Delayed Fluorescence and Phosphorescence from Polyphenylquinoxalines

Diboraanthracene-Doped Polymer Systems for Colour-Tuneable Room-Temperature Organic Afterglow

Organic ultralong room temperature phosphorescence (RTP), or organic afterglow, is a unique phenomenon, gaining widespread attention due to its far-reaching application potential and fundamental interest. Here, two laterally expanded 9,10-dimesityl-dihydro-9,10-diboraanthracene (DBA) derivatives are demonstrated as excellent afterglow materials for red and blue-green light emission, which is traced back to persistent thermally activated delayed fluorescence and RTP. The lateral substitution of polycyclic DBA scaffold, together with weak transversal electron-donating mesityl groups, ensures the optimal molecular properties for (reverse) intersystem crossing and long-lived triplet states in a rigid poly(methyl methacrylate) matrix. The achieved afterglow emission quantum yields of up to 3 % and 15 %, afterglow lifetimes up to 0.8 s and 3.2 s and afterglow durations up to 5 s and 25 s (for red and blue-green emitters, respectively) are attributed to the properties of single molecules.

Identification of excimer delayed fluorescence by Protoporphyrin IX: A novel access to local chromophore concentration?

Protoporphyrin IX (PpIX) is a molecule produced in the mitochondria following the administration of its approved precursor, aminolevulinic acid (ALA). Strong light absorber at different wavelengths in the visible range, PpIX is extensively used as a photosensitizer (PS) for Photodynamic Therapy (PDT). PpIX is also an ideal molecular probe for the quantification of the tissue oxygen partial pressure (pO₂), as its delayed fluorescence (DF) is quenched by oxygen, creating a direct relationship between the DF lifetime and the pO₂. A limitation of both techniques is the ignorance of the PpIX concentration in tissues when the pO₂ is measured or during PDT. In this study, the prompt (PF) and delayed fluorescence of PpIX dissolved in DiMethylFormamide (DMF) were acquired, in absence of oxygen, at different PpIX concentrations. Measurements of the PpIX emission for different excitation energies and temperatures, as well as spectral considerations led to the conclusion that E-type (thermal) DF was the dominant DF mechanism at low PpIX excited states concentrations (density of absorbed energy Hε[PpIX] < 1 μJ. cm^-3, H:excitation radiant exposure per pulse, ε: molar extinction coefficient at excitation wavelength) while P-type (Triplet Triplet Annihilation) DF took place at higher excited states concentrations (Hε[PpIX] > 10 μJ. cm^-3). The gradual development of a strong, red-shifted structureless DF peak at 670 nm, invisible in the PF and absorption spectra, strongly points towards the first observation of PpIX excimer DF (EDF). It appears that, similarly to other aromatic molecules, PpIX excimers can be formed either by the encounter of two molecules in the first excited triplet state T₁, or by the reaction of an excited singlet S₁ with a triplet T₁. Excimer DF could be beneficially used to determine the local concentration of PpIX, as the initial DF intensity ratio I₀⁶⁷⁰/I₀⁶³⁰ is linearly correlated with the local PpIX concentration, and thus rises up to the challenge of PpIX based pO₂ measurement and PDT. This work could also pave the way for a fine comprehension of the production, diffusion and catabolization of PpIX in biological tissues.

The switchable phosphorescence and delayed fluorescence of a new rhodamine-like dye through allenylidene formation in a cyclometallated platinum(ii) system

A new rhodamine-like alkyne-substituted ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. The chemo-induced “ON–OFF” switching capabilities on the spirolactone ring of the Rhodyne ligand with an end-capping platinum(ii) metal centre can modulate the interesting acetylide–allenylidene resonance. The long-lived ³IL excited state of Rhodyne in its ON state as an optically active opened form was revealed via steady-state and time-resolved spectroscopy studies. Exceptional near-infrared (NIR) phosphorescence and delayed fluorescence based on a rhodamine-like structure were observed at room temperature for the first time. The position of the alkyne communication bridge attached to the platinum(ii) unit was found to vary the lead(ii)-ion binding mode and also the possible resonance structure for metal-mediated allenylidene formation. The formation of a proposed allenylidene resonance structure was suggested to rationalize these phenomena.

