Macroscopic Molecular Ordering and Exciton Delocalization in Crystalline Phthalocyanine Thin Films

Abstract

We present spatially-, temporally- and polarization-resolved dual photoluminescence/linear dichroism microscopy experiments that investigate the correlation between long-range order and the nature of exciton states in solution-processed phthalocyanine thin films. The influence of grain boundaries and disorder is absent in these films because typical grain sizes are 3 orders of magnitude larger than focused excitation beam diameters. These experiments reveal the existence of a delocalized singlet exciton, polarized along the high mobility axis in this quasi-1D electronic system. The strong delocalized π orbitals overlap, controlled by the molecular stacking along the high mobility axis, is responsible for breaking the radiative recombination selection rules. Using our linear dichroism scanning microscopy setup, we further established that a rotation of molecules (i.e., a structural phase transition) that occurs above 100 K prevents the observation of this exciton at room temperature.

Exciton Dynamics in Phthalocyanine Molecular Crystals

“…These computed excited states correspond to the experimental peaks recorded at 1.64 and 1.68 eV in solution (commonly known as the Qx and Qy electronic transitions). 21 Higher excited states belonging to the so-called B-band are found to be at least 1.67 eV higher in energy than the S2 excited state. Therefore, the subspace formed by the first two excited states (Qx and Qy) localized on each molecule would be enough to study the exciton dynamics in the H2-OBPc crystal and the excitonic couplings in dimers A and B would stem mainly from the interactions between these localized Qx and Qy states, giving rise to four intermolecular excitonic couplings (Jxx, Jxy, Jyx, and Jyy, vide infra).…”

“…19 Recently, an octa(butyl)-substituted metal-free phthalocyanine, H2-OBPc (1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine, see Figure 1a) has been synthesized and exhibited good crystallinity properties (1D π-stacked arrangements, see Figure 1b) in thin films. 20,21 The high degree of crystallinity of the solution-processed thin films allowed Rawat et al to investigate the correlation between long-range order and the nature of the lowest exciton states of the aggregate. 21 The H2-OBPc compound is therefore an excellent molecular crystal model to study exciton diffusion at room temperature in the absence of amorphous phase (static) disorder and grain boundaries.…”

Importance and Nature of Short-Range Excitonic Interactions in Light Harvesting Complexes and Organic Semiconductors

“…The spectroscopy and exciton dynamics of this system are mainly determined by the couplings in the intra-column dimers A and B. 36,53 The very short π-π intermolecular distances in the 3.5-4.5 Å range (see Figure 1) make these couplings very sensitive to thermal fluctuations. The values of J C and J TOT for 250 MD snapshots are shown in Figure 5.…”

Many-Body Effects in FeN4 Center Embedded in Graphene

“…These, and similar molecules such as phthalocyanine, are polyaromatic complexes that can accommodate a range of atoms at their centers to induce changes in magnetic and optical properties [1][2][3][4]. As a result of their versatility, these complexes have found a range of exciting applications in spintronics [5][6][7][8][9][10][11][12][13][14], optoelectronics [15,16], solar cells [17][18][19][20][21][22], and as building blocks of magnetic materials [23][24][25][26][27][28][29] or highly tunable qubits for quantum computing applications [30]. These complexes display important correlation physics, such as spin and orbital variants of the Kondo effect in phthalocyanine (FePc) molecules deposited on the (111) surface of noble metals [31][32][33][34][35][36][37][38].…”