Model for triplet state engineering in organic light emitting diodes

Abstract

Engineering the position of the lowest triplet state (T1) relative to the first excited singlet state (S1) is of great importance in improving the efficiencies of organic light emitting diodes and organic photovoltaic cells. We have carried out model exact calculations of substituted polyene chains to understand the factors that affect the energy gap between S1 and T1. The factors studied are backbone dimerisation, different donor-acceptor substitutions, and twisted geometry. The largest system studied is an 18 carbon polyene which spans a Hilbert space of about 991 × 10(6). We show that for reverse intersystem crossing process, the best system involves substituting all carbon sites on one half of the polyene with donors and the other half with acceptors.

Indirect Intersystem Crossing (S1 → T3/T2 → T1) Promoted by the Jahn–Teller Effect in Cycloparaphenylenes

“…Identification of various nonradiative decay pathways arising from vibronic coupling effects in the dense excited triplet manifold is quite a challenging task. Nevertheless, detailed investigations of such decay pathways could provide important insights into basic aspects of singlet fission, , triplet–triplet annihilation, , and reverse intersystem crossing processes. − …”

Singlet and Triplet Excited State Energy Ordering in Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons (CP-PAHs) Suitable for Energy Harvesting: An Exact Model and TDDFT Study

“…6−10 In recent times, theoretical and computational investigations have also predicted some conjugated systems with small S 1 –T 1 energy gaps. 11−14 In those cases, the S 1 –T 1 energy gap has been calculated using time-dependent density functional theory (TDDFT), single configuration interaction (SCI), and the full CI technique. Gierschner et al reported carbozole–paraterphenyl systems with donor–acceptor (D–A) substituents.…”

“…In general, for conjugated systems, the lowest triplet energy E (T 1 ) is lower than the lowest singlet energy E (S 1 ), however in the process of RISC, it is very desirable to have a S 1 –T 1 energy gap that is of the order of k B T (0.025 eV) so that S 1 can be populated thermally. There are some experimental reports on such conjugated systems with very small S 1 –T 1 energy gaps (0.04 eV). − In recent times, theoretical and computational investigations have also predicted some conjugated systems with small S 1 –T 1 energy gaps. − In those cases, the S 1 –T 1 energy gap has been calculated using time-dependent density functional theory (TDDFT), single configuration interaction (SCI), and the full CI technique. Gierschner et al reported carbozole–paraterphenyl systems with donor–acceptor (D–A) substituents .…”

“…Theoretical prediction at INDO level reported an S 1 –T 1 energy gap of 0.8 eV in pentamers . Prodhan et al carried out a full CI calculation on model polyenes and showed that the best system with a very low S 1 –T 1 gap involves D substitution in one half of the carbons and A substitution in the other half of the carbon atoms in the polyene chain . The energy ordering of S 1 and T 1 is also very important in photodynamic therapy.…”

“…13 Prodhan et al carried out a full CI calculation on model polyenes and showed that the best system with a very low S 1 −T 1 gap involves D substitution in one half of the carbons and A substitution in the other half of the carbon atoms in the polyene chain. 14 The energy ordering of S 1 and T 1 is also very important in photodynamic therapy. In this technique, ground state (gs) triplet oxygen collides with an S 1 photosensitizer to give rise to singlet oxygen forming the T 1 excited state.…”