Synthesis and Photophysical Characterization of a pH-Sensitive Quadracyclic Uridine (qU) Analogue.

Fluorescent base analogues (FBAs) have become useful tools for applications in biophysical chemistry, chemical biology, live-cell imaging, and RNA therapeutics. Herein, two synthetic routes towards a novel FBA of uracil named qU (quadracyclic uracil/uridine) are described. The qU nucleobase bears a tetracyclic fused ring system and is designed to allow for specific Watson-Crick base pairing with adenine. We find that qU absorbs light in the visible region of the spectrum and emits brightly with a quantum yield of 27 % and a dual-band character in a wide pH range. With evidence, among other things, from fluorescence lifetime measurements we suggest that this dual emission feature results from an excited-state proton transfer (ESPT) process. Furthermore, we find that both absorption and emission of qU are highly sensitive to pH. The high brightness in combination with excitation in the visible and pH responsiveness makes qU an interesting native-like nucleic acid label in spectroscopy and microscopy applications in, for example, the field of mRNA and antisense oligonucleotide (ASO) therapeutics.

Pyrene Functionalized Norbornadiene-Quadricyclane Fluorescent Photoswitches: Characterization of their Spectral Properties and Application in Imaging of Amyloid Beta Plaques.

This study presents the synthesis and characterization of two fluorescent norbornadiene (NBD) photoswitches, each incorporating two conjugated pyrene units. Expanding on the limited repertoire of reported photoswitchable fluorescent NBDs, we explore their properties with a focus on applications in bioimaging of amyloid beta (Aß) plaques. While the fluorescence emission of the NBD decreases upon photoisomerization, aligning with what has been previously reported, for the first time we observed luminescence after irradiation of the quadricyclane (QC) isomer. We deduce how the observed emission is induced by photoisomerization to the excited state of the parent isomer (NBD) which is then the emitting species. Thorough characterizations including NMR, UV-Vis, fluorescence, X-ray structural analysis and density functional theory (DFT) calculations provide a comprehensive understanding of these systems. Notably, one NBD-QC system exhibits exceptional durability. Additionally, these molecules serve as effective fluorescent stains targeting Aß plaques in situ, with observed NBD/QC switching within the plaques. Molecular docking simulations explore NBD interactions with amyloid, unveiling novel binding modes. These insights mark a crucial advancement in the comprehension and design of future photochromic NBDs for bioimaging applications and beyond, emphasizing their potential in studying and addressing protein aggregates.

Seroprevalence of tick-borne encephalitis virus and vaccination coverage of tick-borne encephalitis, Sweden, 2018 to 2019.

BackgroundIn Sweden, information on seroprevalence of tick-borne encephalitis virus (TBEV) in the population, including vaccination coverage and infection, is scattered. This is largely due to the absence of a national tick-borne encephalitis (TBE) vaccination registry, scarcity of previous serological studies and use of serological methods not distinguishing between antibodies induced by vaccination and infection. Furthermore, the number of notified TBE cases in Sweden has continued to increase in recent years despite increased vaccination.AimThe aim was to estimate the TBEV seroprevalence in Sweden.MethodsIn 2018 and 2019, 2,700 serum samples from blood donors in nine Swedish regions were analysed using a serological method that can distinguish antibodies induced by vaccination from antibodies elicited by infection. The regions were chosen to reflect differences in notified TBE incidence.ResultsThe overall seroprevalence varied from 9.7% (95% confidence interval (CI): 6.6-13.6%) to 64.0% (95% CI: 58.3-69.4%) between regions. The proportion of vaccinated individuals ranged from 8.7% (95% CI: 5.8-12.6) to 57.0% (95% CI: 51.2-62.6) and of infected from 1.0% (95% CI: 0.2-3.0) to 7.0% (95% CI: 4.5-10.7). Thus, more than 160,000 and 1,600,000 individuals could have been infected by TBEV and vaccinated against TBE, respectively. The mean manifestation index was 3.1%.ConclusionA difference was observed between low- and high-incidence TBE regions, on the overall TBEV seroprevalence and when separated into vaccinated and infected individuals. The estimated incidence and manifestation index argue that a large proportion of TBEV infections are not diagnosed.

