Intersystem-crossing Yields Research Articles

Switching Singlet Exciton to Triplet for Efficient Pure Organic Room-Temperature Phosphorescence by Rational Molecular Design.

The design and regulation of phosphors are attractive but challenging because of the spin-forbidden intersystem crossing (ISC) process. Here, a new perspective on the enhancement of the ISC is proposed and demonstrated. Different from current strategies, the ISC yield (ΦISC) is enhanced by decreasing the fluorescence radiative transition rate constant (kF) via rational molecular designing rather than boosting the spin-orbit coupling by decorating the molecular skeleton with a heavy atom, heteroatom, or carbonyl. The kF of the designed molecule in this case is associated with the substituent position of the methoxy group, which alters the distribution of the front orbitals. The S0 → S1 transition of these compounds evolves from a bright state to a dark state gradually with the variation of the substituent position, accompanied by the decrease of kF and increase of ΦISC. The fluorescence emission is switched to phosphorescence emission successfully by regulating the kF. This work provides an alternative strategy to design efficient room-temperature phosphorescence material.

Effect of molecular conformation on the efficiency of the spin orbital charge recombination-induced intersystem crossing in bianthryls

Two bianthryl dyads, with the two units connected at either the 2- or 9- positions of the anthryl moiety, were studied to establish the relationship between orientation of the anthryl moieties and the electronic coupling and intersystem crossing (ISC) efficiency. The anthryl moieties in the two dyads adopt a close-to-orthogonal geometry, with dihedral angles of 90° (9,9′-bianthryl) and 106° (2,9′-bianthryl) at the ground state, respectively. The charge transfer (CT) emission efficiency and the fluorescence lifetimes are clearly dependent on the electronic coupling between the two anthryls, and stronger coupling lead to higher fluorescence quantum yields (34% vs. 9%) and shorter luminescence lifetimes (13.9 ns vs. 38.6 ns). The bianthryl with more orthogonal geometry shows higher singlet oxygen quantum yields ΦΔ (9,9′-bianthryl, ΦΔ = 53%) than 2,9′-bianthryl (ΦΔ = 32%). Moreover, highly solvent polarity-dependent fluorescence emission and ΦΔ were observed for the dyads (ΦΔ = 22–53%), which is different from the trend of the monomer anthracene, thus we propose the spin-orbit charge transfer ISC (SOCT-ISC) is responsible for the triplet state productions of the dyads. Interestingly, we found that inducing a heavy atom (Br) does not increase the ISC yield of anthracene. ISC in bianthryls was also confirmed with nanosecond transient absorption spectroscopy, the featured T1→Tn absorption at ca. 433 nm was observed, and the triplet state lifetime are long (9,9′-bianthryl, τT = 353 μs; 2,9′-bianthryl, τT = 493 μs, in acetonitrile).

Photophysics of Oligothiophenes End‐Capped with Platinum(II) Auxochromes

AbstractA series of four oligothiophenes end‐capped with −Pt(PBu3)2Cl moieties on both ends of the oligomers was synthesized, and their excited state properties were investigated. The observation of low fluorescence quantum yield (<2 %) for the oligomers indicates the strong effect of platinum on the intersystem crossing (ISC) efficiency. No phosphorescence was detected for any of the oligomers; however, strong triplet‐triplet absorption was observed by nanosecond transient spectroscopy for oligomers with more than one thiophene unit. The oligomers displayed short triplet lifetimes (∼1–2 μs) compared to the unmetallated oligomers, due to large spin‐orbit coupling induced by the platinum atom. The lower limits of the ISC yields were indirectly determined by measuring the singlet oxygen quantum yields. Femtosecond–picosecond transient absorption studies revealed that the ISC rate ranges from 1012–1010 s−1, decreasing with increasing oligomer length. Electrochemical studies showed that the oligomers exhibit relatively low oxidation potentials (ca. 0.1 V vs. Fc/Fc+). Quenching of the oligomers’ triplet state absorption, simultaneously with the rise of their corresponding cationic radical absorption band in nanosecond transient spectra in the presence of methyl viologen, as an electron acceptor, established that the electron transfer occurs from their triplet state.

