116 or carried out benchmark analyses against reference values that were not well defined and mostly based on experiment. 5,90 Both benchmarks and application-based excimer studies focussed on comparison of methods by the calculation of the dissociation energy, as this explores the binding of the complex, while fewer studies have explored other spectrometric parameters such as the fluorescence, absorption and repulsion energies.…”

Dipole Moment Propels π-Stacking of Heterodimers of Fluorophenylacetylenes

“…In addition, the π-stacking interactions are known to influence the structures of several molecular assemblies including crystal structures . While the literature of π-stacking interactions is ever-increasing, the number of reports on stand-alone stacked structures in the gas phase are considerably sparser. − However, it is important to distinguish “π-stacked” and “stacked” structures. The π-stacked structures correspond to an interaction between the two aromatic rings directly placed over one another.…”