Bulky Substituents Promote Triplet-Triplet Annihilation Over Triplet Excimer Formation in Naphthalene Derivatives.

Visible-to-ultraviolet (UV) triplet-triplet annihilation photochemical upconversion (TTA-UC) has gained a lot of attention recently due to its potential for driving demanding high-energy photoreactions using low-intensity visible light. The efficiency of this process has rapidly improved in the past few years, in part thanks to the recently discovered annihilator compound 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (N-2TIPS). Despite its beneficial TTA-UC characteristics, the success of N-2TIPS in this context is not yet fully understood. In this work, we seek to elucidate what role the specific type and number of substituents in naphthalene annihilator compounds play to achieve the characteristics sought after for TTA-UC. We show that the type of substituent attached to the naphthalene core is crucial for its performance as an annihilator. More specifically, we argue that the choice of substituent dictates to what degree the sensitized triplets form excimer complexes with ground state annihilators of the same type, which is a process competing with that of TTA. The addition of more bulky substituents positively impacts the upconverting ability by impeding excimer formation on the triplet surface, an effect that is enhanced with the number of substituents. The presence of triplet excimers is confirmed from transient absorption measurements, and the excimer formation rate is quantified, showing several orders of magnitude differences between different derivatives. These insights will aid in the further development of annihilator compounds for solar energy applications for which the behavior at low incident powers is of particular significance.

Diarylethene Isomerization by Using Triplet-Triplet Annihilation Photon Upconversion.

Green-to-blue triplet-triplet annihilation photon upconversion with the well-studied upconversion pair 9,10-diphenylanthracene (DPA)/platinum octaethylporphyrin (PtOEP) was used to reversibly drive the photoisomerization of diarylethene (DAE) photoswitches by using visible light. By carefully selecting the kinetic and spectral properties of the molecular system as well as the experimental geometry, a single green light source can be used to selectively trigger both the ring-opening and the ring-closing reactions, whilst also inducing fluorescence from the colored closed isomer that can be used as a readout to monitor the isomerization process in situ. The upconversion solution and the DAE solution are kept physically separated, allowing them to be characterized both concomitantly and individually without further separation processes. The ring-closing reaction using upconverted photons was quantified and compared to the efficiency of direct isomerization with ultraviolet light.

Triplet States of Cyanostar and Its Anion Complexes.

The design of advanced optical materials based on triplet states requires knowledge of the triplet energies of the molecular building blocks. To this end, we report the triplet energy of cyanostar (CS) macrocycles, which are the key structure-directing units of small-molecule ionic isolation lattices (SMILES) that have emerged as programmable optical materials. Cyanostar is a cyclic pentamer of covalently linked cyanostilbene units that form π-stacked dimers when binding anions as 2:1 complexes. The triplet energies, ET, of the parent cyanostar and its 2:1 complex around PF6- are measured to be 1.96 and 2.02 eV, respectively, using phosphorescence quenching studies at room temperature. The similarity of these triplet energies suggests that anion complexation leaves the triplet energy relatively unchanged. Similar energies (2.0 and 1.98 eV, respectively) were also obtained from phosphorescence spectra of the iodinated form, I-CS, and of complexes formed with PF6- and IO4- recorded at 85 K in an organic glass. Thus, measures of the triplet energies likely reflect geometries close to those of the ground state either directly by triplet energy transfer to the ground state or indirectly by using frozen media to inhibit relaxation. Density functional theory (DFT) and time-dependent DFT were undertaken on a cyanostar analogue, CSH, to examine the triplet state. The triplet excitation localizes on a single olefin whether in the single cyanostar or its π-stacked dimer. Restriction of the geometrical changes by forming either a dimer of macrocycles, (CSH)2, or a complex, (CSH)2·PF6-, reduces the relaxation resulting in an adiabatic energy of the triplet state of 2.0 eV. This structural constraint is also expected for solid-state SMILES materials. The obtained T1 energy of 2.0 eV is a key guide line for the design of SMILES materials for the manipulation of triplet excitons by triplet state engineering in the future.