Direct Observation of Long-Lived Upper Excited Triplet States and Intersystem Crossing in Anthracene-Containing PtII Complexes.

Exceptionally long-lived T2 states (7 ns) were observed with the N^N PtII bisacetylide complex (Pt-1) and trans-bis(phosphine) PtII bisacetylide complexes (Pt-2, Pt-3) containing anthryl acetylide ligands. For Pt-1, fluorescence of the anthryl moiety (An) was quenched and phosphorescence was observed. Under 350 nm excitation, the upper long-lived triplet state T2 (3An) was populated via ultrafast intersystem crossing (ISC) of S1 (1An) → T2 (3An) (within 0.2 ps). Interestingly, Pt-3, after population of the S1 state, emits strong fluorescence (ΦF = 89%); the poor ISC is due to the high-lying T2 (3An, 3.36 eV) versus S1 (1An, 2.55 eV) state and the large energy gap between S1 (1An, 2.55 eV) and T1 (3An, 1.32 eV) states. The population of the upper excited state S2 (1LLCT, 3.49 eV) turns to an efficient S2 → T2 → T1, and ISC yield increases by 55% compared with S0 → S1 excitation. These results present new in-depth insights into fundamental photochemistry of upper excited states.

Effects of Naphthyl Connectivity on the Photophysics of Compact Organic Charge-Transfer Photoredox Catalysts.

Modular chromophoric systems with minimal electronic coupling between donor and acceptor moieties are well suited for establishing predictive relationships between molecular structure and excited-state properties. Here, we investigate the impact of naphthyl-based connectivity on the photophysics of phenoxazine-derived orthogonal donor-acceptor complexes. While compounds in this class are themselves interesting as potent organic photocatalysts useful for visible-light-driven organocatalyzed atom-transfer radical polymerization and small-molecule synthesis, many other systems (e.g., phenazine, phenothiazine, and acridinium) exploit charge-transfer excited states involving a naphthyl substituent. Therefore, aided by the facile tunability of the phenoxazine architecture, we aim to provide mechanistic insight into the effects of naphthyl connectivity that can help inform the understanding of other systems. We do so by employing time-resolved and steady-state spectroscopies, cyclic voltammetry, and temperature-dependent studies on two chemical series of phenoxazine compounds. In the first series ( N-aryl 3,7-dibiphenyl phenoxazine), we find high sensitivity of photophysical behavior to naphthyl connectivity at its 1 versus 2 positions, including a drop in the intersystem-crossing yield (ΦISC) from 0.91 ( N-1-naphthyl) to 0.54 ( N-2-naphthyl), which we attribute to the establishment of an excited-state equilibrium in the singlet manifold. Drawing on the synthetic tunability afforded by phenoxazine, a modified series ( N-aryl 3,7-diphenyl phenoxazine) is chosen to circumvent this equilibrium, thereby isolating the impact of naphthyl connectivity on charge-transfer energy and triplet formation. We conclude that donor-acceptor distance is a key design parameter that influences a host of excited-state and dynamical properties and can have an outsized impact on photochemical function.

Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts.

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (ΦISC) in a highly reducing ( E0* = -1.7 V vs SCE), visible-light-absorbing phenoxazine-based PC. Simple substitution of N-phenyl for N-naphthyl is shown to dramatically increase ΦISC from 0.11 to 0.91 without altering catalytically important properties, such as E0*.

Photoexcitation Dynamics of Thymine in Acetonitrile and an Ionic Liquid Probed by Time‐resolved Infrared Spectroscopy

Femtosecond transient IR absorption spectroscopy was used to probe the decay mechanism of electronically excited thymine (a naturally occurring pyrimidine base in DNA) dissolved in an ionic liquid ([Bmim][ BF4 ]) or CD3CN after the absorption of UV light (267 nm). In both solvents, an absorption band grew on a picosecond timescale, along with decaying bleach and evolving red‐shifted absorption signals. A population analysis of the observed kinetic data suggested that most of the photoexcited thymine underwent a sub‐picosecond non‐radiative relaxation to the vibrationally hot ground electronic state. About 4% (16%) of the excited thymine in the ionic liquid (CD3CN) relaxed to an intermediate electronic state, which relaxed into a low‐lying triplet state by intersystem crossing (ISC) (ISC did not relax to the ground electronic state within the experimental period (1 ns)). The low ISC yield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because the ISC to triplet state transition for excited thymine has been considered as a precursor to cyclobutane–pyrimidine dimer formation, which led to functional damage of the base after UV absorption. This finding may shed light on the photostability of DNA in ionic liquids.

