Abstract
The modulation of intermolecular interactions upon aggregation induces changes in excited state properties of organic molecules that can be detrimental for some optoelectronic applications but can be exploited for others. The time-dependent density functional theory (TDDFT) is a cost-effective approach to determining the exciton states of molecular aggregates, and it has been shown to provide reliable results when coupled with the appropriate choice of the functional. Here we apply a general procedure to analyze the aggregates’ exciton states derived from TDDFT calculations in terms of diabatic states chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space. We apply the approach to study energy profiles, interstate couplings, and the charge-transfer character of singlet and triplet exciton states of perylene di-imide aggregates (PDI). We focus on the intermolecular displacement along the longitudinal translation coordinate, which mimics different amounts of slip-stacking observed in PDI crystals. The analysis, in terms of symmetry-adapted Frenkel excitations (FE) and charge-resonance (CR) states and their interactions, discloses how the interchange of the H/J character for small longitudinal shifts, previously reported for singlet exciton states, also occurs for triplet excitons.
Highlights
The photophysical behavior of organic electronic molecular materials is governed by the nature of their low-lying exciton states [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]
We show that for triplet exciton states the modulation of CR/Frenkel excitations (FE) interactions along the longitudinal translation coordinate determines a switch in the symmetry (A g /Bu ) of the lowest triplet exciton state, which corresponds to the unconventional H-/J-character alternation previously documented for singlet excitons [8,9,60]
We have analyzed the modulation of singlet and triplet exciton states of perylene di-imide aggregates (PDI) aggregates computed at the time-dependent density functional theory (TDDFT) level, focusing on the intermolecular displacement along the longitudinal translation coordinate, which mimics different amounts of slip stacking observed in PDI crystals
Summary
The photophysical behavior of organic electronic molecular materials is governed by the nature of their low-lying exciton states [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. The role of CT states on photo-induced processes is documented by a large number of investigations. CT states have a crucial role in exciton dissociation and charge separation in heterojunctions between electron-donating and electron-accepting materials or in homojunctions between crystalline domains with different orientations [41,42,43]. While the modelling and analysis of singlet spin exciton states has received considerable attention [20,22,27,44,45,46,47,48,49,50], triplet excitons have received comparably less attention and only few investigations have been reported [22,51,52]. Enhanced triplet-state generation, following photo-induced charge transfer, was reported by various groups in electron donor–acceptor polymer-blend films in organic photovoltaic devices where nonemissive triplet excitons are responsible for nonradiative-charge recombination [53]. Among others, triplet-state generation in molecular dyads [56] and molecular dimers in which dimerization-induced triplet population has been invoked [57]
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