Competing excitonic couplings as origin of mimicked phase transitions in zinc-phthalocyanine single crystals

TL;DR

This study shows that the abrupt, highly polarized, low-temperature emission observed in β-phase ZnPc single crystals does not require a structural phase transition. By combining polarization-, temperature-, and time-resolved photoluminescence with TD-DFT, the authors demonstrate that a thermally populated dimer exciton model, incorporating fixed H- and J-type couplings across multiple directions, reproduces the spectral evolution. Key findings include a Boltzmann-activated quenching of the low-temperature J-type emission and phonon-assisted activation of higher emissive states, which together mimic a phase-transition signature. The work cautions against attributing discontinuous optical changes to structural transitions in molecular crystals and highlights the complex interplay of anisotropic excitonic couplings and exciton-phonon interactions as a general mechanism for mimicked transitions.

Abstract

The optical properties of molecular crystals are largely determined by the excitonic coupling of neighboring molecules. This coupling is extremely sensitive to the arrangement of adjacent molecular units, as their electronic interaction is defined by the relative orientation of the individual transition dipole moments and their wave function overlap. Hence, the optical properties, such as fluorescence, are usually highly anisotropic and good indicators of structural changes during the variation of intensive thermodynamic parameters like temperature or pressure. Here, we discuss the peculiar though archetypical case of $β$-phase zinc-phthalocyanine: In single crystals, we report a sudden change of spectral emission with temperature from a broad, unpolarized Frenkel-exciton type luminescence to a narrow, highly polarized superradiance-like fluorescence below 80 K. Surprisingly, we find that there is no sign of a discrete structural phase transition in this temperature regime. To understand this apparent contradiction, we perform polarization-, temperature- and time-dependent photoluminescence measurements along different crystallographic directions to fully map the emission characteristics of the crystal-exciton. By means of ab-initio calculations on a density functional theory level we conclude that our observations are consistent with a dimer exciton model when considering thermalized electronic states. As such, our study presents a representative case study on a well-established molecular material class demonstrating that caution is advised when attributing discrete changes in electronic observables to a structural phase transition. As we show for zinc-phthalocyanine in its $β$-phase modification, slowly varying excitonic couplings and thermal redistribution of excitations can mimic the same signatures attributed to a structural phase transition.

Figures (6)

• Figure 1: a) Molecular structure of ZnPc with purple arrows indicating the TDM orientation of the $\text{S}_0 \rightarrow \text{S}_1$ and $\text{S}_0 \rightarrow\text{S}_2$ calculated via TD-DFT. b) Unit cell of $\upbeta$-phase ZnPc with herringbone-like packing. c) Temperature dependent PL spectra measured on the ZnPc $(\bar{1}01)$ crystal facet with a gray line as guide to the eye indicating the evolution of spectral maximum with temperature. The spectra exhibit Fabry-Pérot oscillations at lower energies being indicative for the high crystal quality. d) Luminescence spectra measured at exemplary temperatures of 290 K, 110 K and 6 K.
• Figure 2: Integrated intensity of the spectra displayed in Figure 1 c) as function of temperature showing a Boltzmann activated decay at low temperatures and an increase in emission at higher temperatures following the Bose-Einstein population of phonons.
• Figure 3: Comparison of the integrated intensity as a function of the polarization angle measured in respect to the a) microtome cut $(223)$ plane and (b) naturally occurring $(\bar{1}01)$ facet, illustrated by the schematic above. For the $(223)$ facet at 7 K only the spectral region above 1.45 eV is included for integration. 90 $^\circ$ polarization refers to the normal vector $\hat{n}_{\bar{1}01}$ of $(\bar{1}01)$ plane and the $[010]$ crystallographic direction, respectively. At the bottom, representative spectra for the polarization directions of highest (0 $^\circ$) and lowest (90 $^\circ$) intensity are shown.
• Figure 4: Fluorescence decay of the main emission peak around 1.6 eV at 5 K or 7 K and 295 K for the a) $(223)$ and b) $(\bar{1}01)$ facet with polarization angles of 0 $^\circ$, meaning parallel to the $(\bar{1}01)$ plane and $[010]$-axis, respectively. The instrument response function (IRF) of the setup is shown in gray for comparison. Normalized amplitudes of the one (two) main components with lifetimes $\tau_1$ (and $\tau_2$) at 295 K (5 or 7 K) for both measured polarizations for the c) $(223)$ and d) $(\bar{1}01)$ facets. Polarization angles are defined analogously to Figure \ref{['fgr:3-Pol']}.
• Figure 5: a) Depiction of the neighboring molecules representing the three investigated dimer configurations. b) Arrangement of the dimers in respect to the two measured crystallographic facets, the $(223)$ (left) and $(\bar{1}01)$ facet (right). Exemplary rotated versions of Dimers 1 and 2 are also shown for the $(\bar{1}01)$ facet and labeled Dimers 1’ and 2’ (see main text). c) Projection of the resulting TDMs of the energetically lowest transitions with non-vanishing moment onto the respective crystallographic planes above. The depicted length of the arrows therefore qualitatively correspond to the dipole strength. $\text{S}_3$ of Dimer 1 is indicated by a purple arrow, $\text{S}_1$ for Dimers 2 and 3 in blue and yellow, respectively. Additionally, the TDM of the rotated dimers are shown by dashed arrows, where the different orientations result in substantial changes in TDM orientation.