Plasmonic surface enhancement of dual fluorescence and phosphorescence emission from organic semiconductors: effect of exchange gap and spin–orbit coupling

Abstract

Dual singlet-triplet fluorescence-phosphorescence emitting compounds demonstrate that plasmonic surface enhancement is controlled solely by the underlying oscillator strength of a transition: metal-free compounds with weak spin-orbit coupling show no enhancement in phosphorescence efficiency even though fluorescence is amplified.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. 1 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, 2 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. 3 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. 4 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 (...

Emission enhancement and lifetime modification of phosphorescence on silver nanoparticle aggregates

“…These include metal enhanced rare-earth ion luminescence applied to OLEDs, 17-21 enhancement of organic solar cells with plasmonic nanoparticles, 22,23 and enhanced excitation of phosphorescent dyes related to photodynamic therapy. [27][28][29] Surprisingly, although theory predicts that lifetime modifications similar to that for fluorescence should occur in the case the triplet transition is dipolar in nature (and even larger lifetime modifications in the case it is quadrupolar in nature 30 ), experimental results so far have observed much weaker modification of phosphorescence lifetimes compared to fluorescence lifetimes. [27][28][29] Surprisingly, although theory predicts that lifetime modifications similar to that for fluorescence should occur in the case the triplet transition is dipolar in nature (and even larger lifetime modifications in the case it is quadrupolar in nature 30 ), experimental results so far have observed much weaker modification of phosphorescence lifetimes compared to fluorescence lifetimes.…”

“…[24][25][26] However, only a few examples in the literature look into the basic behaviour of the triplet transition as compared to the normal singlet transition. [27][28][29] Surprisingly, although theory predicts that lifetime modifications similar to that for fluorescence should occur in the case the triplet transition is dipolar in nature (and even larger lifetime modifications in the case it is quadrupolar in nature 30 ), experimental results so far have observed much weaker modification of phosphorescence lifetimes compared to fluorescence lifetimes.…”

Indirect consequences of exciplex states on the phosphorescence lifetime of phenazine-based 1,2,3-triazole luminescent probes

“…Some PZ derivatives can also present steady-state phosphorescence, relatively weak but observable at room temperature. 11 The detection of phosphorescence properties of PZ or PZ derivatives through gated intensified charge-coupled devices (iCCD's), would be also useful as a second experimental signature in order to use them as markers in intra-cellular or related media. In an early work, 12 a PZ derivative was designed and synthesized, being used lately as selective metal sensor and demonstrating selectivity for cadmium ions, which are very dangerous for the human life.…”

Precise Control of Localized Surface Plasmon Wavelengths Is Needed for Effective Enhancement of Triplet–Triplet Annihilation-Based Upconversion Emission

“…800 nm) of Pd-TPTBP. In the presence of plasmonic metal nanoparticles, energy transfer from the sensitizer to the plasmonic metal nanoparticles can be induced, even at a separation distance greater than 10 nm between them through LSP-induced mechanisms including fluorescence resonance energy transfer, nanometal surface energy transfer, and plasmon-enhanced fluorescence energy transfer. − Since the average thickness of TTA-UC thin films was measured as 7.2 nm, there is a high possibility that the TTET and the energy transfer to metal nanoparticles would compete in the deactivation process of the triplet excited sensitizer in the films. Consequently, by overlapping the LSP band with the phosphorescence wavelength of the sensitizer, the appearance of a new pathway of energy transfer to the LSP of metal nanoparticles, k D,LSP , should effectively suppress the TTET efficiency, thereby resulting in a decrease in the upconverted emission intensity, as shown in Figure A and eqs and . , …”