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Observation of a re-entrant phase transition in the molecular complex tris( μ 2 -3,5-diiso ­ propyl-1,2,4-triazolato- κ 2 N 1 : N 2 )trigold(I) under high pressure

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ABSTRACT

We report a molecular crystal that exhibits four successive phase transitions under hydro­static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diiso­propyl-1,2,4-triazolato-κ2N1:N2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040 cm−1 at 2.40 GPa, decreasing steeply to 13550 cm−1 at 3 GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.

No MeSH data available.


Energy gap between the highest-occupied and lowest-unoccupied Kohn–Sham bands in the C2/c, P21/n and P21/a phases as a function of applied hydro­static pressure, , from 0 to 4.2 GPa.
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fig8: Energy gap between the highest-occupied and lowest-unoccupied Kohn–Sham bands in the C2/c, P21/n and P21/a phases as a function of applied hydro­static pressure, , from 0 to 4.2 GPa.

Mentions: Finally, to confirm whether the experimentally observed shift in luminescence is due to changes in the HOMO–LUMO gap with pressure, we extracted the difference between the highest-occupied and lowest-unoccupied Kohn–Sham bands in our optimized structures (Fig. 8 ▸). This analysis suggests that the bandgap of all three phases would decrease significantly under pressure; this is the reverse trend to what would be anticipated from a simple covalent-bonding picture, in which as atoms approach more closely the increased orbital overlap should lead to a larger separation between bonding and antibonding states. It is thus reasonable to infer that the change in the energy gap is dictated more by changes in conformation than by simple compression of interatomic distances. Interestingly, as a qualitative trend, the P21/a phase consistently has a smaller gap than the (ambient) C2/c and P21/n phases, which is consistent with a red shift in the luminescence. This suggests that the transition to the P21/a phase, as well as presumably the transition to the large C2/c phase at higher pressures, may be responsible for the red shift in the luminescence evident in Fig. 8 ▸.


Observation of a re-entrant phase transition in the molecular complex tris( μ 2 -3,5-diiso ­ propyl-1,2,4-triazolato- κ 2 N 1 : N 2 )trigold(I) under high pressure
Energy gap between the highest-occupied and lowest-unoccupied Kohn–Sham bands in the C2/c, P21/n and P21/a phases as a function of applied hydro­static pressure, , from 0 to 4.2 GPa.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5391858&req=5

fig8: Energy gap between the highest-occupied and lowest-unoccupied Kohn–Sham bands in the C2/c, P21/n and P21/a phases as a function of applied hydro­static pressure, , from 0 to 4.2 GPa.
Mentions: Finally, to confirm whether the experimentally observed shift in luminescence is due to changes in the HOMO–LUMO gap with pressure, we extracted the difference between the highest-occupied and lowest-unoccupied Kohn–Sham bands in our optimized structures (Fig. 8 ▸). This analysis suggests that the bandgap of all three phases would decrease significantly under pressure; this is the reverse trend to what would be anticipated from a simple covalent-bonding picture, in which as atoms approach more closely the increased orbital overlap should lead to a larger separation between bonding and antibonding states. It is thus reasonable to infer that the change in the energy gap is dictated more by changes in conformation than by simple compression of interatomic distances. Interestingly, as a qualitative trend, the P21/a phase consistently has a smaller gap than the (ambient) C2/c and P21/n phases, which is consistent with a red shift in the luminescence. This suggests that the transition to the P21/a phase, as well as presumably the transition to the large C2/c phase at higher pressures, may be responsible for the red shift in the luminescence evident in Fig. 8 ▸.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

We report a molecular crystal that exhibits four successive phase transitions under hydro­static pressure, driven by aurophilic interactions, with the ground-state structure re-emerging at high pressure. The effect of pressure on two polytypes of tris(μ2-3,5-diiso­propyl-1,2,4-triazolato-κ2N1:N2)trigold(I) (denoted Form-I and Form-II) has been analysed using luminescence spectroscopy, single-crystal X-ray diffraction and first-principles computation. A unique phase behaviour was observed in Form-I, with a complex sequence of phase transitions between 1 and 3.5 GPa. The ambient C2/c mother cell transforms to a P21/n phase above 1 GPa, followed by a P21/a phase above 2 GPa and a large-volume C2/c supercell at 2.70 GPa, with the previously observed P21/n phase then reappearing at higher pressure. The observation of crystallographically identical low- and high-pressure P21/n phases makes this a rare example of a re-entrant phase transformation. The phase behaviour has been characterized using detailed crystallographic theory and modelling, and rationalized in terms of molecular structural distortions. The dramatic changes in conformation are correlated with shifts of the luminescence maxima, from a band maximum at 14040 cm−1 at 2.40 GPa, decreasing steeply to 13550 cm−1 at 3 GPa. A similar study of Form-II displays more conventional crystallographic behaviour, indicating that the complex behaviour observed in Form-I is likely to be a direct consequence of the differences in crystal packing between the two polytypes.

No MeSH data available.