Synthesis and Ultrafast Dynamics of a DonorâAcceptorâDonor

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Synthesis and Ultrafast Dynamics of a Donor-AcceptorDonor Molecule Having Optoelectronic Properties Mantena V. Niladri Raju, Maneesha Esther Mohanty, Prakriti Ranjan Bangal, and Jayathirtha Rao Vaidya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02063 • Publication Date (Web): 27 Mar 2015 Downloaded from http://pubs.acs.org on March 30, 2015

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The Journal of Physical Chemistry

Synthesis and Ultrafast Dynamics of a Donor-Acceptor-Donor Molecule Having Optoelectronic Properties M.V. Niladari Raju†, Maneesha Esther Mohanty*†, Prakriti Ranjan Bangal*‡, Jayathirtha Rao Vaidya †,§

†

Crop Protection Chemicals Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India.

‡

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India.

§

Network Institute for Solar Energy, New Delhi, India

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ABSTRACT The use of push-pull molecules having donor (D) and acceptor (A) parts arranged in different shapes are being widely studied for application in various optoelectronic devices. In this study three covalently linked D-A-D molecules containing three different carbazole derivatives as donor, anathracene as acceptor, and thiophene as spacer have been synthesized and characterized. A detailed step-wise study has been carried out using anthracene, thiophene-anthracene and carbazole-thiophene-anthracene derivatives so as to indicate the role of each moiety in the molecule. Steady state fluorescence, time resolved fluorescence, transient absorption and cyclic voltammetric methods have been employed to understand the intramolecular charge separation (CS) and charge recombination (CR) dynamics in solvents of different polarity. The thermodynamic free-energy obtained by measuring the redox potential and singlet state energy suggested the possibility of electron transfer from the excited singlet state of carbazole moiety to the anthracene entity. Steady state and timeresolved fluorescence studies showed fluorescence quenching of anthracence moiety upon addition of thiophene while highly efficient fluorescence quenching of anthracene moiety was observed on addition of carbazole derivatives. Femtosecond transient absorption studies confirmed the electron transfer to be the mechanism of fluorescence quenching, in which formation and recombination dynamics of electron-transfer products, anthracene radical anion and carbazole radical cation, were analyzed. The rate of charge separation, kCS, was found to be very high for all the three molecules and it was on the order of 1010 -1011 s-1 while, the rate of charge recombination, kCR, was observed to be much slower and it was on the order of 108-109 s-1. The step-wise structure-property relationship leading to the efficient


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charge separated sate established in the systems studied would help in the improved design of optoelectronic materials that use these moieties.



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INTRODUCTION Push-pull molecules having electron-withdrawing (acceptor) and electron-donating substituents (donor) with spacers that enhance π-conjugation play a vital role in the efficiency of various optoelectronic materials1 such as, organic photovolataics (OPV)2,3, dye sensitized solar cell (DSSC),4-6, organic light emitting diodes (OLED)7-13, and materials showing 2-photon absorption (2-PA)14-17, non-linear optics (NLO)18-24.The separation of charges in the excited state is a prerequisite for the application of these molecules in optoelectronic devices. Small push-pull molecules having different electron-withdrawing and accepting moieties are being widely designed for application as donor molecules in solar cells. Design of small donor molecules for use in bulk heterojunction solar cells has been largely focused on developing donor molecules having appropriate band gap and can readily donate electrons to the standard acceptor molecules, such as fullerene, used in solar cells.2 The properties of these donor molecules in terms of the HOMO / LUMO energy levels and band gap are modified by designing appropriate ‘push-pull’ donor molecules to improve the overall efficiency of the solar cell. Charge injection can be improved by lowering the LUMO and electron mobility depend on the kind of π-conjugation used and inter molecular interaction. Hirade et.al. had designed donoracceptor molecules of different strength to reduce the recombination of geminate pairs which is responsible for limiting the efficiency of BHJ solar cells.3 Push-pull molecules have also been used to increase the absorption in the visible and near infrared region and manipulating the HOMO levels and band gap.2 Better interfacial electron injection for dye sensitized solar cells was achieved by using donor-acceptor metal free dyes.4


