Uncovering Structure-Property Relationships in Push-Pull Chromophores: A Promising Route to Large Hyperpolarizability and Two-Photon Absorption

In this investigation, we report the first hyperpolarizabilities and two-photon absorption cross sections of a large series of 12 push–pull cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of the donor and acceptor units and π-bridge was synthetically tuned to allow insightful comparisons among the molecules. The hyperpolarizability was obtained through a solvatochromic method, by exploiting the rare negative solvatochromism exhibited by the investigated compounds. The two-photon absorption cross sections were determined through two-photon excited fluorescence measurements by means of a tunable nanosecond laser system for sample excitation. The nonlinear optical properties were discussed relatively to the photoinduced intramolecular charge transfer occurring in these donor–acceptor systems, investigated by femtosecond transient absorption experiments. We found a strong increase in hyperpolarizability upon increasing the molecular conjugation. Unexpectedly, the hyperpolarizability is almost unaffected by an increase in donor/acceptor strength and intramolecular charge transfer degree. Differently, the two-photon absorption cross sections of these dyes are enhanced by an increase in both molecular conjugation and intramolecular charge transfer efficiency. Several recent literature works have reported at the same time scattered information about the hyperpolarizability and two-photon absorption of small organic molecules. Our investigation is, to the best of our knowledge, the first attempt to uncover detailed structure–property relationships for these two nonlinear optical properties. Our results represent a promising route to achieve large hyperpolarizability and two-photon absorption in push–pull dyes and may drive the design of new efficient nonlinear optical materials

Transient absorption spectroscopic studies on 4-nitroquinoline N-oxide: From femtoseconds to microseconds timescale

