Femtosecond time-resolved photophysics of 1,4,5,8-naphthalene diimides

The photophysical properties of a tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds were investigated. The absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500 and 650 nm. This band is sensitive to the polarity of the solvent and is attributed to a CT state. The absorption spectra and lifetime (10+/-2 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5-1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran glass.     

Excited state dynamics of a PtII diimine complex bearing a naphthalene-diimide electron acceptor

A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.     

Transient absorption spectra and photochromic mechanism of schiff bases

The transient absorption spectra of salicylideneaniline (SA) and salicylidenebenzylamine (SBA) have been investigated by means of the nanosecond laser flash photolysis. It has been found that the intermediates from the two compounds have different properties. According to the properties of the intermediates for compound SA, the primary photoproduct is a zwitterion produced by the excited singlet state, rather than a trans- keto isomer. The intermediate of compound SBA also was found to be a zwitterion, but produced not only by the excited singlet state but also by the excited triplet state. The photochromic mechanisms of the two compounds are proposed and discussed respectively.     

2-Naphthyl(carbomethoxy)carbene revisited: combination of ultrafast UV-vis and infrared spectroscopic study

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN2CO2CH3) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (tau = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene ((1)NpCCO2CH3). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by approximately 50% of their original intensity in cyclohexane in 3 ns. It is concluded that (1)NpCCO2CH3 has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only (1)NpCCO2CH3 is formed within 50 ps of the laser pulse rather than a singlet-triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.     

Photoreduction of Ru(bpy)(3)2+ by amines in aqueous solution. Kinetics characterization of a long-lived nonemitting excited state

The photochemistry of Ru(bpy)(3)+2 in the presence of amines was investigated in water by laser flash photolysis. N,N'-Dimethylaniline and p-phenylenediamine quench the luminescent metal to ligand charge transfer (MLCT) excited state of the complex by an electron transfer reaction that produces the semireduced form Ru(bpy)3+ in relatively high yields. On the other hand, triethylamine (TEA) and aniline do not quench the MLCT. Nevertheless, when laser flash irradiation at 532 nm is carried out in the presence of these amines, the formation of Ru(bpy)3+ is clearly detected by its transient absorption at 510 nm. These results are interpreted by an electron transfer reaction with the participation of a nonemitting excited state of the complex, formed independently of the MLCT from the Franck-Condon or the relaxed singlet excited state. The rate constants for the quenching of this state by TEA and aniline and the quantum yields for Ru(bpy)(3)+ were determined. The new state is formed in a very fast process and has a lifetime of ca 4 micros in water.     

Photogeneration of Hydrated Electrons, Nitrogen-Centered Radicals and Singlet Oxygen from Naphazoline: A Laser Flash Photolysis Study

The transient intermediates involved in the photochemistry of naphazoline (NP, 2-[1-naphthylme-thyl]imidazoline) have been examined using laser flash photolysis techniques. The photoreactivity of the drug is characterized by a photoionization process occurring through a mixture of mono- and biphotonic pathways. An intramolecular electron transfer involving both the imidazoline and the naphthalene moieties leads to the formation of nitrogen-centered radicals. The generation of singlet oxygen from the lowest excited triplet state of NP is also observed. The results obtained demonstrate the potential for NP to act as a both a type I and type II photosensitizer.     

Allosteric Control of Naphthalene Diimide Encapsulation and Electron Transfer in Porphyrin Containers: Photophysical Studies and Molecular Dynamics Simulation

The complexation processes of N,N’-dibutyl-1,4,5,8-naphthalene diimide ( NDI ) into two types of π-electron-rich molecular containers consisting of two Zn(II)-porphyrins connected by four flexible linkers of two different lengths, were characterized by means of absorption and emission spectroscopies and molecular dynamics simulation. Notably, the addition of NDI leads to a strong quenching of the fluorescence of both cages only when they are in an open conformation suitable for guest encapsulation, a situation triggered by silver(I) ions binding to the lateral triazoles. Molecular dynamics simulations confirm the fast binding of NDI , likely assisted by NDI -silver(I) interactions. Upon NDI complexation, the two porphyrin macrocycles get closer, with an optimized face to face orientation, suggesting an induced-fit mechanism through π–π interactions with the NDI aromatic cycle. Ultrafast transient absorption experiments allowed to identify the process of quenching of the Zn-porphyrin fluorescence as an efficient photoinduced electron transfer reaction between the cage porphyrin and the included NDI guest. The process occurs on fast and ultrafast time scales in the two complexes (1.5 ps and ≤300 fs) leading to a short-lived charge separated state (charge recombination lifetimes in the order of 30–40 ps). The combined computational and experimental approach used here is able to furnish a reliable model of the NDI -cage complexation mechanism and of the corresponding electron transfer reaction, attesting the allosteric control of both processes by the silver(I) ions.     