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The push-pull molecules play an important role as hole transporting materials and electron transporting materials in OLED devices.7,8 Hole transporting material used in OLED should possess electron donating property and their anodic oxidation potential should be reversible to form stable cation radicals9 and electron transporting material should undergo a one electron reduction of the neutral molecule coupled with the oxidation of its anion to form stable anion radical.10 Donor-acceptor molecules have been used to design red emitters to reduce molecular aggregation,11 vapor phase processable blue emitters12 and charge transporters13 for OLED applications. Anthracene, carbazole and thiophene are few among a range of moieties being used in designing new optoelectronic materials. Anthracene, has good photophysical and charge transporting properties and is an excellent blue emitting material25-27 with high carrier mobility, better hole-injection ability and wide band gap28 while carbazole is a strong electron rich molecule29-32 and the two have been widely used as optoelectronic materials. Design of organic materials having promising 2-PA requires efficient intramolecular charge-transfer. Several donor-acceptor molecules have been synthesized with different architectures for better 2-PA and quadropolar molecules have shown the highest potential.14,15

Molecule having D-π-A-π-D

design using carbazole and anthracene16 were recently reported to have high 2-PA cross-section. Anthracene and carbazole derivatives have also been used to design other materials17 for improving the 2-photon absorption property. NLO materials have been designed as linear donorπ-acceptor molecules18, 19 leading to diopole-dipole interactions, two D-A pairs linked together20 and 3-dimensional structures containing acceptor in the centre to which donors are attached at different positions.21 Second order non-linear optical activity of an anthracene based intramolecular charge transfer molecule was studied by Planells et.al.22 Perepichka et.al. 5 ACS Paragon Plus Environment


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synthesized a series of donor-acceptor molecules for NLO applications using intra molecular charge transfer within the D-A molecule and further formation of intermolecular charge-transfer complexes (CTC) with anthracene and other molecules.23,24 Derivatives of carbazole form one of the most efficient hole transporting materials and ambipolar host materials33 in OLEDs. Conjugates of carbazole and fluorene have been used to synthesize host materials for green, red34 and blue35 phosphorescence OLEDs. Li et.al. theoretically calculated the charge mobilities of anthracene based on non donor-acceptor molecule for hole and electron transfer.36 Zhu et.al. had coupled anthracene and carbazole with a fluorine bridge to synthesize emitting material with decreased tendency to crystallize and having high glass transition temperature.37 Anthracene dimers and trimers used as active layers have exhibited good hole mobilities38 based on the design of the molecules. Ando et. al. designed anthracene derivatives having thiophene as an electron-donating part for developing charge transfer materials.39 Anthracene forms radical ions in acids, by gamma irradiation and X-ray radiation and far UV light.40 In the ground state a neutral anthracene molecule can lose an electron to form radical cation and carbonium ion on reaction with sulphuric acid.40 Electron transfers takes place from sodium naphthalene to anthracene in presence of water to form anthracene radical anion.41 Thiophene and polythiophenes are conductive material and used in different molecular electronic devices and their ultrafast dynamics have been studied extensively.42,43 Materials for DSSC consisting of thiophene units asymmetrically functionalized by N-aryl carbazole were used to correlate the photocurrent and recombination effects 5 in DSSC materials. Series of anthracene based DSSC were synthesized having mono and di carboxylic acid group. 6


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Photo-driven charge separation (CS) is a prerequisite initial step in photosynthesis, where light is converted into energy, and other photosynthesis mimicking systems such as organic photovoltaics. Presence of a short-lived excited state that transforms into a longer-lived radical ion pairs is important for optoelectronic materials. However, charge recombination (CR) is a desired process in organic light emitting diode systems and an undesired energy-wasting process for solar cells.44 Understanding the underlying mechanisms involved in CS and CR of these materials and to establish a structure-property relationship is essential to further manipulate the CS and CR dynamics. Flavin et. al. prepared a novel donor-acceptor material, consisting of boron azadipyrromethene donor covalently attached to a highly functionalized single-wall carbon nanotube acceptor which formed a radical ion pair state with a lifetime of about 1.2 ns.45 Zhang and Wang synthesized a dyad, consisting of morpholine as donor and silicon phthalocyanine as acceptor and observed radical cation and anion having a lifetime of 4.8 ns by transient absorption spectra.46 Several fullerene based dyads have been synthesized with the formation of radical ions which recombine in ~1-2 ns.47-49 Due to an incessant need to improve the efficiency of organic optoelectronic devices and the importance of understanding the structure-property relationship of widely used moieties in the design of optoelectronic materials, this work was carried out and is relevant to a wide range of fundamental research and applications. In this work, a series of donor-acceptor-donor materials using different carbazole derivatives as donor, anthracene as acceptor and thiophene as spacer, have designed, synthesized and characterized using steady state fluorescence spectroscopy and cyclic voltametry. The excited state phenomena have been characterized by time-resolved fluorescence and femtosecond transient absorption spectroscopy. The molecules