Emission and transient absorption measurements of substitution effects of C–C triple bonds on relaxation processes of the fluorescent state of naphthalenes

Photophysical and photochemical properties of naphthalenes substituted with trimethylsilylethynyl, tert-butylethynyl, and trimethylsilylbutadiynyl groups were investigated by measurement of fluorescence yields, lifetimes, and triplet absorption. Introducing trimethylsilylethynyl and tert-butylethynyl groups to the 1-position of the naphthalene skeleton substantially enhanced fluorescence and intersystem crossing (ISC). The rates of fluorescence of 2-substituted naphthalenes were low. The effect of ethynyl groups on the 1-substituted naphthalenes was rationalized in terms of an increase of the transition moment along the short axis of the naphthalene skeleton. Substitution of the trimethylsilylbutadiynyl group at the 1 or 2-position of the naphthalene skeleton caused a considerable decrease in the fluorescence yield (approximately 0.01) and an increase in the ISC yield (0.99).

Photophysics and photostability of 9,10-bis(phenylethynyl)anthracene revealed by single-molecule spectroscopy

The photophysics and photostability of 9,10-bis(phenylethynyl)anthracene (BPEA) diluted in a 40-nm-thick Zeonex polymer film have been investigated by single-molecule spectroscopy (SMS). The single-molecule detection of BPEA was verified by recording fluorescence intensity trajectories, fluorescence lifetimes, and fluorescence spectra. The intensity trajectories showed frequent on/off blinking and one-step photobleaching behaviors. The observed blinking was attributed to the temporary occupation of the excited triplet state T1via intersystem crossing (ISC). Assuming a three-state model (e.g., S0, S1, and T1), the distributions of triplet lifetime and S1→T1 ISC quantum yield of BPEA were both derived from the analyses of the blinking statistics and the intensity autocorrelation. We found extremely low ISC yields (on the order of 10−5–10−4), which were theoretically rationalized by the large energy gap between 3B2u and S1(1B1u)/T1(3B1u) states. SMS measurements were also conducted under both air and Ar atmospheres in order to gain insight into the influence of oxygen on photobleaching. The results reveal that, although the presence of oxygen considerably degraded the photostability of BPEA, under deoxygenated conditions, BPEA delivers more than 107 photons before photobleaching and possesses an appreciably low photobleaching yield of 10−9–10−8. This study shows that BPEA has a relatively high degree of photostability at room temperature and can serve as a useful green fluorescent probe for SMS studies.

Single Dibenzoterrylene Molecules in an Anthracene Crystal: Spectroscopy and Photophysics

We study single dibenzoterrylene molecules in an anthracene single crystal at 1.4 K in two insertion sites at 785.1 and 794.3 nm. The single-molecule zero-phonon lines are narrow (about 30 MHz), intense (the detected fluorescence rates at saturation reach 100,000 counts s(-1)), and very photostable. The intersystem-crossing yield is extremely low (10(-7) or lower). All of these features are hallmarks of an excellent system for high-resolution spectroscopy and nanoscale probing at cryogenic temperatures.

Spectral and Photophysical Studies of Poly[2,6-(1,5-dioctylnaphthalene)]thiophenes

A complete spectroscopic and photophysical study of three alternating naphthalene-α-thiophene copolymers was undertaken in solution (room and low temperature) and in the solid state (thin films in a Zeonex matrix). The study comprises absorption, emission, and triplet−triplet spectra together with quantitative measurements of quantum yield (fluorescence, intersystem-crossing, internal conversion, and singlet oxygen formation) lifetimes and singlet and triplet energies. The overall data allow the determination of the rate constants for all the decay processes. Comparison between the behavior of analogous 1-naphthyl(oligo)thiophenes and the 2,6-naphthalene(oligo)thiophene copolymers allows several important observations. First, the polymers display higher fluorescence quantum yields and lower S1∼∼→T1 intersystem-crossing yields than the oligomers. This can be attributed to the presence of the 1,5-dioctyloxynaphthalene groups in the copolymers leading to a more rigid polymer backbone, which decreases radiati...

