Tiago Palmeira

The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

Dinuclear Zinc(II) Macrocyclic Complex as Receptor for Selective Fluorescence Sensing of Pyrophosphate

A new diethylenetriamine-derived macrocycle known as L, bearing 2-methylquinoline arms and containing m-xylyl spacers, was prepared in good yield by a one-pot [2 + 2] Schiff base condensation procedure, followed by reduction with sodium borohydride. Up to now this is the first hexaazamacrocycle with appended fluorophore units. Single-crystal X-ray diffraction determination of the dinuclear zinc(II) complex of L showed that metal centers are located at about 7.20(2) Å from one another. This complex exhibits only weak fluorescence in aqueous solution, but the addition of 1 equiv of pyrophosphate (PPi) caused a 21-fold enhancement of the fluorescence intensity. The sensor response is linear up to a value of 10 μM HPPi(3-) and has a detection limit of 300 nM. The receptor behaves as a highly selective sensor for pyrophosphate as other anions, including phosphate, phenylphosphate (PhP), adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP), failed to induce any fluorescence change and practically do not affect the fluorescence intensity of the sensor in the presence of HPPi(3-). Competition titrations carried out in aqueous solution at pH 7.4 [in 20 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer] by spectrofluorometry revealed a high association constant value of 6.22 log units for binding of PPi by the dinuclear zinc(II) receptor, one of the highest reported values for colorimetric/fluorometric sensors able to work under real aqueous physiological conditions, while association constant values for binding of the other phosphorylated substrates are in the 5.51-4.03 log unit range.

Temperature dependence of the phosphorescence and of the thermally activated delayed fluorescence of 12C70 and 13C70 in amorphous polymer matrices. Is a second triplet involved

The phosphorescence and thermally activated delayed fluorescence (TADF) lifetimes of 12C70 and 13C70 in two different glassy hydrocarbon polymers, one aliphatic (cyclic polyolefin) and one aromatic (polystyrene), were measured between -200 and 100 ºC. The temperature dependence of the lifetimes is equally well described by a three-state mechanism (ground state, S0, and two excited states in thermal equilibrium, T1 and S1, the lifetime of T1 being temperature dependent) and by a four-state mechanism (ground state, S0, and three excited states in thermal equilibrium, T1, T2 and S1, all with temperature-independent lifetimes). The estimated S1-T1 and T2-T1 energy gaps (four-state mechanism) are in good agreement with spectroscopic measurements. These and the determined quantum yield of triplet formation, 0.997 ± 0.001, are found to be essentially independent of the polymer matrix and of the isotopic composition of the fullerene. On the other hand, the lifetimes of both T1 and T2 (four-state mechanism) are weakly dependent on the polymer matrix but strongly vary with the fullerene isotopic composition, nearly doubling when going from 12C70 to 13C70. A parameter useful for the characterization of TADF, the on-set temperature T0, is also introduced.

Distinctive characteristics of the decay function for phosphorescence in the presence of reabsorption

The phosphorescence decay under the effect of significant excited-state absorption has a distinctive signature: it is initially concave, switching to convex (pure exponential decay) after a certain time. A simple one-parameter decay function satisfactorily reproducing the experimental decays is discussed, and some of its peculiar mathematical properties analyzed.

Influence of Excited‐State Absorption on Time‐Resolved Luminescence: General Formalism and Application to the Phosphorescence of Polycyclic Aromatic Hydrocarbons

The luminescence decay of a species in an absorbing medium whose optical thickness changes with time, as occurs with triplet-triplet absorption following excitation cut-off, is studied theoretically and experimentally. A general luminescence decay function based on a distribution of optical thicknesses is presented. A simple decay function previously used empirically is shown to result from an exponential distribution of optical thicknesses. The general approach introduced allows the adequate description of the phosphorescence decays of two polycyclic aromatic hydrocarbons, coronene and triphenylene (normal and perdeuterated forms for both molecules), in polymer films in the presence of excited-state absorption.