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show efficient intramolecular charge separation forming radical ions which remain stable for relatively long time periods. EXPERIMENTAL SECTION Synthesis All chemicals were purchased from Sigma Aldrich and used without further purification. 2,6-Di-tert-butylanthracene50 (DAN, 1). To a mixture of anthracene (12 g, 67.4 mM) and tbutanol (19.3 mL, 202.2 mM) in a 250 mL two necked round bottom flask, trifluoroacetic acid (90 mL) was added at room temperature and the resultant mixture was refluxed for 24 h. After cooling down to room temperature, water was added to the mixture. Brown colored solid obtained was filtered and washed with hexane to procure white solid. Isolated yield: 10.4 g, 53%. 1

H NMR (CDCl3, 500 MHz): δ 8.31 (s, 2H), 7.92 (d, 2H, J = 8.99 Hz), 7.86 (s, 2H), 7.54 (dd,

2H, J1 =8.99 Hz, J2 = 2.00 Hz), 1.44 (s, 18H). ESI-MS m/z Calcd for C22H26: 290; found: 290. 2,6-Di-tert-butylanthracene-9,10-dione51 (2). In a 250 mL two necked round bottom flask, 2,6di-tert-butyl anthracene (6.0 g, 20.7 mM) was dissolved in glacial AcOH (100 mL). The mixture was refluxed for 10 min and cooled down to RT. Separately CrO3 solution was prepared by dissolving CrO3 (6.2 g, 62.0 mM) in 10 mL water and 25 mL glacial AcOH was added to the mixture. The reaction mixture was heated to reflux and stirred at reflux temperature for 4 h. After cooling down to RT, ice- water was added and the light green solid formed was filtered, washed with hot water followed by 1M NaOH (60 mL) solution. Thus obtained solid was further washed with water and re-crystallized from toluene to yield light green crystals of 2,6-di-tertbutylanthracene-9,10-dione. Isolated yield: 5.4 g, 82%. 1H NMR (CDCl3, 300 MHz): δ 8.33 (d,


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2H, J = 1.51 Hz), 8.25 (d, 2H, J = 8.30 Hz), 7.82 (dd, 2H, J1 = 8.30 Hz, J2 = 2.26 Hz), 1.42 (s, 18H). ESI-MS m/z Calcd for C22H24O2: 320; found: 321. 5,5'-(2,6-Di-tert-butylanthracene-9,10-diyl)bis(2-bromothiophene) (3).

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Under nitrogen

atmosphere, n-BuLi (15.5 mL, 2.0 M in cyclohexane) was added drop wise to the solution of 2, 5 dibromo thiophene (3.6 mL, 30.4 mM) in dry THF (80 mL) at -78oC and the mixture was stirred at the same temperature for 30 min. To the above mixture, 2, 6-di-tert-butylanthracene-9, 10dione (3.9 g, 12.1 mM) in dry THF (40 mL) was added drop wise at -78oC and the temperature of the reaction mixture was raised to RT. After stirring at RT for 8h, reaction mixture was concentrated and passed through a small silica gel bed. To the residue obtained, potassium iodide (10.04 g, 60.5 mM), NaH2PO2.H2O (12.7 g, 121 mM) and acetic acid (100 mL) were added and the mixture was refluxed for 12h. After cooling to RT, the pale yellow solid formed was filtered and washed with water. The crude residue was purified using silica gel column chromatography (hexane as eluent) to obtain the target compound as light yellow powder. Isolated yield: 6.76 g, 91 %. 1H NMR (CDCl3, 500 MHz): δ 7.85 (d, 2H J = 9.15 Hz), 7.80 (d, 2H, J = 1.52 Hz), 7.53 (dd, 2H, J1 = 9.30 Hz, J2 = 1.67 Hz), 7.27 (d, 2H, J = 3.66 Hz), 6.95 (d, 2H, J = 3.66 Hz), 1.33 (s, 18H).

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C NMR (127.77 MHz, CDCl3): δ 148.04, 145.55, 137.08, 131.05, 130.83, 130.01,

129.96, 129.75, 126.01, 125.36, 120.54, 35.02, 30.77. IR: νmax/cm-1 3445.26, 3061.59, 2956.11, 2862.61, 1361.06, 960.80, 794.20, 594.39. EI-HRMS m/z Calcd for C30H28Br2S2: 609.99999; found: 609.99900.

3,6-Di-tert-butyl-9H-carbazole.