Photophysical Properties of a Tetraphenoxy‐Substitued Perylene Bisimide Derivative Characterized by Single‐Molecule Spectroscopy

However, the figures detailed in the article result from an analysis where erroneously 〈δI(t)〉2 rather than 〈I(t)〉2 was inserted in the denominator of Equation (4). Using the correct normalization yields λ1=1/20 μs=50000 s−1, λ2=1/300 μs=3300 s−1 and C1=0.034, C2=0.009 for the fit parameters of Equation (6). As a consequence, the deduced photophysical parameters are changed as well and the entries of Tables 1 and 2 of the original article have to be replaced as given below. The main conclusions of the article, that is, the observation of one “fast” and one “slow” triplet sublevel, a low intersystem-crossing yield, and a high fluorescence quantum yield are not affected by these changes. fast slow population rate [s-1] 4500 110 fractional ISC yield 0.98 0.02 decay rate [s-1] 35000 2800 relative steady-state population 0.77 0.23 Parameter fluorescence decay rate[a] [s-1] 1.7⋅108 effective triplet decay rate [s-1] 28 000 ISC rate [s-1] 4610 triplet quantum yield ΦT 2.7⋅10−5 fluorescence quantum yield ΦF 1.3⋅10−5≈1 fully saturated emission rate R∞ [s-1] 7.8⋅107 S1–S0 transition-dipole moment [Debye] 1.5 α=1+kisc 1.1 The authors apologize for any misunderstanding that may have arisen as a result of this mistake.

Picosecond—nanosecond laser photolysis studies of a photoacid generator in solutions: Transient absorption spectroscopy and transient grating measurements

Photochemical reaction profiles of a photoacid generator, N-trifluoromethylsulfonyloxy-1,8-naphthalimide (NI-Sf), in solution phase were investigated by means of picosecond and nanosecond transient absorption spectroscopy as well as transient grating measurements. Picosecond transient absorption spectroscopy directly revealed that the lifetime of the S1 state and the intersystem-crossing yield decreased with increasing solvent polarity. On the other hand, photochemical reaction yield increased with an increase in the solvent polarity. These results indicated that the photochemical reaction started in the S1 state. Transient grating measurement directly detected the diffusion process of the proton and its diffusion coefficient was obtained to be 3.9 x 10(-9) m2 s(-1), which was several times larger than those of the usual solute molecules.

Intersystem crossing (ISC) in DMABN and DMABA: Laser induced optoacoustic spectroscopy and semi-empirical (AM1) calculations

The individual contribution of the intersystem crossing (ISC) process in the total non-radiative deactivation ofthe excited fluorophores, p-N,N-dimethylaminobenzonitrile (DMABN) and p-N,N-dimethylaminobenzaldehyde(DMABA) was determined by laser-induced optoacoustic spectroscopy (LIOAS). It was noted that, for DMABN, the ISC yield is almost insensitive to the solvent polarity, while, it is highly solvent dependent for DMABA. Semi-empirical (AM1) calculations have been performed to optimize the ground state geometry of both the fluorophores. The AM1-SCI (singly excited configuration interaction) method has been adopted to get the energies and the dipole moments of their excited states. From the calculated oscillator strengths for the optical transitions and also from absorption studies in solvents of different polarity it is revealed that DMABN isexcitedtotheS2stateandDMABAtotheS3stateselectivelywithalaserbeamof320nm.Theexperimental results for the large variation of the ISC yield (φISC) in DMABA has been ascribed to the reordering of the nπ* and ππ* states with a change in the solvent polarity.

