Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non-radiative Decay Pathway

To broaden the application of aggregation-induced emission (AIE) luminogens (AIEgens), the design of novel small-molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non-radiative decay pathways, a series of bridged stilbenes was designed, and their non-radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.     

Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S₁(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S₁(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S₁(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [¹ππ*→¹nπ*→³ππ*→S₀] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate.     

Assessing solvent effects on the singlet excited state lifetime of uracil derivatives: A femtosecond fluorescence upconversion study in alcohols and D2O

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ^* and the dark nπ^* states. This relative energy gap controls the non-radiative relaxation of the ππ^* state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D₂O where the decays are faster than in H₂O.     

Shedding new light on the role of the Rydberg state in the photochemistry of aniline

Efficient electronic relaxation following the absorption of ultraviolet light is crucial for the photostability of biological chromophores, so understanding the microscopic details of the decay pathways is of considerable interest. Here, we employ femtosecond time-resolved photoelectron imaging to investigate the ultrafast intramolecular dynamics of aniline, a prototypical aromatic amine, following excitation just below the second absorption maximum. We find that both the second ππ* state and the Rydberg state are populated during the excitation process. Surprisingly, the dominant non-radiative decay pathway is an ultrafast relaxation mechanism that transfers population straight back to the electronic ground-state. The vibrational energy resolution and photoelectron angular distributions obtained in our experiments reveal an interesting bifurcation of the Rydberg population to two non-radiative decay channels. The existence of these competing non-radiative relaxation channels in aniline illustrates how its photostability arises from a subtle balance between dynamics on different electronically excited states and importantly between Rydberg and valence states.     

The dynamics of adiabatic photoreactions as studied by means of the time structure of synchrotron radiation

The decay of the dual fluorescence of the para-substituted dialkylanilines (esters and nitriles) as well as the fluorescence decay of several triphenylmethane (TPM) dyes is investigated. The esters relax several times faster than the nitriles, and in the TPM series, relaxation is accelerated by increasing the substituent acceptor strength. In both cases, non-exponential decay behaviour is observed. It is more pronounced for the compounds with stronger driving forces along the reaction pathways. Freezing of the relaxation process transforms the decay into a mono-exponential one. This is discussed in terms of two models, namely by a log-gaussian distribution of relaxation times, and by a non-radiative sink model.     

Folding and fluorescence enhancement with strong odd–even effect for a series of merocyanine dye oligomers

A series of merocyanine (MC) oligomers with a varying number of chromophores from two to six has been synthesized via a peptide synthesis strategy. Solvent-dependent UV/vis spectroscopic studies reveal folding processes for the MC oligomers driven by strong dipole–dipole interactions resulting in well-defined π-stacks with antiparallel orientation of the dyes. Whilst even-numbered tetramer 4 and hexamer 6 only show partial folding into dimeric units, odd-numbered trimer 3 and pentamer 5 fold into π-stacks of three and five MC units upon decreasing solvent polarity. In-depth 2D NMR studies provided insight into the supramolecular structure. For trimer 3, an NMR structure could be generated revealing the presence of a well-defined triple π-stack in the folded state. Concomitant with folding, the fluorescence quantum yield is increased for all MC oligomers in comparison to the single chromophore. Based on radiative and non-radiative decay rates, this fluorescence enhancement can be attributed to the rigidification of the chromophores within the π-stacks that affords a pronounced decrease of the non-radiative decay rates. Theoretical investigations for the double and triple dye stacks based on time-dependent density functional theory (TD-DFT) calculations indicate for trimer 3 a pronounced mixing of Frenkel and charge transfer (CT) states. This leads to significant deviations from the predictions obtained by the molecular exciton theory which only accounts for the Coulomb interaction between the transition dipole moments of the chromophores.

A series of merocyanine (MC) oligomers with a varying number of chromophores from two to six has been synthesized via a peptide synthesis strategy.     

Femto- and picosecond fluorescence studies of solvation and non-radiative deactivation of ionic styryl dyes in liquid solution

A comparative study of the solvation and non-radiative relaxation dynamics of bridged and unbridged ionic styryl dye compounds is reported. Femto- and picosecond fluorescence transient measurements reveal solvation dynamics on a picosecond timescale of the solutes in ethanol, benzonitrile and decanol. Bridging is found to strongly affect the lifetime of the emissive state. It is shown that the presence of unbridged styryl-group single bonds allows for an effective non-radiative decay process. This decay process is suppressed by double chemical bridging.     

