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Shen Y, An Z, Liu H, Yang B, Zhang Y. Excitation-Dependent Multicolour Luminescence of Organic Materials: Internal Mechanism and Potential Applications. Angew Chem Int Ed Engl 2023; 62:e202214483. [PMID: 36346193 DOI: 10.1002/anie.202214483] [Citation(s) in RCA: 14] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2022] [Revised: 11/07/2022] [Accepted: 11/08/2022] [Indexed: 11/10/2022]
Abstract
Excitation-dependent emission (Ex-de) materials have been of considerable academic interest and have potential applications in real life. Such multicolour luminescence is a characteristic exception to the ubiquitously accepted Kasha's rule. This phenomenon has been increasingly presented in some studies on different luminescence systems; however, a systematic overview of the mechanisms underlying this phenomenon is currently absent. Herein, we resolve this issue by classifying multicolour luminescence from single chromophores and dual/ternary chromophores, as well as multiple emitting species. The underlying processes are described based on electronic and/or geometrical conditions under which the phenomenon occurs. Before we present it in categories, related photophysical and photochemical foundations are introduced. This systematic overview will provide a clear approach to designing multicolour luminescence materials for special applications.
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Affiliation(s)
- Yunxia Shen
- Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Department of Chemistry, Zhejiang Normal University, Yingbin Road No. 688, Jinhua, 321004, P. R. China
| | - Zhongfu An
- Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), Nanjing, 211816, P. R. China
| | - Haichao Liu
- State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China
| | - Bing Yang
- State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, 130012, P. R. China
| | - Yujian Zhang
- Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Department of Chemistry, Zhejiang Normal University, Yingbin Road No. 688, Jinhua, 321004, P. R. China
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2
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Szemik-Hojniak A, Deperasińska I, Erez Y, Gawłowska M, Jerzykiewicz L. Ultrafast excited state dynamics of pyridine N-oxide derivative in solution; femtosecond fluorescence up-conversion and theoretical calculations. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2023; 285:121896. [PMID: 36183536 DOI: 10.1016/j.saa.2022.121896] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/29/2022] [Revised: 08/29/2022] [Accepted: 09/13/2022] [Indexed: 06/16/2023]
Abstract
In this study we have investigated 2-ethylamino-4-nitro-6-methyl pyridine N-oxide (2E6M) molecule that belongs to important group of Proton Coupled Electron Transfer (PCET) compounds where both the charge transfer (CT) and proton transfer processes in excited states may proceed. In this case, this is possible due to the donors and acceptors of electrons and protons in this system, as well as due to the presence of intramolecular {N-H… O [2,566(3) Å}, hydrogen bond.Using stationary and time-resolved spectroscopy, as well as quantum chemical calculations on the DFT and TD DFT B3LYP/6-31G (d,p) level of theory, a partial CT nature of the S0 → S1 transition in both tautomeric forms (N and T) has been revealed. Additionally, the excited state intramolecular proton transfer (ESIPT) process shown to be more favorable in apolar and weakly polar solvents than in strongly polar acetonitrile (EN(S1) > ET(S1). The displacement of charge from the amine group and the ring to the nitro group has been observed on the changing shapes of the HOMO and LUMO orbitals involved in this transition what further quantitatively allowed to realize the increase in the dipole moment of both forms in the electronic excited state. The calculations show that in two solvents with radically different polarity (heptane, acetonitrile), dipole moments of both forms are very similar [in acetonitrile uN(S1) and uT(S1) are 11.0 D and 11.5 D, respectively]. Hence, in polar media both forms can be stabilized in a comparable manner. This made it difficult for us to assign a single fluorescent band in acetonitrile to one of the tautomeric forms. However, it seems that due to application of time-resolved spectroscopy, this problem has been clarified. The TCSPC decay curve in acetonitrile with an ultrafast lifetime assigned to the (N) form, along with the femtosecond up-conversion signals that demonstrated only an ultrafast decay without any rise-time of a new excited (T) species, allowed us to conclude that in 2E6M in strongly polar solvent the ESIPT does not occur.The unique fluorescence band origins from the (N) form. In protic solvents, the significant kinetic isotopic effects have provided us with conclusive evidence for the presence of the solvent-assisted ESIPT process. Furthermore, it was noticed that the fluorescence lifetime in D2O (100-120 fs) estimated from the up-conversion signals is about 40 times shorter relative to methanol. This may suggest that the sine qua non for the ESIPT process in 2E6M in protic solvents is the formation of a complex with a solvent molecule in the hydrogen bridge between the proton donor and proton acceptor, respectively.
