Photophysical Properties and Ultrafast Excited-State Dynamics of a

Article pubs.acs.org/JPCC

Photophysical Properties and Ultrafast Excited-State Dynamics of a New Two-Photon Absorbing Thiopyranyl Probe Kevin D. Belfield,*,†,‡ Mykhailo V. Bondar,§ Alma R. Morales,† Andrew Frazer,† Ivan A. Mikhailov,∥ and Olga V. Przhonska§ †

Department of Chemistry and ‡CREOL, The College of Optics and Photonics, University of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366, United States § Institute of Physics, Prospect Nauki, 46, Kiev-28, 03028, Ukraine ∥ Petersburg Nuclear Physics Institute, 188300, St. Petersburg, Russia ABSTRACT: Comprehensive linear photophysical and photochemical characterization, two-photon absorption (2PA) properties, and femtosecond excited-state dynamics of a symmetrical fluorene derivative 2-(2,6-bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)vinyl)-4H-thiopyran-4-ylidene)malononitrile (1) are reported. The linear one-photon absorption (1PA), steady-state fluorescence, excitation, and excitation anisotropy spectra of 1 were investigated in organic solvents of different polarities at room temperature, exhibiting rather complex absorption and emission behavior. The relatively strong 2PA of thiopyranyl 1 was investigated by the open aperture femtosecond Z-scan technique in the main long wavelength 1PA contour with maxima cross sections up to 600−700 GM. Femtosecond dynamics of the excited-state absorption (ESA) and gain processes in 1 exhibited fast complicated relaxation phenomena with a strong dependence on solvent polarity and a weak dependence on excitation wavelength. The nature of the observed transient absorption kinetics was explained based on the short-lived ESA bands of 1 and solvate relaxation phenomena. Quantum chemical calculations, based on density functional theory, were employed for additional analysis of the 1PA and 2PA properties of 1.

1. INTRODUCTION Investigations of fast dynamic processes in the ground and excited states of organic compounds with efficient two-photon absorption (2PA) play an important role in the development of a number of nonlinear optical technologies, including twophoton fluorescence microscopy (2PFM) biological imaging,1−3 two-photon optical power limiting,4−6 3D microfabrication, and optical data storage,7−9 two-photon photodynamic therapy, 10,11 stimulated emission depletion (STED),12−14 among others. The nature of fast electronic and vibrational relaxations determines a number of important linear photophysical, nonlinear optical, and photochemical parameters of organic molecules, and therefore is a subject of fundamental interest. Ultrafast excited state dynamics have been investigated for numerous organic compounds, including a β-carotene derivative,15 DNA bases,16 ligand−metal complexes,17 spirooxazines and naphthopyrans,18 carotenoids bound to pigment−protein complexes,19 and others. The molecular characteristics of photoinduced, charge-transfer processes,20 transient excited states with efficient absorption and gain properties,21−23 short-lived intermediates of certain photochemical reactions,24 and others were determined via well-developed pump−probe25,26 and time-resolved fluorescence27,28 techniques. Fluorene derivatives with efficient 2PA are promising candidates for most of the applications mentioned above, and, in particular, for two-photon STED microscopy,13,14 where excited-state dynamics processes play an © XXXX American Chemical Society

important role and need to be further investigated. In this paper, the synthesis, comprehensive linear photophysics, 2PA, and femtosecond transient absorption spectroscopy of a new symmetrical fluorene derivative, 2-(2,6-bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl)vinyl)-4H-thiopyran-4ylidene)malononitrile (1), is reported. The complex nature of the linear absorption and fluorescence bands of 1 in a number of aprotic solvents was manifested in steady-state and timeresolved spectroscopic data, including excitation anisotropy. The degenerate 2PA and excited-state absorption (ESA) spectra of 1 were obtained over a broad spectral range by the open-aperture Z-scan29 and pump−probe25 method, respectively, using a tunable 1 kHz femtosecond laser system. Ultrafast transient absorption kinetics of 1 were measured in solvents of different polarity under excitation in the first (S1) and higher excited (Sn) electronic states. The nature of 1PA and 2PA spectra of 1 was also analyzed by quantum chemical calculations based on density functional theory (DFT) with a number of hybrid functionals.

