Generation of the Thymine Triplet State by Through-Bond Energy Transfer

Benzophenone (BP) and drugs containing the BP chromophore, such as the non-steroidal anti-inflammatory drug ketoprofen, have been widely reported as DNA photosensitizers through triplet–triplet energy transfer (TTET). In the present work, a direct spectroscopic fingerprint for the formation of the thymine triplet (³Thy*) by through-bond (TB) TTET from ³BP* has been uncovered. This has been achieved in two new systems that have been designed and synthesized with one BP and one thymine (Thy) covalently linked to the two ends of the rigid skeleton of the natural bile acids cholic and lithocholic acid. The results shown here prove that it is possible to achieve triplet energy transfer to a Thy unit even when the photosensitizer is at a long (nonbonding) distance.     

Steric shielding vs. σ–π orbital interactions in triplet–triplet energy transfer

The influence of non-covalent σ–π orbital interactions on triplet–triplet energy transfer (TTET) through tuning of the donor excitation energy remains basically unexplored. In the present work, we have investigated intermolecular TTET using donor moieties covalently linked to a rigid cholesterol (Ch) scaffold. For this purpose, diaryl ketones of π,π* electronic configuration tethered to α- or β-Ch were prepared from tiaprofenic acid (TPA) and suprofen (SUP). The obtained systems TPA-α-Ch, TPA-β-Ch, SUP-α-Ch and SUP-β-Ch were submitted to photophysical studies (laser flash photolysis and phosphorescence), in order to delineate the influence of steric shielding and σ–π orbital interactions on the rate of TTET to a series of energy acceptors. As a matter of fact, fine tuning of the donor triplet energy significantly modifies the rate constants of TTET in the absence of diffusion control. The experimental results are rationalized by means of theoretical calculations using first principles methods based on DFT as well as molecular dynamics.     

Triplet–Triplet Energy Transfer from a UV‐A Absorber Butylmethoxydibenzoylmethane to UV‐B Absorbers

The phosphorescence decay of a UV‐A absorber, 4‐tert‐butyl‐4′‐methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV‐B absorbers, 2‐ethylhexyl 4‐methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T₁) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet–triplet energy transfer occurs from BMDBM to OMC and OCR. The T₁ state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T₁ state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy‐donor phosphorescence decay measurements can be applied to the study of the triplet–triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet–triplet annihilation was observed in the BMDBM–OMC and BMDBM–OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.     

Remote Sensitized Photoisomerization via Through‐Bond Triplet–Triplet Energy Transfer Mediated by a Salt Bridge in a Supramolecular Dyad

A supramolecular dyad, BP‐(amidinium‐carboxylate)‐NBD is constructed, in which benzophenone (BP) and norbornadiene (NBD) are connected via an amidinium‐carboxylate salt bridge. The photophysical and photochemical properties of the assembled BP‐(amidinium‐carboxylate)‐NBD dyad are examined. The phosphorescence of the BP chromophore is efficiently quenched by the NBD group in BP‐(amidinium‐carboxylate)‐NBD via the salt bridge. Time‐resolved spectroscopy measurements indicate that the lifetime of the BP triplet state in BP‐(amidinium‐carboxylate)‐NBD is shortened due to the quenching by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane (QC). All of these observations suggest that the triplet–triplet energy transfer occurs efficiently in the BP‐(amidinium‐carboxylate)‐NBD salt bridge system. The triplet–triplet energy transfer process proceeds with efficiencies of approximately 0.87, 0.98 and the rate constants 1.8×10³ s^?1, and 1.3×10⁷ s^?1 at 77 K and room temperature, respectively. The mechanism for the triplet–triplet energy transfer is proposed to proceed via a “through‐bond” electron exchange process, and the non‐covalent bonds amidinium‐carboxylate salt bridge can mediate the triplet–triplet energy transfer process effectively for photochemical conversion.     

Study of the Light‐Induced Spin Crossover Process of the [FeII(bpy)3]2+ Complex

Ab initio calculations have been performed on [Fe^II(bpy)₃]²⁺ (bpy=bipyridine) to establish the variation of the energy of the electronic states relevant to light‐induced excited‐state spin trapping as a function of the Fe? ligand distance. Light‐induced spin crossover takes place after excitation into the singlet metal‐to‐ligand charge‐transfer (MLCT) band. We found that the corresponding electronic states have their energy minimum in the same region as the low‐spin (LS) state and that the energy dependence of the triplet MLCT states are nearly identical to the ¹MLCT states. The high‐spin (HS) state is found to cross the MLCT band near the equilibrium geometry of the MLCT states. These findings give additional support to the hypothesis of a fast singlet–triplet interconversion in the MLCT manifold, followed by a ³MLCT–HS (⁵T₂) conversion accompanied by an elongation of the Fe? N distance.     

Organic triplet sensitizer library derived from a single chromophore (BODIPY) with long-lived triplet excited state for triplet-triplet annihilation based upconversion

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.     

