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Excitement in the area of plasmonics stems from the ability to detect processes that are otherwise undetectable. Collective electron oscillations at metal surfaces experience strong spatial localization, which results in extremely intense and localized electromagnetic fields. This phenomenon is at the heart of all surface-enhanced spectroscopic processes. The intense local field can relax several selection rules, thereby allowing transitions that are otherwise forbidden, and are therefore inaccessible to regular spectroscopy. While observation of forbidden Raman modes in surface-enhanced Raman scattering (SERS) is well documented, the same is not as common for electronic transitions. Particularly noteworthy is a report on the observation of dipole-forbidden, but quadrupole-allowed transitions in conjugated oligoenes near silver films. A crucial question to explore is hence whether metal nanostructures can also enhance dipole-forbidden radiative recombination from triplet excited states, phosphorescence. The necessity arises because 75% of radical-pair recombination events in an organic light-emitting diode (OLED) lead to triplet excited states that generally have poor radiative recombination efficiency in the absence of heavy-metal atoms. There have been a few reports on plasmon-enhanced phosphorescence. One aspect common to these studies is that the effect was investigated in materials that are strongly spin–orbit (SO) coupled and thus highly phosphorescent to begin with. Most hydrocarbon organic semiconductors are weakly phosphorescent. OLED electrodes provide a natural environment for surface enhancement. If surface enhancement were to apply to transitions involving pure triplet and singlet states, in the absence of SO-induced spin mixing, the mechanism could open a new intrinsic radiative channel in an OLED, changing the way triplet harvesting is achieved and removing present limitations on triplet emitters posed by organometallic chemistry. Unfortunately, as we demonstrate here, this approach will not succeed. Phosphorescence can only be enhanced by plasmonics when intersystem crossing (ISC) is already strong. Surprisingly, phosphorescence due to transitions between pure triplet and singlet states is not enhanced to any measurable extent by plasmonic effects, even though a strong increase in fluorescence is observed in dual singlet–triplet-emitting compounds. To assess the possibility of surface enhancement of phosphorescence, we need an independent observable to confirm the presence of an enhancement effect. This observable is given by the dipole-allowed singlet transition in the dual-emitting compounds shown in Fig. 1a. We chose four materials with variable triplet yield (controlled by ISC) and singlet–triplet gap. The steady-state photoluminescence (PL) (at 300 K) and delayed luminescence (25 K) of the compounds dispersed at low concentration in polystyrene films are shown in Fig. 1b–e. The phosphorescence yield varies strongly across the sample series. The PL spectrum of 1, a Pt–porphyrin derivative, is dominated by the 637 nm phosphorescence (Fig. 1b). Strong SO coupling mediated by the Pt centre ensures efficient and near-complete ISC to the triplet manifold. Consequently, 1 exhibits very weak fluorescence (531 nm) that can only be detected by using a colour filter (Schott BG39) to attenuate the phosphorescence signal. 2 is a phenazine derivative that shows strong fluorescence and weaker but detectable steadystate phosphorescence at 300 K that is quite unusual for an organic compound devoid of any heavy atom. The phosphorescence is shifted to the red by the exchange gap DEST = 0.6 eV.We recently reported a series of triphenylene copolymers with tuneable singlet–triplet energy difference DEST. 6 To investigate whether plasmonic field-mediated singlet–triplet crossover is sensitive to DEST, we extended our study to compounds 3 Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741252, India. E-mail: [email protected] c Kekulé-Institut für Organische Chemie und Biochemie der Universität Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany d Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93053 Regensburg, Germany w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cc31843a ChemComm Dynamic Article Links