Antibody Responses to Severe Acute Respiratory Syndrome Coronavirus 2 in the Serum and Cerebrospinal Fluid of Patients With Coronavirus Disease 2019 and Neurological Symptoms.

Antibody responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in serum and cerebrospinal fluid (CSF) samples from 16 patients with coronavirus disease 2019 and neurological symptoms were assessed using 2 independent methods. Immunoglobulin G (IgG) specific for the virus spike protein was found in 81% of patients in serum and in 56% in CSF. SARS-CoV-2 IgG in CSF was observed in 2 patients with negative serological findings. Levels of IgG in both serum and CSF were associated with disease severity (P < .05). All patients with elevated markers of central nervous system damage in CSF also had CSF antibodies (P = .002), and CSF antibodies had the highest predictive value for neuronal damage markers of all tested clinical variables.

Approaching the Spin-Statistical Limit in Visible-to-Ultraviolet Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process in which triplet excitons combine to form emissive singlets and holds great promise in biological applications and for improving the spectral match in solar energy conversion. While high TTA-UC quantum yields have been reported for, for example, red-to-green TTA-UC systems, there are only a few examples of visible-to-ultraviolet (UV) transformations in which the quantum yield reaches 10%. In this study, we investigate the performance of six annihilators when paired with the sensitizer 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), a purely organic compound that exhibits thermally activated delayed fluorescence. We report a record-setting internal TTA-UC quantum yield (ΦUC,g) of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene, demonstrating the first example of a visible-to-UV TTA-UC system approaching the classical spin-statistical limit of 20%. Three other annihilators, of which 2,5-diphenylfuran has never been used for TTA-UC previously, also showed impressive performances with ΦUC,g above 12%. In addition, a new method to determine the rate constant of TTA is proposed, in which only time-resolved emission measurements are needed, circumventing the need for more challenging transient absorption measurements. The results reported herein represent an important step toward highly efficient visible-to-UV TTA-UC systems that hold great potential for driving high-energy photochemical reactions.

Optically Switchable NIR Photoluminescence of PbS Semiconducting Nanocrystals using Diarylethene Photoswitches.

Precisely modulated photoluminescence (PL) with external control is highly demanded in material and biological sciences. However, it is challenging to switch the PL on and off in the NIR region with a high modulation contrast. Here, we demonstrate that reversible on and off switching of the PL in the NIR region can be achieved in a bicomponent system comprised of PbS semiconducting nanocrystals (NCs) and diarylethene (DAE) photoswitches. Photoisomerization of DAE to the ring-closed form upon UV light irradiation causes substantial quenching of the NIR PL of PbS NCs due to efficient triplet energy transfer. The NIR PL fully recovers to an on state upon reversing the photoisomerization of DAE to the ring-open form with green light irradiation. Importantly, fully reversible switching occurs without obvious fatigue, and the high PL on/off ratio (>100) outperforms all previously reported assemblies of NCs and photoswitches.

Best practice in determining key photophysical parameters in triplet-triplet annihilation photon upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process in which low-energy light is transformed into light of higher energy. During the last two decades, it has gained increasing attention due to its potential in, e.g., biological applications and solar energy conversion. The highest efficiencies for TTA-UC systems have been achieved in liquid solution, owing to that several of the intermediate steps require close contact between the interacting species, something that is more easily achieved in diffusion-controlled environments. There is a good understanding of the kinetics dictating the performance in liquid TTA-UC systems, but so far, the community lacks cohesiveness in terms of how several important parameters are best determined experimentally. In this perspective, we discuss and present a "best practice" for the determination of several critical parameters in TTA-UC, namely triplet excited state energies, rate constants for triplet-triplet annihilation ([Formula: see text]), triplet excited-state lifetimes ([Formula: see text]), and excitation threshold intensity ([Formula: see text]). Finally, we introduce a newly developed method by which [Formula: see text], [Formula: see text], and [Formula: see text] may be determined simultaneously using the same set of time-resolved emission measurements. The experiment can be performed with a simple experimental setup, be ran under mild excitation conditions, and entirely circumvents the need for more challenging nanosecond transient absorption measurements, a technique that previously has been required to extract [Formula: see text]. Our hope is that the discussions and methodologies presented herein will aid the photon upconversion community in performing more efficient and manageable experiments while maintaining-and sometimes increasing-the accuracy and validity of the extracted parameters.