Relaxation processes in the excited state of phenylsiloxanes

The relaxation processes in the excited state of phenylsiloxanes and related compounds have been studied by measurements of fluorescence decay, quantum yields, time-resolved fluorescence spectra and intersystem-crossing yields. The absorption and emission spectra of the phenylsiloxanes used correspond to the π,π* transition (1B2u–1A1g) in benzene. The local (1Lb) emissions for the silicon compounds used are very weak: the quantum yields are less than 0.033. For the compounds 2–4 with phenyl groups, very weak intramolecular excimer emissions (ΦfE < 0.017) are observed together with weak local (1Lb) emissions (ΦfM < 0.025) at 298 K. The intersystem-crossing quantum yields of the siloxane compounds obtained by the thermal lensing method are very small (ΦISC < 0.1) at 298 K. The fluorescence decay kinetics of the compounds 1–4, and 6 and 7 with an Si—O bond give double-exponential decay functions. The decay rates of the siloxane compounds are significantly faster than those of the carbon analogues. The siloxane compounds are photochemically stable (the reaction quantum yields at 254 nm are less than 1 × 10–3). On the basis of these results, it can be said that the main channel of the deactivation processes in the excited singlet states (local and excimer states) at room temperature is internal conversion to the ground state. The fast decay rate (i.e. the rate of internal conversion) of the siloxane compounds may be due to the perturbation of a pseudo-π system of the Si—O bond to the S1(1Lb) state.

Laser-flash-photolysis study of aliphatic thioketone triplets. Self-quenching and singlet-oxygen sensitization

Seven aliphatic thioketones, several of them possessing bicyclo[2.2.1]heptane geometries, have been examined for triplet-state behaviour in benzene using laser excitation into their low-lying 1(n, π*) band systems (485–490 and 532 nm). In the case of di-t-butylthioketone (DTBTK) a transient absorption (280–500 nm) was attributable to its triplet (IµT≈ 1.5 × 103 dm3 mol–1 cm–1 at 280 nm). The thioketone triplets were, however, probed in all cases by energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH). Results are presented for triplet lifetimes (0.1–0.6 µs at 1 × 10–3 mol dm–3 thioketone concentration), intersystem-crossing yields (0.85–1.0), self-quenching kinetics [(0.1–7)× 109 dm3mol–1 s–1] and kinetics of energy transfer to DPH [(6–8)× 109 dm3 mol–1 s–1]. The steric crowding at α positions severely hinders the self-quenching interaction in DTBTK. The efficiency of singlet-oxygen formation as a result of DTBTK triplet quenching by oxygen is close to unity (0.85 ± 0.17 in benzene).

Vibronic-level dependence of radiative and nonradiative processes in naphthalene vapor

The vibronic-level dependence of radiative and nonradiative processes in naphthalene vapor has been investigated under collision-free conditions, with particular attention to the effect of molecular rotation and to the contamination caused by simultaneous excitation of levels other than the one under study. Fluorescence excitation spectra, lifetimes, and sensitized biacetyl–phosphorescence excitation spectra were measured by using a pulsed dye laser with a bandwidth of 0.5 cm−1. From the resulting data, the radiative and intersystem-crossing rate constants (kf and kisc,respectively), as well as the fluorescence and intersystem-crossing yields, were determined for 20 single vibronic levels of S1 up to an excess vibrational energy of about 2000 cm−1. The value of kf varies greatly with vibronic level. The levels involving the 8(b1g) vibrational mode have particularly large kf values, and those involving 8(ag), 6(ag), and 4(ag) modes moderately large ones. The observed variation of kf is explained in terms of the transition moment induced by vibronic interaction of S1 with the other singlet states. The value of kisc also varies with vibronic level. A sudden increase and pronounced fluctuation of the kisc value are found at an excess energy of about 1000 cm−1, suggesting the presence of a new crossing channel related presumably to T3. The fluorescence quantum yield measured along the rotational contour of the 8(b1g)01 absorption band shows a rather small variation; moreover, the lifetimes are nearly constant over the contour of the 8(b1g)10 band and of the 0–0 band. The observed variation of the fluorescence yield along the rotational contour is interpreted as due largely to excitation of sequence bands.