Variable Temperature Single‐Molecule Dynamics of MEH‐PPV

Herein, we continue our investigation of the single‐molecule spectroscopy of the conjugated polymer poly[2‐methoxy,5‐(2′‐ethylhexyloxy)‐p‐phenylene‐vinylene] (MEH‐PPV) at cryogenic temperatures. First, the low temperature microsecond dynamics of single MEH‐PPV conjugated polymer molecules are compared to the dynamics at room temperature revealing no detectible temperature dependence. The lack of temperature dependence is consistent with the previous assignment of the dynamics to a mechanism that involves intersystem crossing and triplet–triplet annihilation. Second, the fluorescence spectra of single MEH‐PPV molecules at low temperature are studied as a function of excitation wavelength (i.e. 488, 543, and 568 nm). These results exhibit nearly identical fluorescence spectra for different excitation wavelengths. This strongly suggests that electronic energy transfer occurs efficiently to a small number of low‐energy sites in the multichromophoric MEH‐PPV chains.     

Vibrational relaxation of triplet chlorotoulenes studied by photosensitized phosphorescence of biacetyl

The photosensitized phosphorescence of biacetyl by chlorotoulene vapors has been investigated under stationary conditions. Quantum yields of stable triplet formation for chlorotoulenes increased with increasing foreign gas (ethane) pressure, suggesting that vibrational relaxation of initially formed triplet levels is competitive with photodissociation of the CCl bond. Foreign-gas pressure effects on the triplet formation yield by the 0-0 excitation has lead to photodecomposition rate constants of chlorotoluenes ( ≈ 10² s^?1). Step-ladder collisional relaxation processes have been postulated for the excitation at shorter wavelengths.     

New Anthracene Derivatives as Triplet Acceptors for Efficient Green‐to‐Blue Low‐Power Upconversion

Three new anthracene derivatives [2‐chloro‐9,10‐dip‐tolylanthracene (DTACl), 9,10‐dip‐tolylanthracene‐2‐carbonitrile (DTACN), and 9,10‐di(naphthalen‐1‐yl)anthracene‐2‐carbonitrile (DNACN)] were synthesized as triplet acceptors for low‐power upconversion. Their linear absorption, single‐photon‐excited fluorescence, and upconversion fluorescence properties were studied. The acceptors exhibit high fluorescence yields in DMF. Selective excitation of the sensitizer Pd^IIoctaethylporphyrin (PdOEP) in solution containing DTACl, DTACN, or DNA‐CN at 532 nm with an ultralow excitation power density of 0.5 W cm^?2 results in anti‐Stokes blue emission. The maximum upconversion quantum yield (Φ_UC=17.4 %) was obtained for the couple PdOEP/DTACl. In addition, the efficiency of the triplet–triplet energy transfer process was quantitatively studied by quenching experiments. Experimental results revealed that a highly effective acceptor for upconversion should combine high fluorescence quantum yields with efficient quenching of the sensitizer triplet.     

FORMATION OF CYCLOBUTANE THYMINE DIMERS PHOTOSENSITIZED BY PYRIDOPSORALENS: A TRIPLET-TRIPLET ENERGY TRANSFER MECHANISM

The 365 nm irradiation of thymine thin films in the presence of pyridopsoralens is shown to induce the formation of cyclobutane thymine dimers, in contrast to other compounds such as 8- and 5-methoxypsoralen. In order to elucidate the mechanism of such a photosensitized reaction, we have determined the energy of the lowest triplet state (T1) of these compounds, using phosphorescence spectroscopy and CNDO/S quantum chemistry calculations. The T1 energy values were found to be significantly higher for pyridopsoralens--up to 0.3 eV--than for 8- and 5-methoxypsoralen (approximately 2.8 eV), which are not able to photoinduce cyclobutane thymine dimers. The determination of the relative efficiency of cyclobutane thymine dimer formation was performed using chromatographic analysis. A good correlation was found between the energy of the T1 state of the psoralen derivatives and the related cyclobutane thymine dimer formation. Moreover, the photosensitized cyclobutane thymine dimer formation appeared to be temperature-dependent. Our results are consistent with a mechanism involving a triplet energy transfer from the pyridopsoralen to thymine.     

Elucidation of the Dexter‐Type Energy Transfer in DNA by Thymine–Thymine Dimer Formation Using Photosensitizers as Artificial Nucleosides

C‐nucleosides of 4‐methylbenzophenone, 4‐methoxybenzophenone, and 2′‐methoxyacetophenone were synthetically incorporated as internal photosensitizers into DNA double strands. This structurally new approach makes it possible to study the distance dependence of thymidine dimer formation because the site of photoinduced triplet energy transfer injection is clearly defined. The counterstrands to these modified strands lacked the phosphodiester bond between the two adjacent thymidines that are supposed to react with each other. Their dimerization could be evidenced by gel electrophoresis because the covalent connection by cyclobutane formation between the two thymidines changes the mobility. A shallow exponential distance dependence for the formation of thymidine dimers over up to 10 A‐T base pairs was observed that agrees with a Dexter‐type triplet–triplet energy transfer mechanism. Concomitantly, a significant amount of photoinduced DNA crosslinking was observed.     