A new rhodamine-like ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. Allenylidene formation could trigger NIR phosphorescence at 740 nm originating from Rhodyne ³IL, as well as delayed fluorescence at 620 nm.

Understanding delayed fluorescence and triplet decays of Protoporphyrin IX under hypoxic conditions

Photosensitizers of singlet oxygen exhibit three main types of reverse intersystem-crossing (RISC): thermally activated, triplet-triplet annihilation, and singlet oxygen feedback. RISC can be followed by delayed fluorescence (DF) emission, which can provide important information about the excited state dynamics in the studied system. An excellent model example is a widely used clinical photosensitizer Protoporphyrin IX, which manifests all three mentioned types of RISC and DF. Here, we estimated rate constants of individual RISC and DF processes in Protoporphyrin IX in dimethylformamide, and we showed how these affect triplet decays and DF signals under diverse experimental conditions, such as a varying oxygen concentration or excitation intensity. This provided a basis for a general discussion on guidelines for a more precise analysis of long-lived signals. Furthermore, it has been found that PpIX photoproducts and potential transient excited complexes introduce a new overlapping delayed luminescence spectral band with a distinct lifetime. These findings are important for design of more accurate biological oxygen sensors and assays based on DF and triplet lifetime.

The interplay between fluorescence and phosphorescence with luminescent gold(i) and gold(iii) complexes bearing heterocyclic arylacetylide ligands

The photophysical properties of a series of gold(i) [LAu(C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CR)] (L = PCy₃ (1a-4a), RNC (5a), NHC (6a)) and gold(iii) complexes [Au(C^N^C)(CCR)] (1b-4b) bearing heterocyclic arylacetylide ligands with narrow band-gap are compared. The luminescence of both series are derived from an intraligand transition localized on the arylacetylide ligand (ππ*(CCR)) but 1a-3a displayed prompt fluorescence (τ_PF = 2.7-12.0 ns) while 1b-3b showed mainly phosphorescence (τ_Ph = 104-205 μs). The experimentally determined intersystem crossing (ISC) rate constants (k_ISC) are on the order of 10⁶ to 10⁸ s^-1 for the gold(i) series (1a-3a) but 10¹⁰ to 10¹¹ s^-1 for the gold(iii) analogues (1b-3b). DFT/TDDFT calculations have been performed to help understand the difference in the k_ISC between the two series of complexes. Owing to the different oxidation states of the gold ion, the Au(i) complexes have linear coordination geometry while the Au(iii) complexes are square planar. It was found from DFT/TDDFT calculations that due to this difference in coordination geometries, the energy gap between the singlet and triplet excited states (ΔE_ST) with effective spin-orbit coupling (SOC) for Au(i) systems is much larger than that for the Au(iii) counterparts, thus resulting in the poor ISC efficiency for the former. Time-resolved spectroscopies revealed a minor contribution (<2.9%) of a long-lived delayed fluorescence (DF) (τ_DF = 4.6-12.5 μs) to the total fluorescence in 1a-3a. Attempts have been made to elucidate the mechanism for the origins of the DF: the dependence of the DF intensity with the power of excitation light reveals that triplet-triplet annihilation (TTA) is the most probable mechanism for the DF of 1a while germinate electron-hole pair (GP) recombination accounts for the DF of 2a in 77 K glassy solution (MeOH/EtOH = 4 : 1). Both 4a and 4b contain a BODIPY moiety at the acetylide ligand and display only ¹IL(ππ*) fluorescence with negligible phosphorescence being observed. Computational analyses attributed this observation to the lack of low-lying triplet excited states that could have effective SOC with the S₁ excited state.

Graphene quantum dots from graphite by liquid exfoliation showing excitation-independent emission, fluorescence upconversion and delayed fluorescence

Schematic representation of GQD formation from graphite.