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Affiliation(s)
- Anna Szemik-Hojniak
- Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland; The Angelus Silesius State University of Applied Sciences, Institute of Health, Zamkowa 4 str 58-300 Wałbrzych, Poland.
| | - Irena Deperasińska
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46,02-668 Warsaw, Poland
| | - Yuval Erez
- Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
| | - Monika Gawłowska
- The Angelus Silesius State University of Applied Sciences, Institute of Health, Zamkowa 4 str 58-300 Wałbrzych, Poland
| | - L Jerzykiewicz
- Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland
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3
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Prediction of structural, vibrational and nonlinear optical properties of small organic conjugated molecules derived from pyridine. J Mol Struct 2019. [DOI: 10.1016/j.molstruc.2019.01.043] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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4
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Investigation of the Intermolecular Hydrogen Bonding Effects on the Intramolecular Charge Transfer Process of Coumarin 340 in Tetrahydrofuran Solvent. J CLUST SCI 2018. [DOI: 10.1007/s10876-018-1371-9] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
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Demchenko AP, Tomin VI, Chou PT. Breaking the Kasha Rule for More Efficient Photochemistry. Chem Rev 2017; 117:13353-13381. [DOI: 10.1021/acs.chemrev.7b00110] [Citation(s) in RCA: 210] [Impact Index Per Article: 30.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Affiliation(s)
- Alexander P. Demchenko
- Palladin
Institute of Biochemistry, National Academy of Sciences of Ukraine, 9 Leontovicha Street, Kyiv 01030, Ukraine
| | - Vladimir I. Tomin
- Institute
of Physics, Pomeranian University in Słupsk, ul. Arciszewskiego, 22b, Słupsk 76-200, Poland
| | - Pi-Tai Chou
- Department
of Chemistry, National Taiwan University, 1 Roosevelt Road Section 4, Taipei 106, Taiwan
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6
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Shimizu T, Manita S, Yoshikawa S, Hashimoto K, Miyazaki M, Fujii M. The mechanism of excited-state proton transfer in 1-naphthol–piperidine clusters. Phys Chem Chem Phys 2015; 17:25393-402. [DOI: 10.1039/c5cp03620h] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Photoexcitation directly triggers proton transfer in 1-naphthol–(piperidine)n. This mechanism is essentially different from 1-naphthol–(NH3)n in which the internal conversion process is required to promote excited-state proton transfer.
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Affiliation(s)
- Toshihiko Shimizu
- Chemical Resources Laboratory
- Tokyo Institute of Technology
- Yokohama 226-8503
- Japan
| | - Shun Manita
- Chemical Resources Laboratory
- Tokyo Institute of Technology
- Yokohama 226-8503
- Japan
| | - Shunpei Yoshikawa
- Chemical Resources Laboratory
- Tokyo Institute of Technology
- Yokohama 226-8503
- Japan
| | - Kenro Hashimoto
- Department of Chemistry
- Graduate School of Science and Engineering
- Tokyo Metropolitan University
- Hachioji
- Japan
| | - Mitsuhiko Miyazaki
- Chemical Resources Laboratory
- Tokyo Institute of Technology
- Yokohama 226-8503
- Japan
| | - Masaaki Fujii
- Chemical Resources Laboratory
- Tokyo Institute of Technology
- Yokohama 226-8503
- Japan
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Szemik-Hojniak A, Wiśniewski Ł, Deperasińska I, Makarewicz A, Jerzykiewicz L, Puszko A, Erez Y, Huppert D. The impact of solvent polarity on intramolecular proton and electron transfer in 2-alkylamino-4-nitro-5-methyl pyridine N-oxides. Phys Chem Chem Phys 2012; 14:8147-59. [DOI: 10.1039/c2cp40591a] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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8
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Klerk JD, van Stokkum IHM, Szemik-Hojniak A, Deperasińska I, Gooijer C, Zhang H, Buma WJ, Ariese F. Excited State Processes of 2-Butylamino-6-methyl-4-nitropyridine N-oxide in Nonpolar Solvents. A Transient Absorption Spectroscopy Study. J Phys Chem A 2010; 114:4045-50. [DOI: 10.1021/jp909468h] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Joost de Klerk
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Ivo H. M. van Stokkum
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Anna Szemik-Hojniak
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Irena Deperasińska
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Cees Gooijer
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Hong Zhang
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Wybren-Jan Buma
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
| | - Freek Ariese
- Laser Centre Vrije Universiteit Amsterdam, The Netherlands, Department of Chemistry, University of Wroclaw, Poland, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland, and Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, The Netherlands
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9
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Makarewicz A, Szemik-Hojniak A, van der Zwan G, Deperasińska I. Excited-State Forms of 2-Methylamino-6-methyl-4-nitropyridine N-Oxide and 2-Butylamino-6-methyl-4-nitropyridine N-Oxide. J Phys Chem A 2009; 113:3438-46. [DOI: 10.1021/jp8073927] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Artur Makarewicz
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland, Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland, and Department of Analytical Chemistry and Applied Spectroscopy, Laser Centre, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
| | - Anna Szemik-Hojniak