Photophysical processes in dimers of thiacarbocyanine cations

The results of quantum-chemical calculations of absorption, fluorescence, and induced T-T absorption spectra and the rate constants of photophysical processes for the monomer and dimers of the 3,3′-diethylthiacarbocyanine cation are presented. It has been shown that the formation of the dimers leads to a substantial reduction in the efficiency of the radiative pathways and enhancement of the efficiency of the non-radiative pathways of deactivation of the fluorescent state, thereby resulting in a decrease in the quantum yield of fluorescence. The fluorescence quantum yield depends in this case on the relative orientation of the molecules in the dimer with respect to each other. The calculation results satisfactorily describe experimental data on the spectral and luminescent properties of the dimers of the 3,3′-diethylthiacarbocyanine cation and their photophysical processes.     

Dual excited state deactivation pathways in TPZ2: A centrosymmetric dye with both high fluorescence and triplet state quantum yield

Dual excited state deactivation pathways in TPZ2 leading to 50% fluorescence quantum yield and 50% triplet state generation yield, suggest TPZ2 is a molecule has potential application in fluorescence imaging and photodynamic therapy in the same time.     

Solvent effect on the non-radiative S2(ππ*) decay of xanthione

The fluorescence quantum yield of the S₂(ππ*) state of xanthione has been measured in various solvents at room temperature. With these data, the strong solvent effect on the subnanosecond, non-radiative S₂ decay has been assessed.     

Hybridized local and charge-transfer excited state fluorophores enabling organic light-emitting diodes with record high efficiencies close to 20%

Pure organic emitters with full utilization of triplet excitons are in high demand for organic light-emitting diodes (OLEDs). Herein, through modulation of electron donors and introduction of phenyl rings as π spacers, we present three pure organic fluorophores (BCz, BTCz and BPTCz) with the hybridized local and charge-transfer (HLCT) excited state feature for OLED fabrication. Importantly, the introduction of π spacers in BPTCz not only enhances locally excited character with a fast radiative decay but also promotes intermolecular interactions to suppress non-radiative decays, contributing to a high solid-state fluorescence efficiency over 90%. Significantly, BPTCz not only endows its doped OLEDs with an external quantum efficiency (EQE) up to 19.5%, but also its non-doped OLED with a high EQE of 17.8%, and these outstanding efficiencies are the state-of-the-art performances of HLCT-based OLEDs.

Three purely organic fluorophores with a hybridized local and charge-transfer excited state feature are presented and enabled organic light-emitting diodes with record high external quantum efficiencies close to 20%.     

Photophysics, photochemistry and photobiology of curcumin: Studies from organic solutions, bio-mimetics and living cells

Curcumin, with its recent success as an anti-tumor agent, has been attracting researchers from wide ranging fields of physics, chemistry, biology and medicine. The chemical structure of curcumin has two o-methoxy phenols attached symmetrically through α,β-unsaturated β-diketone linker, which also induces keto–enol tautomerism. Due to this, curcumin exhibits many interesting photophysical and photochemical properties. The absorption maximum of curcumin is 408–430 nm in most of the organic solvents, while the emission maximum is very sensitive to the surrounding solvent medium (460–560 nm) and the Stokes’ shift varied from 2000 to 6000 cm⁻¹. The fluorescence quantum yield in most of the solvents is low and reduced significantly in presence of water. The fluorescence lifetime is short (<1 ns) and displayed multi-exponential decay profile. The singlet excited states of curcumin decay by non-radiative processes contributed mainly by intra- and intermolecular proton transfer with very low intersystem crossing efficiency. Polarity, π-bonding nature, hydrogen bond donating and accepting properties of the solvent influence the excited state photophysics of curcumin in a complex manner. The triplet excited states of curcumin absorb at 720 nm and react with oxygen to produce singlet molecular oxygen. The photodegradation of curcumin produces smaller phenols and the photobiological activity of curcumin is due to the generation of reactive oxygen species.Being lipophilic in nature, the water solubility of curcumin could be enhanced upon the addition of surfactants, polymers, cyclodextrins, lipids and proteins. Changes in the absorption and fluorescence properties of curcumin have been found useful to follow its interaction and site of binding in these systems. Curcumin fluorescence could be employed to follow the unfolding pattern and structural changes in proteins. The intracellular curcumin showed more fluorescence in tumor cells than in normal cells and fluorescence spectroscopy could be used to monitor its preferential localization in the membrane of tumor cells. This review, presents the current status of research on the photophysical, photochemical and photobiological processes of curcumin in homogeneous solutions, bio-mimetics and living cells. Based on these studies, the possibility of developing curcumin, as a bimolecular sensitive fluorescent probe is also discussed.     