2. EXPERIMENTAL SECTION 2.1. General. Chemicals were purchased from Aldrich and Acros Chemical and were used without any further purification. Received: February 15, 2013 Revised: May 15, 2013

A

dx.doi.org/10.1021/jp4016454 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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22.8. HRMS-ESI theoretical m/z [M]+ = 188.0403, found, 188.0406, [M+Na]+ = 211.0300, found, 211.0307. 7-(Diphenylamino)-9,9-dihexyl-9H-fluorene-2-carbaldehyde (E). Under a nitrogen atmosphere, a mixture of 7-bromo9,9-dihexyl-9H-fluorene-2-carbaldehyde (3.5 g, 7.92 mmol), diphenylamine (2 g, 11.87 mmol), Pd(OAC)2 (0.043 g, 0.19 mmol), P(tBu)3 (0.086 g, 0.42 mmol), and Cs2CO3 (3.86 g, 11.84 mmol) in dry toluene (20 mL) was stirred and heated to 120 °C for 48 h. After cooling to room temperature, the reaction mixture was passed through a short plug (silica gel), and the filtered solution was concentrated to obtain a yellow− brownish oil. Purification was carried out by flash column chromatography (silica gel, first hexanes, then hexanes/CH2Cl2 1:1). A yellow oily compound was obtained (3.7 g, 88% yield). 1 H NMR (500 MHz, CDCl3) δ: 9.69 (s, 1H), 7.50 (dd, J = 7.5 Hz, 2H), 7.40 (dd, J = 8 Hz, 1H), 7.30 (dd, J = 8.5 Hz, 1H), 6.97−6.93 (m, 4H), 6.81 (dd, J = 6.5 Hz, 4H), 6.77 (d, J = 2 Hz, 1H), 6.74−6.71 (m, 4H), 1.72- 1.65 (m, 2H), 1.61−1.53 (m, 2H), 0.02 (t, J = 14.5, 6H). 13C NMR (125 MHz, CDCl3) δ: 192.5, 153.8, 151.4, 149.0, 147.8, 134.6, 134.2, 131.0, 129.4, 124.5, 123.3, 122.9, 121.9, 119.3, 118.4, 55.3, 40.2, 31.7, 29.7, 23.9, 22.7, 14.2. HRMS-ESI theoretical m/z [M]+ = 529.3339, found 529.3349; theoretical m/z [M+Na]+ = 552.3237, found 552.3251. Synthesis of 2-(2,6-Bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)vinyl)-4H-thiopyran-4-ylidene)malononitrile (1). A mixture of aldehyde E (0.38 g, 0.71 mmol) and commercial 2-(2,6-dimethyl-4H-thiopyran-4ylidene)malononitrile (0.067 g, 0.38 mmol) was dissolved in EtOH (100 mL). After piperidine (0.2 mL) was slowly added through a syringe while stirring, the reaction mixture was refluxed for 4 days. Reddish precipitate was obtained after cooling the reaction to room temperature. Compound 1 was purified through a silica gel column using hexanes/ethyl acetate (95:5) as eluent, resulting in 0.26 g of red solid (60% yield); mp 221−222 °C. 1H NMR (500 MHz, CDCl3) δ: 7.66 (d, J = 7.9 Hz, 1H), 7.59 (d, J = 8.2 Hz, 1H), 7.52 (d, J = 7.6 Hz, 2H), 7.41 (d, J = 16.1 Hz, 1H), 7.35 (s, 1H), 7.32−7.25 (m, 2H), 7.18−7.10 (m, 6H), 7.09−7.02 (m, 3H), 2.03−1.84 (m, 4H), 1.22−1.04 (m, 11H), 0.82 (t, J = 7.2 Hz, 6H), 0.73−0.64 (m, 5H). 13C NMR (125 MHz, CDCl3) δ: 156.3, 152.8, 151.6, 148.1, 147.7, 147.6, 143.6, 137.4, 134.8, 132.6, 129.2, 127.5, 124.1, 123.3, 123.1, 122.8, 121.3, 121.1, 120.9 119.5, 118.6, 115.6, 77.2, 77.2, 77.1, 77.0, 76.7, 64.7, 55.1, 40.2, 31.5, 29.6, 23.8, 22.5, 14.0. HRMS-ESI theoretical m/z [M+H]+ = 1211.6959, found 1211.6958. 2.3. Linear Photophysical Characterization. All steadystate spectral properties and fluorescence lifetimes of 1 were investigated at room temperature in spectroscopic grade cyclohexane (CHX), toluene (TOL), tetrahydrofuran (THF), dichloromethane (DCM), and acetonitrile (ACN) that were purchased from Aldrich and used without further purification. Linear 1PA spectra were measured with UV−vis spectrophotometer (Agilent model 8453) using 10 mm path length quartz cuvettes and molecular concentrations, C ≈ 10−5 M. Steadystate fluorescence measurements were performed with a PTI Quantamaster spectrofluorimeter for low-concentrated solutions (C ≤ 10−6 M) in 10 mm spectrofluorometric quartz cuvettes, and an additional correction for the spectral responsivity of PTI emission monochromator and PMT detector was performed. The fundamental values of excitation anisotropy of 1, r0(λ), were obtained in viscous polytetrahydrofuran (pTHF), where relatively slow rotational movement