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland, Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland, and Department of Analytical Chemistry and Applied Spectroscopy, Laser Centre, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
| | - Gert van der Zwan
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland, Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland, and Department of Analytical Chemistry and Applied Spectroscopy, Laser Centre, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
| | - Irena Deperasińska
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland, Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 st, 50-383 Wrocław, Poland, and Department of Analytical Chemistry and Applied Spectroscopy, Laser Centre, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
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Crystal structure, spectroscopic, and theoretical studies of 2-methylamino-6-methyl-4 nitropyridine N-oxide. J Mol Struct 2009. [DOI: 10.1016/j.molstruc.2008.10.029] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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de Klerk JS, Szemik-Hojniak A, Ariese F, Gooijer C. Cryogenic fluorescence and absorption spectroscopy studies on monomeric and dimeric species of 2-butylamino-6-methyl-4-nitropyridine N-oxide. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2009; 72:144-150. [PMID: 18977168 DOI: 10.1016/j.saa.2008.09.003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/20/2008] [Revised: 08/25/2008] [Accepted: 09/11/2008] [Indexed: 05/27/2023]
Abstract
2-Butylamino-6-methyl-4-nitropyridine-N-oxide (2B6M) belongs to a group of compounds that can undergo not only excited-state intra-, but also intermolecular proton transfer. The latter of course requires the presence of dimeric species. Previously, we have shown that for 2B6M in aprotic non-polar solvents in the liquid state such dimers play no role. Under these conditions, only one single monomeric species exists, exhibiting anomalous fluorescence behavior, i.e. proton transfer not only starting from the lowest excited electronic singlet state, but also from higher excited states. However, we also noted that under frozen, crystalline matrix conditions more species show up in the spectra. In order to study this multi-species system in more detail, we present absorption and fluorescence experiments on 2B6M, recorded in n-octane at various temperatures between 293 and 5 K. High-resolution spectra are included, not only in fluorescence but also in absorption. We demonstrate that under cryogenic conditions three species can be discerned, two of these providing high-resolution spectra with their main 0-0 lines around 452 and 465 nm, respectively. A detailed vibrational analysis of their emission spectra is included. The third species gives broad-banded spectra, in absorption extending to about 520 nm with its long-wavelength maximum around 460 nm, in emission with a maximum around 535 nm. We tentatively assign the three species to a monomer, a H-bonded dimer and a strongly interacting (pi-pi-stacked) dimer, respectively. We conclude from the excitation spectra that (anomalous) intramolecular proton transfer at higher excited states is still operative under cryogenic conditions. Indications for excited-state intermolecular proton transfer in the stacked dimeric species were not found.
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Affiliation(s)
- Joost S de Klerk
- Analytical Chemistry & Applied Spectroscopy, Laser Centre Vrije Universiteit Amsterdam, De Boelelaan 1083, NL-1081 HV Amsterdam, The Netherlands
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12
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High-Resolution Fluorescence Studies on Excited-State Intra- and Intermolecular Proton Transfer. REVIEWS IN FLUORESCENCE 2009. [DOI: 10.1007/978-0-387-88722-7_12] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
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13
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Ganesan P, Baggerman J, Zhang H, Sudhölter EJR, Zuilhof H. Femtosecond time-resolved photophysics of 1,4,5,8-naphthalene diimides. J Phys Chem A 2007; 111:6151-6. [PMID: 17585850 DOI: 10.1021/jp0719558] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
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.
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Affiliation(s)
- Palaniswamy Ganesan
- Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands
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de Klerk JS, Szemik-Hojniak A, Ariese F, Gooijer C. Intramolecular Proton-Transfer Processes Starting at Higher Excited States: A Fluorescence Study on 2-Butylamino-6-methyl-4-nitropyridine N-Oxide in Nonpolar Solutions. J Phys Chem A 2007; 111:5828-32. [PMID: 17559196 DOI: 10.1021/jp0672813] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations.
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Affiliation(s)
- Joost S de Klerk
- Analytical Chemistry & Applied Spectroscopy, Laser Centre Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
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Szemik-Hojniak A, Deperasiñska I, Jerzykiewicz L, Sobota P, Hojniak M, Puszko A, Haraszkiewicz N, van der Zwan G, Jacques P. Crystal Structure, Spectroscopic, and Theoretical Investigations of Excited-State Proton Transfer in the Doubly Hydrogen-Bonded Dimer of 2-Butylamino-6-methyl-4-nitropyridine N-Oxide. J Phys Chem A 2006; 110:10690-8. [PMID: 16970358 DOI: 10.1021/jp062405x] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.
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Affiliation(s)
- Anna Szemik-Hojniak
- Faculty of Chemistry, University of Wrocław, ul. Joliot-Curie 14, 50-383 Wrocław, Poland.
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