Charge separation in DNA via consecutive adenine hopping

Charge transfer in DNA is of current interest because of the involvement of charge transfer in oxidative DNA damage and electronic molecular devices. We have investigated the charge separation process via the consecutive adenine (A)-hopping mechanism using laser flash photolysis of DNA conjugated with naphthaldiimide (NDI) as an electron acceptor and phenothiazine (PTZ) as a donor. Upon the 355-nm laser flash excitation of NDI, the charge separation and recombination process between NDI and PTZ was observed. The yields of the charge separation via the consecutive A-hopping were slightly dependent upon the number of A bases between the two chromophores, while the charge recombination rate was strongly dependent upon the distance. The charge-separated state persisted over 300 micros when NDI was separated from PTZ by eight A bases. Furthermore, the rate constant of the A-hopping process was determined to be 2 x 10(10) s(-1) from an analysis of the yield of the charge separation depending on the number of A-hopping steps.     

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二苯硒醚(TPDASe)间光诱导电子转移过程的激光光解研究

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二 苯硒醚(TPDASe)间在激光光诱导条件下，发生了分子间的电子转移过程．在可见- 近红外区(600-1200nm)，观测到了TPDASe阳离子自由基、富勒烯(C60／C70)激发三 线态和阴离子自由基，在苯腈溶液中，观测瞬态谱测定了电子从TPDASe转移到富勒 烯(C60／C70)激发三线态的量子转化产率(Φet^T)和电子转移常数(Ket)．     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

Photochemistry of thioxanthones—III. Spectroscopic and flash photolysis study on hydroxy and methoxy derivatives

The spectroscopic properties of 9 oil soluble hydroxy and methoxy thioxanthone derivatives have been examined in various solvents and the data compared to their photopolymerization efficiency and flash photolysis behaviour in solution. Absorption maxima, extinction coefficients, fluorescence and phosphorescence spectra and quantum yields have been measured. Generally, most of the compounds exhibit low fluorescence and high phosphorescence quantum yields except 1-substituted derivatives where intra-molecular hydrogen bonding is involved. These observations are consistent with the high photoreactivity of the molecules occurring via the lowest excited triplet state. Photopolymerization rates of n-butyl methacrylate, using N-diethylmethylamine as co-initiator, correlate to some extent with the absorption maxima and extinction coefficients of the thioxanthones. Transient formation on micro-second flash photolysis is associated with the ketyl radical formed by the lowest excited triplet state of the thioxanthones abstracting a hydrogen atom from the solvent. In the presence of a tertiary amine, a new longer wavelength transient absorption is produced and is assigned to a radical-anion formed by the lowest excited triplet state of the thioxanthones abstracting an electron from the amine. A correlation was observed between the transient absorption due to the radical-anion and the ionisation potential of various amines. Flash photolysis studies in acid and base media confirmed the identity of the radical and radical-ion species. Intra-molecular hydrogen bonding in the α-position to the carbonyl group deactivates both the lowest excited singlet and triplet states of thioxanthone but has little effect on polymerization efficiency. The latter is associated with competition of the carbonyl group with the amine co-initiator for hydrogen bonding and consequent electron abstraction to give an active radical-anion. This is confirmed using micro-second flash photolysis.     

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.     

Intramolecular triplet energy transfer via higher triplet excited state during stepwise two-color two-laser irradiation

We studied the energy transfer processes in the molecular array consisting of pyrene (Py), biphenyl (Ph2), and bisphthalimidethiophene (ImT), (Py-Ph2)2-ImT, during two-color two-laser flash photolysis (2-LFP). The first laser irradiation predominantly generates ImT in the lowest triplet excited state (ImT(T1)) because of the efficient singlet energy transfer from Py in the lowest singlet excited state to ImT and, then, intersystem crossing of ImT. ImT(T1) was excited to the higher triplet excited state (Tn) with the second laser irradiation. Then, the triplet energy was rapidly transferred to Py via a two-step triplet energy transfer (TET) process through Ph2. The efficient generation of Py(T1) was suggested from the nanosecond-picosecond 2-LFP. The back-TET from Py(T1) to ImT was observed for several tens of microseconds after the second laser irradiation. The estimated intramolecular TET rate from Py(T1) to ImT was as slow as 3.1 x 104 s-1. Hence, long-lived Py(T1) was selectively and efficiently produced during the 2-LFP.     

Photochromism of cis(z)-1,2-bispyrryl-substituted ethene derivatives

Photochromism of two bispyrryl-substituted ethenes, 2, 3-bis-(1-p-methoxyphenyl-5-phenyl-2-methyl-3-pyrryl)-2-butene (BPE1) and 2,3-bis(1-p-bromophenyl-4-phenyl-2-methyl-3-pyrryl)-2-butene (BPE2), was studied by laser flash photolysis technique. The results indicate that photocyclization of these compounds proceeds mainly via the excited triplet state, and the cis-trans isomerization proceeds mainly via the excited singlet state. After UV laser pulse irradiation, both photocylization and cis-trans isomerization of BPEl occur, but photocydization is the main reaction. On the other hand, laser photolysis of BPE2 leads mainly to photocydization. The effects of the substituents on the photochromic mechanism are also discussed.     