Calixarenes as High Temperature Matrices for Thermally Activated Delayed Fluorescence: C70 in Dihomooxacalix[4]arene

Thermally activated delayed fluorescence (TADF) of 12C70 and 13C70 was observed up to 140 °C in a p-tert-butyldihomooxacalix[4]arene solid matrix, a temperature range significantly higher than that of previous TADF quantitative studies. An effective singlet–triplet energy gap of 29 kJ/mol and triplet formation quantum yields of 0.97 and 0.99 were measured for 12C70 and 13C70, respectively. The photophysical properties of the two fullerenes in this new matrix are comparable to those obtained in polystyrene at a lower temperature range. Calixarenes are proposed to be suitable matrices for high temperature TADF studies and applications.

Resonance energy transfer (RET) with excited-state acceptors (Conference Presentation)

Resonance energy transfer (RET), both radiative and nonradiative, is a well-known process in the molecular fluorescence field. On the other hand, RET has been much less explored in connection with phosphorescent systems. We have recently studied in detail phosphorescence reabsorption (radiative transfer) of polycyclic aromatic hydrocarbons in the presence of excited-state absorption. In this case, reabsorption of phosphorescence results from triplet-triplet (T-T) absorption overlapping the emission spectrum, i.e. Tn ← T1 radiative transitions, and not from absorption by ground state molecules, owing to the forbidden nature of the Tn←S0 radiative transitions. In this way, all absorbing molecules are already in the T1 state, and are generated in the first place by the external source, whose intensity and spatial distribution completely determines the set of allowed sites for excitation diffusion. The excitation moves from one triplet, that returns to the ground state, to another, that undergoes the fast sequence T1→Tn→T1 and thus remains unchanged, hence the elementary process for radiative transfer is T1 + T1 → S0 + T1. The process may take place a number of times. In this mechanism there is no radiation imprisonment, only a peculiar type of inner filter effect, as the emitted photon is lost and does not reach the detector. Unlike the fluorescence reabsorption process, phosphorescence reabsorption depends significantly on the excitation intensity, which determines the number and spatial distribution of triplets. Furthermore, the reabsorption probability is time-dependent, as the T-T absorption contribution to the optical thickness of the medium continuously decreases with time, after excitation cut-off: For sufficiently long times, the phosphorescence absorption probability is negligible, and the decay becomes exponential. However, for shorter times the decay has a distinctive form, displaying an initial concavity when reabsorption is significant [3,4]. For sufficiently high concentrations (typically around 0.01 M), nonradiative transfer of the type T1 + T1 → S0 + Tn (the final triplet state after relaxation being T1) by the dipole-dipole mechanism can also occur. Here, the phosphorescence decay is affected owing to an increase of the nonradiative decay rate. This process is sometimes called long-range triplet-triplet annihilation. For even higher concentrations (typically around 0.1 M) a significant fraction of molecules is so close that the exchange interaction is now operative. In this way, for high excitation intensities short-range triplet-triplet annihilation, T1 + T1 → S0 + S1, eventually preceded by triplet energy migration, T1 + S0 → S0 + T1, comes into play. The phosphorescence decay is again affected and delayed fluorescence may be observed. In this work, we discuss radiative and nonradiative transfer of energy owing to triplet-triplet absorption, including the effect of dimensionality. [1] D. L. Andrews and A. A. Demidov eds., Resonance Energy Transfer, Wiley, Chichester, 1999. [2] B. Valeur and M. N. Berberan-Santos, Molecular Fluorescence. Principles and Applications, Wiley-VCH, Weinheim, 2nd ed., 2012. [3] T. Palmeira, M. N. Berberan-Santos, J. Lumin. 158 (2015) 510-518. [4] T. Palmeira, A. Fedorov, M. N. Berberan-Santos, ChemPhysChem 16 (2015) 640-648.