[EN] The singlet excited state of 4-nitroquinoline N-oxide ((1)NQNO*) has been characterized by different spectroscopic techniques, combining transient absorption with steady state and time-resolved emission spectroscopy. The energy of (1)NQNO* has been established as 255 kJ/mol from the fluorescence spectrum, whereas its lifetime has been found to be 10 ps in the femto-laser flash photolysis (LFP) experiments, where a characteristic S-1-S-n absorption band with maximum centered at 425 nm is observed. In a first stage, the triplet excited state of NQNO ((3)NQNO*) has also been characterized by emission spectroscopy in solid matrix, at low temperature. Thus, from the steady state phosphorescence spectrum the triplet energy has been estimated as 183 kJ/mol, whereas the setup with time resolution has allowed us to determine the phosphorescence lifetime as 3 ms. Formation of (3)NQNO* by intersystem crossing in solution at room temperature, has been monitored by femto-LFP, which shows the appearance of a band with maximum at 560 nm (T-1-T-n). It increases with the decreasing intensity of its precursor 425 nm(S-1-S-n) band, giving rise to an isosbestic point at 500 nm. The characterization of (3)NQNO* has been completed by nano-LFP, using xanthone as photosensitizer and oxygen as well as beta-carotene as quenchers. In addition, quenching of (3)NQNO* by electron donors (DABCO) is also observed in aprotic solvents, leading to the radical anion of NQNO (.(-)NQNO). If there is a proton source in the medium (Et3N as electron donor or MeCN:H2O/4:1 as solvent system) protonation of the radical anion results in formation of the neutral radical of NQNO (.NQNOH). In general, all processes are slower in protic solvents because of the solvation sphere. Overall, this information provides a deeper insight into the formation and behavior of excited states and radical ionic species derived from the title molecule NQNO. (C) 2019 Elsevier B.V. All rights reserved.The work was financially supported by the Regional Government of Generalitat Valenciana (PROMETEO/2017/075) as well as the Spanish Government Science Department with the CTQ-2016-78875-P and CTQ-2009-13699 projects and the N. D.-G. fellowship (BES-2010-035875). BC, CC and FE acknowledge MIUR and the University of Perugia for financial support to the project AMIS, through the program "Dipartimenti di Eccellenza - 2018-2022"Duran-Giner, N.; Carlotti, B.; Clementi, C.; Elisei, F.; Encinas Perea, S.; Miranda Alonso, MÁ. (2019). Transient absorption spectroscopic studies on 4-nitroquinoline N-oxide: From femtoseconds to microseconds timescale. Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy. 216:265-272. https://doi.org/10.1016/j.saa.2019.02.105S265272216Fuchs, T., Gates, K. S., Hwang, J.-T., & Greenberg, M. M. (1999). Photosensitization of Guanine-Specific DNA Damage by a Cyano-Substituted Quinoxaline Di-N-oxide. Chemical Research in Toxicology, 12(12), 1190-1194. doi:10.1021/tx990149sDaniels, J. S., & Gates, K. S. (1996). DNA Cleavage by the Antitumor Agent 3-Amino-1,2,4-benzotriazine 1,4-Dioxide (SR4233):  Evidence for Involvement of Hydroxyl Radical. Journal of the American Chemical Society, 118(14), 3380-3385. doi:10.1021/ja9510774Jerina, D. M., Boyd, D. R., & Daly, J. W. (1970). Photolysis of pyridine-N-oxide: an oxygen atom transfer model for enzymatic oxygenation, arene oxide formation, and the NIH shift. Tetrahedron Letters, 11(6), 457-460. doi:10.1016/0040-4039(70)80074-0Albini, A., & Alpegiani, M. (1984). The photochemistry of the N-oxide function. Chemical Reviews, 84(1), 43-71. doi:10.1021/cr00059a004Winkle, S. A., & Tinoco, I. (1978). Interactions of 4-nitroquinoline 1-oxide with four deoxyribonucleotides. Biochemistry, 17(7), 1352-1356. doi:10.1021/bi00600a033Sheng, Z., Song, Q., Gao, F., Zhou, X., Li, J., Dai, J., … Ma, X. (2000). A laser flash photolysis study of amino acids and dipeptides using 4-nitroquinoline 1-oxide as a photosensitizer: The pH dependence. Research on Chemical Intermediates, 26(7-8), 715-725. doi:10.1163/156856700x00633Yamakawa, M., Kubota, T., Ezumi, K., & Mizuno, Y. (1974). Absorption and phosphorescence spectra of 4-nitropyridine N-oxides and 4- and 3-nitroquinoline N-oxides. Spectrochimica Acta Part A: Molecular Spectroscopy, 30(11), 2103-2119. doi:10.1016/0584-8539(74)80061-9Kubota, T., Yamakawa, M., & Mizuno, Y. (1972). The Singlet-triplet Absorption Spectra of Heterocyclic AmineN-Oxides (I). Bulletin of the Chemical Society of Japan, 45(11), 3282-3286. doi:10.1246/bcsj.45.3282Kasama, K., Takematsu, A., Yamamoto, S., & Arai, S. (1984). Photochemical reactions of 4-nitroquinoline 1-oxide with DNA and related compounds. The Journal of Physical Chemistry, 88(21), 4918-4921. doi:10.1021/j150665a025Dutta Choudhury, S., & Basu, S. (2006). Interaction of 4-Nitroquinoline-1-oxide with Indole Derivatives and Some Related Biomolecules:  A Study with Magnetic Field. The Journal of Physical Chemistry B, 110(17), 8850-8855. doi:10.1021/jp055971lSeki, H., Takematsu, A., & Arai, S. (1987). Photoinduced electron transfer from amino acids and proteins to 4-nitroquinoline 1-oxide in aqueous solutions. The Journal of Physical Chemistry, 91(1), 176-179. doi:10.1021/j100285a038Shi, X., & Platz, M. S. (2004). Time Resolved Spectroscopy of Some Aromatic N-Oxide Triplets, Radical Anions, and Related Radicals. The Journal of Physical Chemistry A, 108(20), 4385-4390. doi:10.1021/jp037708vEzumi, K., Kubota, T., Miyazaki, H., & Yamakawa, M. (1976). Electronic spectra of the anion radicals of heterocyclic amine N-oxides and related substances. The Journal of Physical Chemistry, 80(9), 980-988. doi:10.1021/j100550a012Daniels, J. S., Gates, K. S., Tronche, C., & Greenberg, M. M. (1998). Direct Evidence for Bimodal DNA Damage Induced by Tirapazamine. Chemical Research in Toxicology, 11(11), 1254-1257. doi:10.1021/tx980184jBosca, F., Encinas, S., Heelis, P. F., & Miranda, M. A. (1997). Photophysical and Photochemical Characterization of a Photosensitizing Drug:  A Combined Steady State Photolysis and Laser Flash Photolysis Study on Carprofen. Chemical Research in Toxicology, 10(7), 820-827. doi:10.1021/tx9700376Carlotti, B., Cesaretti, A., Fortuna, C. G., Spalletti, A., & Elisei, F. (2015). Experimental evidence of dual emission in a negatively solvatochromic push–pull pyridinium derivative. Physical Chemistry Chemical Physics, 17(3), 1877-1882. doi:10.1039/c4cp04963bCesaretti, A., Carlotti, B., Gentili, P. L., Clementi, C., Germani, R., & Elisei, F. (2014). Spectroscopic Investigation of the pH Controlled Inclusion of Doxycycline and Oxytetracycline Antibiotics in Cationic Micelles and Their Magnesium Driven Release. The Journal of Physical Chemistry B, 118(29), 8601-8613. doi:10.1021/jp502278zCarlotti, B., Cesaretti, A., & Elisei, F. (2012). Complexes of tetracyclines with divalent metal cations investigated by stationary and femtosecond-pulsed techniques. Phys. Chem. Chem. Phys., 14(2), 823-834. doi:10.1039/c1cp22703