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To the mixture of 9-H-Carbazole (4.0 g, 23.92 mM) and

ZnCl2 (9.78 g, 90 mM) dissolved in nitrometahne (80 mL), 2-chloro-2-methylpropane (5.54 mL, 63.0 mM) was added drop wise under inert atmosphere. The reaction mixture was stirred overnight at RT and hydrolyzed with water (100 mL). The resultant solution was extracted with



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dichloromethane (3 x 60 mL) and the combined organic extract was washed with brine, dried over anhydrous MgSO4, filtered and concentrated under reduced pressure. The crude product obtained was purified using silica gel column chromatography using petroleum ether and dichloromethane in 10:1 (v/v) ratio. Isolated yield: 5.95 g, 89 %. δ 8.07 (d, 2H, J = 1.07 Hz), 7.83 (s b, 1H), 7.46 (dd, 2H, J = 8.49, J = 1.88), 7.33 (d, 2H, J = 8.49 Hz), 1.45(S, 18H).ESI-MS m/z Calcd for C20H25N: 279; found: 280.

11H-Benzo[a]carbazole. 54 To the mixture of α-Tetrlone (1 g, 6.84 mM) and phenylhydrazene hydrochloride (0.613 g, 4.24 mM) in ethanol (20 mL), glacial acetic acid (5 drops) was added and the mixture was refluxed for 8 h under nitrogen atmosphere. After cooling to RT, the mixture was concentrated under reduced pressure and partitioned between ethyl acetate and water. Ethyl acetate layer was collected, dried over anhydrous Na2SO4, filtered and solvent was evaporated to obtain dihydrobenzocarbazole as an orange color residue. Yield: 1.47 g. To the obtained dihydrobenzocarbazole (1.4 g, 6.7 mM) in m-xylene (30 mL), chloranil (2.4 g, 9.8 mM) was added and the mixture was refluxed under nitrogen atmosphere for 10 h. The temperature of the reaction mixture was allowed to reach RT and the excess chloranil added was quenched with the addition of 10% NaOH solution. The resulting solution was extracted with ethyl acetate; ethyl acetate layer was collected and concentrated under reduced pressure. The crude substance obtained was purified using silica gel column chromatography with 95:5 (v/v) of petroleum ether and ethyl acetate. Combined yield for the two steps: 1.11 g, 74%. 1H NMR (CDCl3, 500 MHz): δ 8.78 (s, 1H), 8.10-8.15(m, 3H), 8.01(d, 1H, J = 8.54 Hz), 7.68-7.51 (m, 4H), 7.46-7.40 (m, 1H), 7.33-7.28 (m, 1H). ESI-MS m/z Calcd for C16H11N: 217; found: 217. CTATC

(5).

To

the

mixture

of

5,5'-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-

bromothiophene) (1.0 g, 1.63 mM), carbazole (0.82 g, 4.9 mM) and K2CO3 (2.23 g, 16.3 mM) in 10 ACS Paragon Plus Environment

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DMF (40 mL) copper(I)iodide (46 mg,15 mol%), 1,10 phenanthroline (44 mg,15 mol%) and 18crown-6 (15 mg) were sequentially added and the mixture was refluxed for 24 h under nitrogen atmosphere. After cooling down to RT, solvent was removed using rotary evaporator and the residue obtained was suspended in ice-water and extracted with dichloromethane (3 x 50 mL). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered and solvent was evaporated. The crude product obtained was purified using silica gel column chromatography with hexane eluent, to get white solid. Isolated yield: 0.874 g, 68 %. 1H NMR (CDCl3, 500 MHz): δ 8.16 (d, 4H J = 8.30 Hz), 8.11 (d, 2H, J = 9.06 Hz), 8.03 (d, 2H, J1 = 1.51 Hz), 7.68 (d, 6H, J = 9.06 Hz), 7.54 (t, 4H, J = 8.30 Hz), 7.44 (d, 2H, J = 3.02 Hz).7.36 (t, 4H, J = 8.30 Hz), 7.3 (d, 2H, J = 3.02 Hz), 1.44(s, 18H).

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C NMR (127.77 MHz, CDCl3): δ 148.03,

142.08, 139.50, 138.28, 131.27, 130.12, 129.06, 128.35, 126.33, 126.19, 125.57, 125.31, 123.60, 120.79, 120.65, 120.31, 110.17, 35.14, 30.77. MALDI-MS m/z Calcd for C54H44N2S2: 784; found: 783.9. IR: νmax/cm-1: 2951, 1553, 1446, 1228, 744. [Element analysis] Calcd for C54H44N2S2: C, 82.61; H, 5.65; N, 3.57; S, 8.17; found: C, 80.96; H, 5.53; N, 3.51; S, 8.03.