Vapor-phase fluorescence emissions of pyridine and its methyl derivatives: Excitation-energy dependence of nonradiative electronic relaxation

Vapor-phase fluorescence from the S1 (n, π*) states of pyridine-h5 and -d5, α- and β-picolines, and 2,6-lutidine has been studied. The fluorescence quantum yields of these compounds are, respectively, 5.9×10−5, 6.0×10−5, 3.5×10−5, 5.4×10−5, and 2.5×10−5 for excitation to the zero-point level of S1. The quantum yields of the first four compounds decrease sharply as the excitation energy is raised to the value corresponding to the S2 (π, π*) state, whereas the yield of 2,6-lutidine is fairly constant throughout the S1←S0 and S2←S0 absorption regions. The fluorescence quantum yields and their excitation-energy dependence undergo no significant change upon pressure increase from 0.5 to 20 Torr or addition of a foreign gas up to 760 Torr. The rate constants for the intersystem crossing (kISC) and the other nonradiative processes (kFQ) are evaluated as functions of the excitation energy for pyridine-h5 and α-picoline with the aid of the intersystem-crossing yield values which are available from the literature. The results show that kFQ increases rapidly with increasing excitation energy, while kISC is nearly constant. The fast nonradiative decay responsible for the sharp decrease of the fluorescence quantum yield is attributed to the S1→S0 internal conversion.

Fluorescence transient and optical free induction decay spectroscopy of pentacene in mixed crystals at 2 K. Determination of intersystem crossing and internal conversion rates

In this paper the recently reported fluorescence transient, appearing during the onset of cw dye laser excitation of the energetically lower sites of pentacene in p-terphenyl at 2 K, is analyzed quantitatively. The role of the lowest triplet state in causing the long (compared to the fluorescence lifetime) transient, is firmly established. For the lower sites a kinetic analysis yields a T1←S1 intersystem crossing (ISC) yield of about 0.4% and an S0←T1 ISC rate of (2.2±0.1) × 104 s−1. Using these data in calculating the S1←S0 transition dipole moment from our earlier OFID experiments yields for the O1 site (proto) μ=0.71±0.24 D. The S0→S1 internal conversion yield is estimated to be 22%. The fluorescence transients of the energetically higher sites of pentacene in p-terphenyl exhibit a much faster decay, which indicates a drastic increase in ISC yield from S1. Optical free induction decay (OFID) experiments performed on these sites confirm this and imply that the T1←S1 ISC yield for these sites is ≳60%. We further discuss results of an OFID experiment on pentacene in naphthalene and derive in this case a T1←S1 ISC yield of ≲2.8% and an S0←S1 internal conversion yield of 31.5±1.5%. The off-resonance pumping effect in a 3-level system was also studied, using the parameter values for pentacene in p-terphenyl. For the lower sites, with the excitation intensities used (∼1 MW/m2) the part of the inhomogeneous absorption line that is pumped has a width of about 2.5 GHz (effective excitation bandwidth &amp;lt;1 MHz).

Kinetics of Triplet–Triplet Annihilation in the Vapor Phase of Naphthalene, Anthracene, and Phenanthrene

The triplet–triplet annihilation process has been investigated in naphthalene, anthracene, and phenanthrene vapor. The observation of spontaneous and delayed fluorescence produced by flash excitation when combined with steady-state fluorescence measurements permits the determination of the temperature and pressure dependence of cSTΦ (absolute) and γ / c (in arbitrary units) (ΦST = S1 → T intersystem-crossing yield; γ, triplet–triplet annihilation rate constant; c, probability of a triplet–triplet annihilation reaction leading to an excited singlet A*1). An attempt is made to separate the observed temperature and pressure dependence of cΦST and γ / c into the individual contributions of c, γ, and ΦST. The behavior of c with respect to temperature is interpreted in terms of the Stevens–Parker mechanism, where excimers are involved as an intermediate product in the annihilation process of two triplets: in molecules where ΔE = 2E(T0) − E(S1) is smaller than the excimer binding energy Ebinding, c is temperature dependent (anthracene). In molecules where ΔE &amp;gt; Ebinding, c has a constant “saturation value” near unity (napthalene, phenanthrene). In anthracene the temperature dependence of c shows a marked parallelism to that of fluorescence self-quenching. Napthalene is found to exhibit a temperature-dependent γ in contrast to phenanthrene and anthracene.