Comprehensive Evaluation of the Absorption, Photophysical, Energy Transfer, Structural, and Theoretical Properties of α-Oligothiophenes with One to Seven Rings

A large basis set of R-oligothiophenes with two to seven rings (R2-R7), also including thiophene, R1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence, quantum yields of fluorescence ( F) and triplet formation ( T), lifetimes of fluorescence and the triplet state, quantum yields of singlet oxygen production ( ¢), all rate constants kF, kIC, kISC, and several of the foregoing as a function of temperature. Ten different theoretical calculations across several levels including three levels of ab initio have been carried out regarding which conformer is lowest in energy and the ¢H’s among all conformers of R2, R3 and R5, as well as calculations of transitions energies of the R-oligothiophenes. We have shown that the (l) 1Bu state is the lowest singlet state for all R2-R7 in any solvent, in contradiction to previous predictions for the higher members. Based on absorption and fluorescence data and calculations of atomic charges in S0 and S1, the ground state is twisted while the excited state is planar (quinoidal-like). Significant charge transfer occurs between S0 and S1 but not S0 and T1. For all R2-R7, IC is small, k0 F is approximately constant while kISC decreases significantly from R2 to R7. The decrease is kISC is believed to arise from a decrease in matrix elements of the type á1¾CTjH¢j3¾1ñ. The essential lack of phosphorescence is assigned as originating from inter-ring twisting mode coupling between T1 and S0. Triplet energy transfer to 3O2 to produce 1O2 is highly efficient for R2-R5. Based on all data, the first Rn representative of R-polythiophene is R5

Hydrogen Production from Water by Photolysis, Sonolysis and Sonophotolysis with Solid Solutions of Rare Earth, Gallium and Indium Oxides as Heterogeneous Catalysts

In this work, we present the hydrogen production by photolysis, sonolysis and sonophotolysis of water in the presence of newly synthesized solid solutions of rare earth, gallium and indium oxides playing as catalysts. From the experiments of photolysis, we found that the best photocatalyst is the solid solution Y0.8Ga0.2InO3 doped by sulphur atoms. In experiments of sonolysis, we optimized the rate of hydrogen production by changing the amount of water, adding ethanol and tuning the power of our piezoelectric transducer. Finally, we performed sonolysis and sonophotolysis experiments in the presence of S:Y0.8Ga0.2InO3 finding a promising synergistic effect of UV-visible electromagnetic waves and 38 kHz ultrasound waves in producing H2