Synthesis and investigation of intra-molecular charge transfer state properties of novel donor-acceptor-donor pyridine derivatives: the effects of temperature and environment on molecular configurations and the origin of delayed fluorescence

A novel series of donor-acceptor-donor (D-A-D) structured pyridine derivatives were synthesised and detailed photo-physical investigations were made using mainly steady-state and time-resolved spectroscopy techniques at varying temperatures. The investigations showed that the molecules have solvent polarity and temperature dependent excited-state configurations, confirmed in two different polarity solvents (295-90 K), i.e. methyl cyclohexane (MCH) and 2-methyltetrahdrofurane (2-MeTHF). In MCH, the investigations revealed dual fluorescence over the temperature range of 295-90 K. At 295 K, the ground-state configuration of the molecules has a partially twisted geometry as determined by DFT calculation, yet the emission originates totally from a locally excited (LE) state, however once the temperature is lowered to 90 K, the twisted molecular configuration is stabilised, and the emission originates from a fully-relaxed intramolecular charge transfer state (ICT), this is contrary to the systems where structural reorganisation stabilises ICT and this is frozen out at low temperatures. The DFT calculations revealed different ground state molecular configurations due to the presence of different electron-donating groups, e.g. the molecule including anthracene groups has a near 90° twisted geometry whereas the triphenylamine including molecule has a pyramidal geometrical folding, therefore, the decrease in temperature restricts the donor degree of rotational freedom. In 2-MeTHF solution, the fluorescence spectrum of both molecules is always of ICT character, but gradually red-shifts through the fluid to glass transition temperature (∼135 K), in this case, the fluorescence occurs after structural and solvent-shell relaxations, however, upon cooling below 135 K, the spectra dramatically shift back to blue giving rise to strong emission from an ICT excited-state (but not the LE state) where the molecules have unrelaxed geometries. This significant change in the nature of the emitting species was explained with specific solvent-solute dynamic interactions in the vicinity of the solvation shell and the effect of thermal excitation of molecular vibrational modes of the C-C bond linking donor and acceptor units. Finally, we confirmed that the molecules have ICT ground-state geometry in the solid-state phase (spin-coated films), and the time-resolved decay dynamics were investigated comparing the spin-coated films (at RT and 25 K) and MCH solutions (at 295 K and 90 K).

Visible room-temperature phosphorescence of pure organic crystals via a radical-ion-pair mechanism

The afterglow of phosphorescent compounds can be distinguished from background fluorescence and scattered light by a time-resolved observation, which is a beneficial property for bioimaging. Phosphorescence emission accompanies spin-forbidden transitions from an excited singlet state through an excited triplet state to a ground singlet state. Since these intersystem crossings are facilitated usually by the heavy-atom effect, metal-free organic solids are seldom phosphorescent, although these solids have recently been refurbished as low-cost, eco-friendly phosphorescent materials. Here, we show that crystalline isophthalic acid exhibits room-temperature phosphorescence with an afterglow that lasts several seconds through a nuclear spin magnetism-assisted spin exchange of a radical ion pair. The obvious afterglow that facilitates a time-resolved detection and the unusual phosphorescence mechanism that enables emission intensification by nuclear spin managements are promising for exploiting the phosphorescence materials in novel applications such as bioimaging.

The key role of geminate electron-hole pair recombination in the delayed fluorescence in rhodamine 6G and ATTO-532

In this paper we investigate the delayed fluorescence (DF) phenomena in the widely used laser dye, rhodamine 6G, and its derivative ATTO-532 as a function of excitation energy using highly sensitive time-resolved gated nanosecond spectroscopy. Excitation with UV laser radiation results in delayed emission, which arises from singlet states created from geminate pair recombination, not triplet annihilation. For the first time the origins and photo-physical properties of delayed fluorescence in these highly fluorescent molecules are elucidated.