Diphenylanthracene Dimers for Triplet-Triplet Annihilation Photon Upconversion: Mechanistic Insights for Intramolecular Pathways and the Importance of Molecular Geometry.

Novel approaches to modify the spectral output of the sun have seen a surge in interest recently, with triplet-triplet annihilation driven photon upconversion (TTA-UC) gaining widespread recognition due to its ability to function under low-intensity, noncoherent light. Herein, four diphenylanthracene (DPA) dimers are investigated to explore how the structure of these dimers affects upconversion efficiency. Also, the mechanism responsible for intramolecular upconversion is elucidated. In particular, two models are compared using steady-state and time-resolved simulations of the TTA-UC emission intensities and kinetics. All dimers perform TTA-UC efficiently in the presence of the sensitizer platinum octaethylporphyrin. The meta-coupled dimer 1,3-DPA2 performs best yielding a 21.2% upconversion quantum yield (out of a 50% maximum), which is close to that of the reference monomer DPA (24.0%). Its superior performance compared to the other dimers is primarily ascribed to the longer triplet lifetime of this dimer (4.7 ms), thus reinforcing the importance of this parameter. Comparisons between simulations and experiments reveal that the double-sensitization mechanism is part of the mechanism of intramolecular upconversion and that this additional pathway could be of great significance under specific conditions. The results from this study can thus act as a guide not only in terms of annihilator design but also for the design of future solid-state systems where intramolecular exciton migration is anticipated to play a major role.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes.

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm-1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Humoral and cellular response to SARS-CoV-2 BNT162b2 mRNA vaccine in hemodialysis patients.

BACKGROUND: Hemodialysis (HD) patients have an increased risk of acquiring infections due to many health care contacts and may, in addition, have a suboptimal response to vaccination and a high mortality from Covid-19 infection. METHODS: In 50 HD patients (mean age 69.4 years, 62% men) administration of SARS-CoV-2BNT162b2 mRNA vaccine began in Dec 2020 and the immune response was evaluated 7-15 weeks after the last dose. Levels of Covid-19 (SARS-CoV-2) IgG antibody against the nucleocapsid antigen (anti-N) and the Spike antigen (anti-S) and T-cell reactivity testing against the Spike protein using ELISPOT technology were evaluated. RESULTS: Out of 50 patients, anti-S IgG antibodies indicating a vaccine effect or previous Covid-19 infection, were detected in 37 (74%), 5 (10%) had a borderline response and 8 (16%) were negative after two doses of vaccine. T-cell responses were detected in 29 (58%). Of the 37 patients with anti-S antibodies, 25 (68%) had a measurable T-cell response. 2 (40%) out of 5 patients with borderline anti-S and 2 (25%) without anti-S had a concomitant T-cell response. Twenty-seven (54%) had both an antibody and T-cell response. IgG antibodies to anti-N indicating a previous Covid-19 disease were detected in 7 (14%) patients. CONCLUSIONS: Most HD patients develop a B- and/or T-cell response after vaccination against Covid-19 but approx. 20% had a limited immunological response. T-cell reactivity against Covid-19 was only present in a few of the anti-S antibody negative patients.

Multi-laboratory evaluation of ReaScan TBE IgM rapid test, 2016 to 2017.

