High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission

One of the major drawbacks of organic‐dye‐modified self‐assembled monolayers on metal nanoparticles when employed for efficient use of light energy is the fact that singlet excited states on dye molecules can be easily deactivated by means of energy transfer to the metal surface. In this study, a series of 6,13‐bis(triisopropylsilylethynyl)pentacene–alkanethiolate monolayer protected gold nanoparticles with different particle sizes and alkane chain lengths were successfully synthesized and were employed for the efficient generation of excited triplet states of the pentacene derivatives by singlet fission. Time‐resolved transient absorption measurements revealed the formation of excited triplet states in high yield (172±26 %) by suppressing energy transfer to the gold surface.     

A Pentacene‐based Nanotube Displaying Enriched Electrochemical and Photochemical Activities

Unlike previously well‐studied, acyclic pentacene oligomers, the first synthesis of a cyclic pentacene trimer with a fixed tubular conformation is reported. A short‐step synthesis starting from common pentacenequinone yielded the target molecule with a 1.5 nanometer length and a subnanometer pore. Steady‐state spectroscopic analyses revealed that the close proximity of the non‐conjugated, three pentacene chromophores allows the nanotube to display stepwise electrochemical/chemical oxidation characteristics. Furthermore, time‐resolved transient absorption measurements elucidated the generation of an excited triplet state of the nanotube, with high quantum yield reaching about 180 % through intramolecular singlet fission and a very long triplet lifetime.     

Photochemical synthesis of pentacene and its derivatives

A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-dibromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

Singlet Heterofission in Tetracene–Pentacene Thin-Film Blends

Heterofission is a photophysical process of fundamental and applied interest whereby an excited singlet state is converted into two triplets on chemically distinct chromophores. The potential of this process lies in the tuning of both the optical band gap and the splitting between singlet and triplet energies. Herein, we report the time-domain observation of heterofission in mixed thin films of the prototypical singlet fission chromophores pentacene and tetracene using excitation wavelengths above and below the tetracene band gap. We found a time constant of 26 ps for endothermic heterofission of a singlet exciton on pentacene in blends with low pentacene fractions, which was outcompeted by pentacene homofission for increasing pentacene concentrations. Direct excitation of tetracene lead to fast energy transfer to pentacene and subsequent singlet fission, which prevented homo- or heterofission of a singlet exciton on tetracene.     

Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

Excited singlet state kinetics on a subnanosecond time-scale

It is possible to study directly the absorption time-profiles of short-lived excited singlet states by sampling the transient excitation spectra for fluorescence from upper states. This approach has many advantages over direct absorption measurements, since the effects of triplet state absorption can be suppressed and the population time-profiles of excited singlet states can be studied in detail. The first direct measurements are reported of singlet state absorption time profiles in a number of aromatic aza- and carbonyl compounds.     

Ultrafast spectroscopy and computational study of the photochemistry of diphenylphosphoryl azide: direct spectroscopic observation of a singlet phosphorylnitrene

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ～ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ～ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Conformational Planarization versus Singlet Fission: Distinct Excited‐State Dynamics of Cyclooctatetraene‐Fused Acene Dimers

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited‐state dynamics in solution. While the anthracene dimer showed a fast V‐shaped‐to‐planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.     

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t₄-PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr₄). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ_exc=670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q_x-band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.     

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time‐Resolved Spectroscopy

Detailed investigations by time‐resolved transient absorption and fluorescence spectroscopies with nano‐ and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes ( 1 – 3 ) and three ethynyl anthracenes ( 4 – 6 ) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1 – 4 , thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge‐transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO₂ derivative 1 . No push–pull character is found for 5 and 6 , which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.     

Singlet excitation energies of thiophene derivatives of fluorene: TD‐DFT study

Fluorene‐thiophene (FT)‐based oligomers and polymers and their derivatives are good candidates for organic blue light‐emitting diodes. In this work, the intrinsic properties of the ground and excited states of FT monomer and its derivatives are studied. The ground‐state optimized structures and energies are obtained using molecular orbital theory and density functional theory (DFT). The ground‐state potential energy curves or surfaces of FT and its derivatives are also obtained. All derivatives are nonplanar in their electronic ground states. The character and energy of the first 20 singlet–singlet electronic transitions are investigated by applying the time‐dependent density functional theory (TD‐DFT) approximations to the correspondingly optimized ground‐state geometries. The lowest singlet state is studied with the configuration interaction (singles) approach (CIS). Excitation energies are red shifted when the FT unit or its derivatives are extended longitudinally. CIS results suggest geometry relaxation in the first singlet excited state. When available, a comparison is made with experimental results. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Ultrafast study of p-biphenylyldiazomethane and p-biphenylylcarbene

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.     

Singlet energy migration along an alternating block copolymer of oligothiophene and oligosilylene in solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the F?rster theory, the energy migration process was attributed to an incoherent hopping mechanism.     

Singlet Fission for Photovoltaics with 130 % Injection Efficiency

A novel pentacene dimer ( P2 ) and a structurally analogous monomer ( P1 ) were synthesized for use in n‐type dye‐sensitized solar cells. In P2 , the triplet excited states formed by the rapid, spin‐allowed process singlet fission were expected to enable carrier multiplication in comparison to the slow, spin‐forbidden intersystem crossing seen in P1 . A meta‐positioning of the two pentacenes and the carboxylate anchor were chosen in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low‐lying triplet excited states of pentacene units necessitated the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Indium‐zinc oxide in the presence of Li⁺ was found to be the optimum choice for the photoelectrodes. Efficient electron injection from the triplet excited states of P1 and P2 was found, with a carrier multiplication of nearly 130 %.     

Role of excited state intramolecular charge transfer in the photophysical properties of norfloxacin and its derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.     

Generation and decay dynamics of triplet excitons in Alq3 thin films under high-density excitation conditions

We studied the generation and decay dynamics of triplet excitons in tris-(8-hydroxyquinoline) aluminum (Alq3) thin films by using transient absorption spectroscopy. Absorption spectra of both singlet and triplet excitons in the film were identified by comparison with transient absorption spectra of the ligand molecule (8-hydroxyquinoline) itself and the excited triplet state in solution previously reported. By measuring the excitation light intensity dependence of the absorption, we found that exciton annihilation dominated under high-density excitation conditions. Annihilation rate constants were estimated to be gammaSS = (6 +/- 3) x 10(-11) cm3 s(-1) for single excitons and gammaTT = (4 +/- 2) x 10(-13) cm3 s(-1) for triplet excitons. From detailed analysis of the light intensity dependence of the quantum yield of triplet excitons under high-density conditions, triplet excitons were mainly generated through fission from highly excited singlet states populated by singlet-singlet exciton annihilation. We estimated that 30% of the highly excited states underwent fission.     

四-α(β)-(4-吡啶氧基)酞菁锌的光物理性质研究

测定了新合成的α位取代和β位取代的四-(4-吡啶氧基)酞菁锌配合物的UV-Vis吸收光谱、荧光光谱及激发单重态寿命、纳秒瞬态吸收光谱与激发三重态寿命.在此基础上,与相关配合物进行了比较,探讨了取代基及其取代位置对酞菁锌配合物的吸收光谱、激发单重态寿命及激发三重态寿命的影响.