Direct Measurements of Intersystem Crossing Rates and Triplet Decays of Luminescent Conjugated Oligomers in Solutions

Photothermal calorimetry and fluorescence spectroscopy were used to determine the relaxations of the photoexcited singlet state of two PPV and polyfluorene oligomers, (E,E)-1,4-bis[(2-benzyloxy)styryl]benzene (PVDOP) and ter(9,9'-spirobifluorene) (TSBF). The decay rates of different S1 relaxation channels, which include intersystem crossing (ISC), radiative, and nonradiative decay can be determined by the combination of photoacoustic calorimetry (PAC) and the time-correlated single photon counting (TCSPC) technique. The triplet state energy level is determined by the phosphorescence (Ph) spectra recorded at 77 K. The ISC yields are approximately 3% and 6% for PVDOP and TSBF, respectively. The T1 to S0 transition decay rate is acquired by PAC and photothermal beam deflection (PBD) measurements. The triplet state decay rate is 17 and 21 ms(-1) at room temperature. The Ph intensity decay at 77 K shows that the triplet state lifetime increases by 4 orders of magnitude, as compared to room temperature.

What determines the mobility of charge carriers in conjugated polymers?

In a conjugated polymer, the mobility of charge carriers is not a well-defined coefficient of a particular material as it is in an inorganic crystalline semiconductor but depends on the time domain of detection. On a time-scale of typically 100 fs, the on-chain mobility is ultra-high and controlled by the electronic band width of the polymer chain. When a carrier hits a chain imperfection, subsequent mesoscopic on-chain motion is retarded and controlled by intrachain disorder to which the chain environment contributes. Macroscopic transport commences after a time when interchain carrier jumps become rate limiting. It is routinely probed by time-of-flight experiments and can be rationalized in terms of random walk within a rough energy landscape. Experimental signatures of the various modes of transport are discussed.

Phosphorescence and triplet state energies of oligothiophenes

The phosphorescence spectra of a series of small oligothiophenes (nT, n = 1-3) incorporating a variety of substituents, end cappers, and functional groups have been recorded for the first time using gated detection in combination with nanosecond excitation in frozen solution at 80 K. The vibrationally resolved emission spectra provide accurate estimates of the T(1) and S(1) levels, and the singlet-triplet energy gap. Theoretical quantum chemical calculations performed at the DFT (B3LYP/6-31G*) level reproduce all experimental trends accurately and provide quantitative description of the S(0)-T(1) energy difference. The geometry relaxation in the excited state shows that the "natural" size of the triplet exciton is about 3-4 thiophene units.

Exciplex−Exciplex Energy Transfer and Annihilation in Solid Films of Porphyrin−Fullerene Dyads

Exciplex-exciplex annihilation was observed for the first time in porphyrin-fullerene molecular films. The films were prepared using Langmuir-Blodgett and drop casting methods. The exciplex-exciplex interactions were studied using femtosecond pump-probe method. The exciplex-exciplex annihilation can be seen as a fast (within few picoseconds) decay of the transient absorption at excitation densities higher than 0.4 mJ/cm2. Analysis of the excitation density dependences indicates that in average four dyads are involved in the exciplex-exciplex interaction, suggesting that an exciplex-exciplex energy transfer may precede the annihilation.

Regarding the origin of the delayed fluorescence of conjugated polymers

In the first part of this work we revisit and reevaluate the experimental data that lead to the assignment of the origin of the delayed fluorescence (DF) to triplet-triplet annihilation for polyfluorene and to geminate pair recombination in the case of the ladder-type polyparaphenylene (MeLPPP); the ambiguity of this classification is unveiled. Next, new data about the DF of MeLPPP under applied electric field are presented. Here, the DF intensity completely recovers once the field is turned off, which rules out geminate pairs as the origin of the DF and in turn provides clear evidence of the triplet-triplet annihilation picture. Finally, we show and discuss how recombination of space charge layers may also give rise to electric field induced delayed fluorescence, whereby the formation of these space charge layers strongly depends on device configuration and purity of the materials.

Ultrafast intrachain photoexcitation of polymeric semiconductors

We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embedding the polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are detected by pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. We show that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain charge separation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to the lowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrational redistribution are also determined.