Effect of solvents on the photophysical properties of substituted imidazonaphthyridine derivatives

The absorption and fluorescence characteristics of three substituted imidazonaphthyridine derivatives are studied in a series of organic solvents. The variation of Stokes shift with the polarity parameter of the solvent is studied and the excited state dipole moment of these derivatives is higher than the ground state dipole moment. The fluorescence lifetime profile shows single exponential decay in all the solvents. The fluorescence quantum yield, radiative and non-radiative rate constants are also calculated and these parameters show much variation in the change in substitution of these derivatives.     

Dual Fluorescence and Photoprototropic Characteristics of 2-Aminodiphenylsulphone-β-Cyclodextrin Inclusion Complex

The absorption and fluorescence spectral characteristics of 2-aminodiphenylsulphone (2ADPS) have been investigated in the presence of β-cyclodextrin (β-CDx) in water. Dual emission is observed upon the complexation of 2ADPS in β-CDx. The stoichiometry of the host:guest inclusion complex is found to be 2:1. Steady state and time-resolved fluorescence spectral analysis support the formation of 2:1 complex between β-CDx and 2ADPS. The large enhancement in fluorescence intensity of twisted intramolecular charge transfer (TICT) band in aqueous β-CDx solution is due to the decrease in non-radiative processes. The ground and the excited state pK_a values for the monocation-neutral equilibrium of 2ADPS in β-CDx are found to be different from the pK_a values in aqueous solution. In the presence of β-CDx, 2ADPS is found to be less basic in the ground and the excited states.     

Time-resolved fluorescence study on the photomerocyanine form of spiropyran and its derivative with azobenzene

The excited state dynamics of the photomerocyanine (PMC) form originating from spirobenzopyran and the bi-functional photochromic compound spirobenzopyran–azobenzene (SpAz), containing typical photochromic molecules of spirobenzopyran (Sp) and azobenzene (Az), were investigated using picosecond time-resolved fluorescence measurements in solution at 200 and 285 K and in a PMMA polymer film at 298 K. While the fluorescence lifetimes of PMC were about twice as long as those of SpAz under all experimental conditions, both lifetimes showed similar strong dependence on viscosity rather than temperature. These results suggest that non-radiative decay to an intermediate state could be accompanied by a significant conformational change. The effect of the Az moiety in this relaxation process is also discussed. It is unlikely that an energy transfer from the PMC moiety to the Az moiety occurs. It was concluded that the PMC moiety in the bi-functional SpAz is independent from the Az moiety.     

Fluorescence lifetime properties of near-infrared cyanine dyes in relation to their structures

Structurally diverse near-infrared (NIR) absorbing polymethine dyes were prepared and their fluorescence lifetimes (FLT) were evaluated in relation to their structural features. Comparative FLT analysis based on the modification of methine chain length and heterocyclic system showed that indolium or benzo[e]indolium heptamethine dyes exhibited longer FLT than the benzo[c,d]indolium trimethine dye. Modification of heterocyclic system alone with an intact chain length showed that indolium-based heptamethine dyes showed approximately 30% longer FLT than the benzo[e]indolium-based dyes. In general, the FLT of polymethine dyes increased from polar to non-polar solvents. In addition, correlation study between the theoretical and the experimental FLT for indocyanine green (ICG) suggests that the lack of structural rigidity for these cyanine dyes is primarily responsible for the loss of the excited state energy via non-radiative pathway.     

On the Origin of the Heterogeneous Emission from Pyrene Sequestered Within Tetramethylorthosilicate-Based Xerogels: A Decay-Associated Spectra and O2 Quenching Study