BackgroundTick-borne encephalitis (TBE) is a potentially severe neurological disease caused by TBE virus (TBEV). In Europe and Asia, TBEV infection has become a growing public health concern and requires fast and specific detection.AimIn this observational study, we evaluated a rapid TBE IgM test, ReaScan TBE, for usage in a clinical laboratory setting.MethodsPatient sera found negative or positive for TBEV by serological and/or molecular methods in diagnostic laboratories of five European countries endemic for TBEV (Estonia, Finland, Slovenia, the Netherlands and Sweden) were used to assess the sensitivity and specificity of the test. The patients' diagnoses were based on other commercial or quality assured in-house assays, i.e. each laboratory's conventional routine methods. For specificity analysis, serum samples from patients with infections known to cause problems in serology were employed. These samples tested positive for e.g. Epstein-Barr virus, cytomegalovirus and Anaplasma phagocytophilum, or for flaviviruses other than TBEV, i.e. dengue, Japanese encephalitis, West Nile and Zika viruses. Samples from individuals vaccinated against flaviviruses other than TBEV were also included. Altogether, 172 serum samples from patients with acute TBE and 306 TBE IgM negative samples were analysed.ResultsCompared with each laboratory's conventional methods, the tested assay had similar sensitivity and specificity (99.4% and 97.7%, respectively). Samples containing potentially interfering antibodies did not cause specificity problems.ConclusionRegarding diagnosis of acute TBEV infections, ReaScan TBE offers rapid and convenient complementary IgM detection. If used as a stand-alone, it can provide preliminary results in a laboratory or point of care setting.

Electron transfer reactions in sub-porphyrin-naphthyldiimide dyads.

A series of donor-acceptor compounds based on a sub-porphyrin (SubP) as an electron donor and naphthyldiimide (NDI) as an acceptor has been designed, synthesized and investigated by time-resolved emission and transient absorption measurements. The donor and acceptor are separated by a single phenyl spacer substituted by methyl groups in order to systematically vary the electronic coupling. The electron transfer reactions in toluene are found to be quite fast; charge separation is quantitative and occurs within 5-10 ps and charge recombination occurs in 1-10 ns, depending on the substitution pattern. As expected, when steric bulk is introduced on the adjoining phenyl group, electron transfer rates slow down because of smaller electronic coupling. Quantum mechanical modelling of the potential energy for twisting the dihedral angles combined with a simplified model of the electronic coupling semi-quantitatively explains the observed variation of the electron transfer rates. Investigating the temperature variation of the charge separation in 2-methyltetrahydrofuran (2-MTHF) and analyzing using the Marcus model allow experimental estimation of the electronic coupling and reorganization energies. At low temperature, relatively strong phosphorescence is observed from the donor-acceptor compounds with onset at 660 nm signaling that charge recombination occurs, at least partially, through the sub-porphyrin localized triplet excited state. Finally, it is noted that charge separation in all SubP-NDI dyads is efficient even at cryogenic temperatures (85 K) in 2-MTHF glass.

Singlet and triplet energy transfer dynamics in self-assembled axial porphyrin-anthracene complexes: towards supra-molecular structures for photon upconversion.

Energy and electron transfer reactions are central to many different processes and research fields, from photosynthesis and solar energy harvesting to biological and medical applications. Herein we report a comprehensive study of the singlet and triplet energy transfer dynamics in porphyrin-anthracene coordination complexes. Seven newly synthesized pyridine functionalized anthracene ligands, five with various bridge lengths and two dendrimer structures containing three and seven anthracene units, were prepared. We found that triplet energy transfer from ruthenium octaethylporphyrin to an axially coordinated anthracene is possible, and is in some cases followed by back triplet energy transfer to the porphyrin. The triplet energy transfer follows an exponential distance dependence with an attenuation factor, ß, of 0.64 Å-1. Further, singlet energy transfer from anthracene to the ruthenium porphyrin appears to follow a R6 Förster distance dependence. Porphyrin-anthracene complexes are also used as triplet sensitizers for triplet-triplet annihilation (TTA) based photon upconversion, demonstrating their potential for photophysical and photochemical applications. The triplet lifetime of the complex is extended by the anthracene ligands, resulting in a threefold increase in the upconversion efficiency, ΦUC to 4.5%, compared to the corresponding ruthenium porphyrin-pyridine complex. Based on the results herein we discuss the future design of supra-molecular structures for TTA upconversion.

Turn-off mode fluorescent norbornadiene-based photoswitches.

Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes have been exploited for biological imaging and optical memory storage applications. However, there is a lack of understanding if negative photochromic systems can be used for such type of applications. Hence, to explore the potential of negative photochromic molecules for possible optical memory storage applications, we have here synthesized and studied a series of four norbornadiene-quadricyclane (NBD-QC) photoswitching molecules. These molecules feature either linearly conjugated or cross-conjugated pi-electron systems. Upon photoisomerization, the UV-vis absorption spectra of the molecules revealed a strong blue shift in the QC-form, with a photoisomerization quantum yield close to 80% for the cross-conjugated systems. In contrast, a strong intrinsic emission (up to Φf = 49%) for the linearly conjugated compounds in the NBD form was observed. Upon light-induced isomerization, the emission was completely turned off in the QC-form in all the compounds studied. Further, the robustness of the system was evaluated by performing several switching cycles. Under nitrogen, the emission can be turned off and recovered with almost no loss of emission. We also show that the QC-form can be photochemically triggered to convert back to the NBD-form using a low energy UV light (340 nm), allowing an all optical conversion to both species. The demonstrated properties can make the NBD-QC system attractive for potential applications such as optical memory storage devices.

Distinction between serological responses following tick-borne encephalitis virus (TBEV) infection vs vaccination, Sweden 2017.

Tick-borne encephalitis virus (TBEV) is an important European vaccine-preventable pathogen. Discrimination of vaccine-induced antibodies from those elicited by infection is important. We studied anti-TBEV IgM/IgG responses, including avidity and neutralisation, by multiplex serology in 50 TBEV patients and 50 TBEV vaccinees. Infection induced antibodies reactive to both whole virus (WV) and non-structural protein 1 (NS1) in 48 clinical cases, whereas 47 TBEV vaccinees had WV, but not NS1 antibodies, enabling efficient discrimination of infection/vaccination.

Robust triplet-triplet annihilation photon upconversion by efficient oxygen scavenging.

We hereby present a simple method for reducing the effect of oxygen quenching in Triplet-Triplet Annihilation Upconversion (TTA-UC) systems. A number of commercially available thioethers and one thiol have been tested as singlet oxygen scavengers. Recording of the upconverted emission from a well-studied PdOEP (sensitizer)-DPA (annihilator/emitter) couple has been made over time with steady-state excitation capturing the steady-state kinetics of the TTA-UC process as the solubilized oxygen is depleted by reaction with the scavengers. The efficiency of the TTA-UC process is compared between chemical oxygen scavenging and mechanical removal by inert gas purging or the freeze-pump-thaw method. Selected methods are combined to explore the highest attainable TTA-UC quantum yield. A maximum TTA-UC quantum yield of 21% with the shortest UC onset time was obtained with dimethylthiomethane (DMTM) as the scavenger in an air-saturated solvent and slightly higher quantum yields were obtained in combination with other deoxygenation techniques. Samples containing DMTM displayed little decrease in the quantum yield over four hours of continuous high intensity irradiation, which illustrates the robustness of applying chemical oxygen removal in TTA-UC instead of more time-consuming mechanical processes that usually require specialized equipment.

Loss channels in triplet-triplet annihilation photon upconversion: importance of annihilator singlet and triplet surface shapes.

Triplet-triplet annihilation photon upconversion (TTA-UC) can, through a number of energy transfer processes, efficiently combine two low frequency photons into one photon of higher frequency. TTA-UC systems consist of one absorbing species (the sensitizer) and one emitting species (the annihilator). Herein, we show that the structurally similar annihilators, 9,10-diphenylanthracene (DPA, 1), 9-(4-phenylethynyl)-10-phenylanthracene (2) and 9,10-bis(phenylethynyl)anthracene (BPEA, 3) have very different upconversion efficiencies, 15.2 ± 2.8%, 15.9 ± 1.3% and 1.6 ± 0.8%, respectively (of a maximum of 50%). We show that these results can be understood in terms of a loss channel, previously unaccounted for, originating from the difference between the BPEA singlet and triplet surface shapes. The difference between the two surfaces results in a fraction of the triplet state population having geometries not energetically capable of forming the first singlet excited state. This is supported by TD-DFT calculations of the annihilator excited state surfaces as a function of phenyl group rotation. We thereby highlight that the commonly used "spin-statistical factor" should be used with caution when explaining TTA-efficiencies. Furthermore, we show that the precious metal free zinc octaethylporphyrin (ZnOEP) can be used for efficient sensitization and that the upconversion quantum yield is maximized when sensitizer-annihilator spectral overlap is minimized (ZnOEP with 2).