DTATD (6). DTATD was synthesized by adopting the similar procedure used for the synthesis of CTATC using 5,5'-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-bromothiophene) (0.5gm, 0.82 mM), 3,6-di-tert-butyl-9H-carbazole (0.572 g, 2.0 mM), K2CO3 (1.3 g, 8.2 mM) copper(I)iodide (22 mg,15 mol%) 1,10 phenanthroline (22 mg,15 mol%) and 18-crown-6 (5mg) as starting materials. White solid was obtained after column chromatography using a 98:2 (v/v) mixture of petroleum ether and ethyl acetate. Isolated yield: 0.52g, 63%. 1H NMR (CDCl3, 500 MHz): δ 8.16 (s, 4H), 8.09 (d, 2H, J = 9.15 Hz), 8.02 (dd, 2H, J1 = 1.67 Hz, J2 = 1.67 Hz), 7.66 (dd, 2H, J1 = 9.15 Hz, J2 = 1.83 Hz), 7.62-7.56 (m, 8H), 7.39 (d, 2H, J = 3.50 Hz) 7.27 (d, 2H, J = 3.50 Hz), 1.50 (s, 18H), 1.44 (s,18H).

13

C NMR (127.77 MHz, CDCl3): δ 147.93, 143.58, 140,38, 11

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140.11, 137.54, 131.24, 130.17, 129.14, 128.25, 126.22, 125.48, 124.48, 123.94, 123.53, 120.81, 116.29, 109.59, 35.13, 34.79, 32.01, 30.92, 30.80. MALDI-MS m/z Calcd for C70H76N2S2: 1008 found 1007.92. IR: νmax/cm-1: 2957,1495,1362,1296,808 [Element analysis] Calcd for C70H76N2S2: C, 82.61; H, 5.65; N, 2.71; S, 8.17; found: C, 81.64; H, 7.48; N, 3.51; S, 6.23.

BTATB (7). BTATB was synthesized by adopting a procedure similar to that used for the synthesis of CTATC using 5,5'-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-bromothiophene) (0.30 g, 0.49 mM), 11H-benzo carbazole (0.267 g, 1.23 mM), K2CO3 (0.68 g, 4.9 mM) copper(I)iodide (13 mg, 15 mol%) 1,10 phenanthroline (12 mg, 15 mol%) and 18-crown-6 (5mg) as starting materials. White solid was obtained after column chromatography and petroleum ether-ethyl acetate (97:3 v/v) as eluent. Isolated yield: 0.23g, 52%. 1H NMR (CDCl3, 500 MHz): δ 8.28-8.18 (m, 10H), 8.07 (d, 2H, J = 9.30 Hz), 7.80 (d, 2H, J = 8.39 Hz), 7.74 (dd, 2H, J1 = 9.15 Hz, J2 = 1.83 Hz), 7.60-7.50 (m, 10H), 7.46-7.41 (m, 2H), 7.39 (d, 2H, J = 3.5 Hz), 1.50 (s,18H).

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C NMR (127.77MHz,CDCl3): δ 148.19, 143.17, 142.36, 140.18, 136.04, 133.60,

131.06,129.30, 129.04, 128.64, 127.83, 126.07, 125.78, 125.55, 125.48, 125.03, 123.75, 122.22, 122.00, 121.80, 121.10, 120.85, 119.91, 119.57, 118.96, 110.61, 35.23, 30.95. MALDI-MS m/z Calcd for C70H76N2S2: 884 found 884 IR: νmax/cm-1: 2958, 1559, 1459, 1376, 744. TAT (4): Under nitrogen atmosphere, n-BuLi (15.5 mL, 2.0 M in cyclohexane) was added drop wise to the solution of 2-bromo thiophene (2.9 mL, 30mM) in dry THF (80 mL) at -78oC and the mixture was stirred at the same temperature for 30 min. To the above mixture, 2,6-di-tertbutylanthracene-9,10-dione (3.9 g, 12.1 mM) in dry THF (40 mL) was added drop wise at -78oC and the temperature of the reaction mixture was raised to RT. After stirring at RT for 8 hours, reaction mixture was concentrated and passed through a small silica gel bed. To the residue obtained, potassium iodide (10.04 g, 60.5 mM), NaH2PO2.H2O (12.7 g, 121 mM) and acetic acid 12 ACS Paragon Plus Environment