Broadband Visible‐Light‐Harvesting trans‐Bis(alkylphosphine) Platinum(II)‐Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

A heteroleptic bis(tributylphosphine) platinum(II)‐alkynyl complex ( Pt‐1 ) showing broadband visible‐light absorption was prepared. Two different visible‐light‐absorbing ligands, that is, ethynylated boron‐dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt‐2 and Pt‐3 , which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt‐1 , with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M ^?1 cm^?1. Long‐lived triplet excited states lifetimes were observed for Pt‐1 – Pt‐3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady‐state and time‐resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time‐resolved transient difference absorption spectra. A triplet‐state equilibrium was observed for Pt‐1 . The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt‐1 .     

Analytical nuclear excited‐state gradients for the Tamm–Dancoff approximation using uncoupled frozen‐density embedding

We report the derivation and implementation of analytical nuclear gradients for excited states using time‐dependent density functional theory using the Tamm–Dancoff approximation combined with uncoupled frozen‐density embedding using density fitting. Explicit equations are presented and discussed. The implementation is able to treat singlet as well as triplet states and functionals using the local density approximation, the generalized gradient approximation, combinations with Hartree–Fock exchange (hybrids), and range‐separated functionals such as CAM‐B3LYP. The new method is benchmarked against supermolecule calculations in two case studies: The solvatochromic shift of the (vertical) fluorescence energy of 4‐aminophthalimide on solvation, and the first local excitation of the benzonitrile dimer. Whereas for the 4‐aminophthalimide–water complex deviations of about 0.2 eV are obtained to supermolecular calculations, for the benzonitrile dimer the maximum error for adiabatic excitation energies is below 0.01 eV due to a weak coupling of the subsystems. © 2017 Wiley Periodicals, Inc.     

The triplet state of cytosine and its derivatives: electron impact and quantum chemical study

The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first C's vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second C's vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated.     

Density functional theory study of 1,2‐dioxetanone decomposition in condensed phase

The decomposition of 1,2‐dioxetanone into a CO₂ molecule and into an excited state formaldehyde molecule was studied in condensed phase, using a density functional theory approach. Singlet and triplet ground and excited states were all included in the calculations. The calculations revealed a novel mechanism for the chemiluminescence of this compound. The triplet excitation can be explained by two intersystem crossings (ISCs) with the ground state, while the singlet excitation can be accounted by an ISC with the triplet state. The experimentally verified small excitation yield can then be explained by the presence of an energy barrier present in the potential energy surface of the triplet excited state, which will govern both triplet and singlet excitation. It was also found that the triplet ground state interacts with both the triplet excited and singlet ground states. A MPWB1K/mPWKCIS approach provided results in agreement with the existent literature. © 2012 Wiley Periodicals, Inc.     

The Femtochemistry of a Ferracyclobutadiene

The eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo‐induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid‐infrared spectroscopy. The time‐resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation–association mechanism. Following optical excitation, the system relaxes non‐radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe−CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl–solvent product complex.     

Theoretical Investigation of the Excited States of 2‐Nitrobenzyl and 4,5‐Methylendioxy‐2‐nitrobenzyl Caging Groups†

The photochemistry of caged compounds of the o‐nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol. 84 162–171] that we found long‐lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5‐methylendioxy‐2‐nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o ‐nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5‐methylendioxy‐2‐nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro‐group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o ‐nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.     

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

A series of directly meso‐meso‐linked Pd–porphyrin oligomers (PdDTP‐M, PdDTP‐D, and PdDTP‐T) have been prepared. The absorption region and the light‐harvesting ability of the Pd–porphyrin oligomers are broadened and enhanced by increasing the number of Pd–porphyrin units. Triplet–triplet annihilation upconversion (TTA‐UC) systems were constructed by utilizing the Pd–porphyrin oligomers as the sensitizer and 9,10‐diphenylanthracene (DPA) as the acceptor in deaerated toluene and green‐to‐blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet–triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP‐M]/DPA, [PdDTP‐D]/DPA, and [PdDTP‐T]/DPA systems, respectively, under an excitation power density of 500 mW cm^?2. The photophysical processes of the TTA‐UC systems have been investigated in detail. The higher triplet–triplet annihilation upconversion quantum efficiency observed in the [PdDTP‐D]/DPA system can be rationalized by the enhanced light‐harvesting ability of PdDTP‐D at 532 nm. Under the same experimental conditions, the [PdDTP‐D]/DPA system produces more ³DPA* than the other two TTA‐UC systems, benefiting the triplet–triplet annihilation process. This work provides a useful way to develop efficient TTA‐UC systems with broad spectral response by using Pd–porphyrin oligomers as sensitizers.     

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.     

Photosensitized Thymine Dimerization via Delocalized Triplet Excited States

A new mechanism of photosensitized formation of thymine (Thy) dimers is proposed, which involves generation of a delocalized triplet excited state as the key step. This is supported by chemical evidence obtained by combining one benzophenone and two Thy units with different degrees of freedom, whereby the photoreactivity is switched from a clean Paternò–Büchi reaction to a fully chemo‐, regio‐, and stereoselective [2+2] cycloaddition.