Steady-state and time-resolved fluorescence spectroscopy are used to determine the local microheterogeneity surrounding pyrene molecules sequestered within tetramethylorthosilicate-derived xerogels. After compensation for the intrinsic background emission from the xerogel, we find that the pyrene intensity decay kinetics are best described by a two-term rate law. This is consistent with the pyrene molecules distributing primarily into two microenvironments. Under ambient conditions, the individual pyrene microenvironments exhibit excited-state fluorescence lifetimes that differ by 100 ns. However, the pyrene I₁ to I₃ band ratios that are associated with each microenvironment are statistically equivalent to one another. These results show that the local dipolarity surrounding these pyrene microenvironments are similar, but the decay rates associated with each microenvironment are very different. The longer-lived pyrene species (Environment #1) constitutes 1/2 of the total fluorescence and it exhibits an O₂ quenching sensitivity (K_sv1) of (5.19 ± 0.52 × 10^–3 %O₂ ^–1 and a bimolecular quenching constant (k_q1) of (2.30 ± 0.23) × 10⁴ %O₂ ^–1 s^–1. Environment #2, associated with the shorter-lived pyrene species, exhibits an O₂ quenching sensitivity (K_sv2) of (2.31 ± 0.16) × 10^–2 %O₂ ^–1 and a bimolecular quenching constant (k_q2) of (2.11 ± 0.23) × 10⁵ %O₂ ^–1 s^–1. These results are interpreted as follows: Environment #1 consists of pyrene molecules sequestered within a relatively rigid siloxane network wherein non-radiative decay pathways are lessened, but these pyrene molecules are not quenched readily by O₂. Environment #2 consists of pyrene molecules adsorbed onto surface silanols within the xerogel. These pyrene molecules are quenched by the silanols and they are simultaneously more accessible to O₂ compared to Environment #1.     

Non-radiative decay processes in isolated SO2 molecules excited within the first allowed absorption band (2500–3200 Å)

New and previously published SO₂ fluorescence emission data related to non-radiative decay processes are considered in light of the recent observations of Brus and McDonald. All the present data are consistent with the previous conclusion of Mettee that non-radiative processes in SO₂ singlet photochemistry are unimportant. It appears that any small inefficiency in the emission of light quanta for SO₂ excited at short wavelengths (2650 A) is largely due to the population of a second very short-lived state which is quenched effectively even at pressures down to 1 μ. The very low efficiency of quanta production which we observed at long wavelengths (3020 Å) appears to have its origin not only in the second easily quenched state, but more importantly, in the diffusional loss of the long-lived singlet which for these conditions has a 20-fold greated lifetime than was expected previously.     

Two-step anti-cooperative self-assembly process into defined π-stacked dye oligomers: insights into aggregation-induced enhanced emission

Aggregation-induced emission enhancement (AIEE) phenomena received great popularity during the last decade but in most cases insights into the packing structure – fluorescence properties remained scarce. Here, an almost non-fluorescent merocyanine dye was equipped with large solubilizing substituents, which allowed the investigation of it''s aggregation behaviour in unpolar solvents over a large concentration range (10⁻² to 10⁻⁷ M). In depth analysis of the self-assembly process by concentration-dependent UV/Vis spectroscopy at different temperatures revealed a two-step anti-cooperative aggregation mechanism. In the first step a co-facially stacked dimer is formed driven by dipole–dipole interactions. In a second step these dimers self-assemble to give an oligomer stack consisting of about ten dyes. Concentration- and temperature-dependent UV/Vis spectroscopy provided insight into the thermodynamic parameters and allowed to identify conditions where either the monomer, the dimer or the decamer prevails. The centrosymmetric dimer structure could be proven by 2D NMR spectroscopy. For the larger decamer atomic force microscopy (AFM), diffusion ordered spectroscopy (DOSY) and vapour pressure osmometric (VPO) measurements consistently indicated that it is of small and defined size. Fluorescence, circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy provided insights into the photofunctional properties of the dye aggregates. Starting from an essentially non-fluorescent monomer (Φ_Fl = 0.23%) a strong AIEE effect with excimer-type fluorescence (large Stokes shift, increased fluorescence lifetime) is observed upon formation of the dimer (Φ_Fl = 2.3%) and decamer (Φ_Fl = 4.5%) stack. This increase in fluorescence is accompanied for both aggregates by an aggregation-induced CPL enhancement with a strong increase of the g_lum from ∼0.001 for the dimer up to ∼0.011 for the higher aggregate. Analysis of the radiative and non-radiative decay rates corroborates the interpretation that the AIEE effect originates from a pronounced decrease of the non-radiative rate due to π–π-stacking induced rigidification that outmatches the effect of the reduced radiative rate that originates from the H-type exciton coupling in the co-facially stacked dyes.

The self-assembly of a dipolar merocyanine into preferred dimers and small-sized chiral aggregates leads to enhanced emission due to a reduced non-radiative rate as well as amplified circular polarized luminescence.