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(100 mL) were added and the mixture was refluxed for 12h. After cooling down to RT, the pale yellow solid formed was filtered and washed with water. The crude residue was purified using silica gel column chromatography (hexane as eluent) to obtain the target compound as light yellow powder. Isolated yield: 5.86 g, 85 %. 1H NMR (CDCl3, 500 MHz): δ 7.81 (d, 2H J = 9.25 Hz), 7.76 (d, 2H, J = 1.51 Hz), 7.61 (d, 2H, J1 = 5.09 Hz), 7.48 (d, 2H, J = 9.25 Hz), 7.3 (t, 2H, J = 3.39 Hz),7.2 (d, 2 H,J = 3.39 Hz), 1.30 (s, 18H).

13

C NMR (127.77 MHz, CDCl3): δ 147.43,

139.26, 131.08, 130.12, 129.35,129.26, 127.02, 126.57, 126.23, 124.93, 12090, 34.94,30.76. IR: νmax/cm-1: 2957, 2864, 1458, 1369, 821, 697. ESI-MS m/z Calcd for C30H30S2: 454 found 454, ESI-HR-MS m/z Calcd for C30H30S2: 454.1789 found 454.17654. Cyclic Voltammetry Cyclic voltammetric measurements were performed on a PC-controlled CHI 620C electrochemical analyzer (CH instruments) in 1 mM solution of degassed dry dichloromethane at a scan rate of 100 mV s-1 using 0.1 M tetrabutylammoniumperchlorate (TBAP) as the supporting electrolyte. The glassy carbon was used as the working electrode, Ag/AgCl as the reference electrode and platinum wire as the counter electrode. The working electrode surface was first polished with 1 mm alumina slurry, followed by 0.3 mm alumina slurry on a micro cloth. It was then rinsed with millipore water and also sonicated in water for 5 min. The polishing and sonication steps were repeated twice. Steady State Measurements All solvents used were HPLC grade and were used without further purification. Stock solutions containing 10-3M samples were prepared in chloroform and were further diluted with the required solvents to get the 10-5 M final concentration in the cuvettes. All experiments were repeated at least thrice on different days using freshly prepared samples. The experiments were 13 ACS Paragon Plus Environment


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reproducible and the data shown is a representative set. UV-Vis measurements were carried out on Cary Series UV-Vis-NIR spectrophotometer, Agilent Technologies. Fluorescence measurements were carried out using Cary Eclipse Fluorescence Spectrophotometer, Agilent Technologies with excitation and emission slits at 5 nm. Time Resolved Measurement Fluorescence Up-Conversion/ Fluorescence Optical Gate. Detail description of our femtosecond laser apparatus was described elsewhere.55 However, in brief, for fluorescence up-conversion study FOG 100-DX system (CDP System Corp. Moscow, Russian Federation ) was used. Fundamental laser output (~500 mW at 800 nm) of Ti:sapphire oscillator (Mai Tai HP, Spectra Physics) was steered into CDP2015 frequency conversion unit to have second harmonic (SH). A beam splitter (BS) is used to split the input beam (SH of fundamental) to excitation and gate (fundamental residual pulses) beams. The excitation beam directed to a rotating sample cell with the help of six mirrors and one BS. A lens (f=40 mm) was used to focus excitation beam into the sample. A neutral density (ND) filter was used for the excitation attenuation. The gate beam was directed by two mirrors to gold-coated retro- reflector mirror connected to 8 ns optical delay line before being focused together with the fluorescence (collected by an achromatic doublet, f=80 mm) on 0.5 mm type-I BBO crystal. The angle of the crystal was adjusted to phase matching conditions at the fluorescence wavelength of interest. The up-converted signal (in the UV range) was focused with a lens (f=60 nm) to an input slit of the monochromator (CDP2022D). The intensity of the up-converted radiation was measured with a photomultiplier tube operating in the photon counting mode. Proper filters were used before the detector to eliminate parasitic light from the up-converted signal if any. The polarization of the excitation pulses was set at magic angle relative to that of the gate pulses using Berek's variable 14 ACS Paragon Plus Environment

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wave plate. The sample solutions were placed in a 0.6 mm or 1 mm rotating cell and absorbance of about ~0.6 at excitation wavelength generally used (yielding a concentration around 100-200 µM). The FWHM of the instrument response function (IRF) in this setup was calculated about 240 fs in the 0.4 mm cell and 280 fs in the 1 mm cell. Hence, a time resolution of

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