Comprehensive Investigation of the Photophysical Behavior of Oligopolyfurans

The photophysical properties of (oligo)polyfurans (Fn, where n = 1−5 represents the number of rings) are presented for the first time for 2−4 rings and compared to parallel data for the corresponding (oligo)polythiophenes (Tn). The quantum yields of fluorescence of the polyfurans are consistently considerably greater (5−50-fold), and that of the triplet occupation, considerably smaller (2.5−4) than for the polythiophenes. The kF of the Fn set vary from about equal (n = 4) to ≈4-fold greater (n = 2) than for the Tn set. The kISC of the Fn set are from 10−25-fold smaller than for the Tn set. The Fn set shows very little internal conversion (except some for F2) and less than for the Tn set. Triplet lifetimes of the Fn set are ≈3 fold less than for the Tn set. The lowest excited singlet state of the polyfurans is of 1Bu (or 1B1) character and not 1Ag. The magnitude of the intersystem crossing is essentially constant as a function of n for the Fn set whereas there is a large decrease for the Tn set as n increases. This indicates a difference in the mechanism for intersystem crossing as is discussed. The π-delocalization is greater for the Tn set than for the Fn set. It is not possible to clearly distinguish whether cis and trans conformers simultaneously exist (most likely for F2), or the magnitude of inter-ring bond twisting but the latter appears to be no more than 20−30 degrees and the virtual molecules at 77 K are clearly more planar than at room temperature

Sulfur radical cations. Kinetic and product study of the photoinduced fragmentation reactions of (phenylsulfanylalkyl)trimethylsilanes and phenylsulfanylacetic acid radical cations

Laser and steady-state photolysis, sensitized by NMQ(+), of PhSCH(R)X 1-4 (R = H, Ph; X =SiMe3 CO2H) Was carried out in CH3CN. The formation of 1(+.)-4(+.) was clearly shown. All radical cations undergo a fast first-order fragmentation reaction involving C-Si bond cleavage with 1(+.) and 2(+.) and C-C bond cleavage with 3(+.) and 4(+.). The desilylation reaction of 1(+.) and 2(+.) was nucleophilically assisted, and the decarboxylation rates of 3(+.) and 4(+.) increased in the presence of H2O. A deuterium kinetic isotope effect of 2.0 was observed when H2O was replaced by D2O. Pyridines too were found to accelerate the decarboxylation rate of 3(+.) and 4(+.). The rate increase, however, was not a linear function of the base concentration, but a plateau was reached. A fast and reversible formation of a H-bonded complex between the radical cation and the base is suggested, which undergoes C-C bond cleavage. It is probable that the H-bond complex undergoes first a rate determining proton-coupled electron transfer forming a carboxyl radical that then loses CO, The steady-state photolysis study showed that PhSCH3 Was the exclusive product formed from I and 3 whereas [PhS(Ph)CH-](2) was the only product with 3 and 4

Photophysical properties of quinones and their interaction with the photosynthetic reaction centre

Photophysical properties of tetramethyl-1,4-benzoquinone (TMBQ) and 2,6-dimethoxy-1,4-benzoquinone (DMOBQ) in solution and their interactions with the photosynthetic reaction centre (RC) isolated from the photosynthetic bacterium Rhodobacter sphaeroides have been investigated in this work. For these two benzoquinone derivatives an efficient ISC process which leads to the population of the lowest triplet state of the molecules upon direct excitation was observed. The presence of RC does not alter the properties of the triplet state of DMOBQ suggesting that interactions are negligible; on the other side RC efficiently quenched the T1 state of TMBQ. The behavior is rationalized in terms of redox potentials of quinones and kinetic characteristics of their transients