Reaction of triarylphosphine radical cations generated from photoinduced electron transfer in the presence of oxygen

[reaction: see text] The 9,10-dicyanoanthracene (DCA)-sensitized photoreaction of triarylphosphines (1) was carried out in acetonitrile under aerobic conditions. Phosphine 1 was oxidized to the corresponding phosphine oxide with no appreciable side reactions. Product analysis and laser flash photolysis experiments suggest that the radical cation of 1 formed by the electron transfer from 1 to DCA in the singlet excited state ((1)DCA) reacts with O(2) to eventually afford the phosphine oxide.     

Laser induced photochromic mechanism of 3-phenyl-3-[1,2-dimethylindol-3-YL]-3H-naphtho[2,1-B]pyran

The photochromic process of 3-phenyl-3-[1,2-dimethylindol-3-yl]-3H-naphtho[2,1-b]pyran [I] has been examined with nanosecond laser flash photolysis techniques in cyclohexane and acetonitrile respectively. Both excited singlet state and triplet state are involved in the photocoloration process. The decay kinetics of photoproducts are also studied. The maximum absorption wavelength and lifetime of the transient species are solvent dependent.     

DETECTION OF THE EXCITED SINGLET STATE OF A DEPROTONATED, REDUCED FLAVIN

Abstract— The excited singlet state of a deprotonated, reduced flavin [1, 5-dihydro- N (3)-carboxymethyllumiflavin] in aqueous solution at pH 8 has been detected by laser flash photolysis. The broad absorption band maximized at ∼ 490 nm (ε= 9.9 × 10³ M ^-1 cm^-1). The lifetime of the transient was found to be 100 ± 15 ps. The lifetime was not affected by the presence of pyrimidine dimers, which would be monomerized under these conditions. A longer-lived transient, tentatively identified as the solvated electron, was also detected. The neutral reduced flavin did not give a detectable transient.     

Hole transfer in DNA and photosensitized DNA damage: importance of adenine oxidation

Photosensitized DNA damage reactions were investigated for two well-known DNA-damaging photosensitizers (Sens), naphthalimide (NI) and napthaldiimide (NDI), which have similar photophysical properties but differ in their redox properties. NI and NDI derivatives (NIN, NDIN), which have cationic side chains and electrostatically binding to DNA due to favorable electrostatic interactions between the negatively charged phosphate groups of DNA and cationic groups, and NIP and NDIP, which possess phosphate groups and do not bind to DNA, were synthesized. NIN and NDIN can oxidize A and G via their singlet excited state, and NDIP oxidizes A and G via its triplet excited state, whereas NIP oxidizes only G. A combination of laser flash photolysis kinetic studies and quantitative HPLC analyses of photosensitized DNA damage was performed for several DNA sequences in the presence of Sens. NIN, NDIN, and NDIP, which oxidizes A, caused significant DNA damage upon photoirradiation, and DNA damage yield increased with the length of the consecutive A stretch. In contrast, NIP, which oxidizes only G, caused only moderate damage to DNA and showed no preference for the consecutive A sequences. These results clearly demonstrate the importance of A-oxidation, especially in consecutive A sequences, which triggers the rapid hole transfer between A's.     

Photoinduced electron transfer in crystal violet (CV)–bovine serum albumin (BSA) system: evaluation of reaction paths and radical intermediates

Photoinduced electron transfer (PET) from excited probes attached to proteins is of considerable current interest. Photochemical processes following 532 nm excitation of triphenyl methane dye, crystal violet (CV⁺) bound to a protein, bovine serum albumin (BSA), have been investigated in picosecond (ps) to microseconds (μs) time scales by flash photolysis technique. The excited singlet state lifetime of CV⁺ is found to be increased to 130 ps as compared to 1–5 ps for the unbound dye in low viscosity solvents. From flash photolysis studies in microsecond region, transient absorption in the region 650 nm is observed which is attributed to the dication radical CV√²⁺ formed by electron transfer from ³CV^+* to BSA, contrary to electron transfer from BSA to the excited dye as proposed in a recent report. Supporting spectral evidence for the electron transfer from ³CV^+* to BSA is obtained from pulse radiolysis studies.     

Changing DNA grooves--a 1,4,5,8-naphthalene tetracarboxylic diimide bis-intercalator with the linker (beta-Ala)(3)-Lys in the minor groove

We have been investigating a modular, threading DNA polyintercalator design based upon the 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) intercalating unit. Previously, we have reported the NMR analysis of a bis-intercalator-DNA complex in which the peptide linker between NDI units was found to occupy the DNA major groove (Guelev, Lee, Sorey, Hoffman, Iverson, Chem. Biol. 2001, 8, 415-425). Here we describe the NMR analysis of a complex between a related bis-intercalator known to display altered DNA sequence specificity. In this case, the linker resides in the DNA minor groove. We have thus shown that within this set of sequence specific bis-intercalators, both DNA grooves can be accessed, setting the stage for longer threading polyintercalators designed to have linkers occupying both grooves in an alternating fashion.