Piperidine was dried with appropriate drying agent, distilled under reduced pressure, and stored over 4 Å molecular sieves. 7-Bromo-9,9-dihexyl-9H-fluorene-2-carbaldehyde was synthesized according to the literature.30 Physical characterization was conducted on all new compounds by 1H and 13C NMR spectroscopy and high-resolution mass spectroscopy (HRMS). 2.2. Synthetic Procedures. Hepta-2,5-diyn-4-ol (A). A dried 250 mL round-bottomed flask was charged with 150 mL of dry THF, and the mixture was cooled to −78 °C. n-BuLi (60 mL, 96 mmol, and 1.6 M solution in hexane) was added via a syringe. Propyne was condensed into the flask in excess (until there was a visible change in volume in the flask). After the addition of methyl formate (2.05 mL, 33.4 mmol), the mixture was stirred at −78 °C for 2.0 h. The mixture was allowed to warm to −40 °C and was maintained there for 1.5 h. Water was added and the reaction mixture was warmed to room temperature. The mixture was rinsed into a separatory funnel with H2O and ether. The aqueous phase was extracted with ether (3 × 300 mL). The combined organic extracts were dried with MgSO4, filtered, and concentrated under reduced pressure. The residue was purified via flash chromatography (10% CH3CN/CH2Cl2). A white solid (2.88 g) was obtained in 80% yield; m.p. 152−153 °C dec (lit.31 150−154 °C dec). 1H NMR (500 MHz, CDCl3) δ: 5.07−5.01 (m, 1H), 2.35 (d, J = 6.1 Hz, 1H), 1.85 (d, J = 2.3 Hz, 6H). 13C NMR (125 MHz, CDCl3) δ 77.1, 73.5, 48.7, and 3.5. HRMS-ESI theoretical m/z [M+H]+ = 109.0652, found, 109.0653. Hepta-2,5-diyn-4-one (B). A mixture of 200 mL of dry CH2Cl2 and BaMnO4 (12 g, 46.8 mmol) was prepared and added to a 100 mL round-bottomed flask containing alcohol A (1.9 g, 10.09 mmol). After stirring under nitrogen at room temperature for 5 h, the reaction was complete, and the progress was monitored by TLC. The contents of the flask were flash-filtered through a short Celite plug, and the filtrate was dried with MgSO4. The filtrate was concentrated under reduced pressure and purified by flash column chromatography (CH2Cl2) resulting in 1/71 g of pure-white solid in 92% yield; mp 77−78 °C (lit.32 79.7−80 °C). 1 H NMR (500 MHz, CDCl3) δ 1.99 (s, 6H). 13 C NMR (125 MHz, CDCl3) δ: 161.2, 90.5, 81.4, and 4.2. HRMS-ESI theoretical m/z [M +NH4]+ = 124.0757, were found, and 124.0750, [2M+NH4]+ = 230.1176, found, 230.1169. 2,6-Dimethyl-4H-thiopyran-4-one (C). After a solution of B (1.2 g, 11.30 mmol) and thiourea (0.85 g, 11.10 mmol) in 30 mL of dry DMF was allowed to stir at room temperature for 14 h, the reaction mixture was poured in ice water and extracted with CHCl3, yielding reddish-brown semisolid. Compound C was obtained as colorless solid after recrystallization in cyclohexane (1.11 g, 70% yield); mp 104 °C (lit.33 mp 104 °C). 1H NMR (500 MHz, CDCl3) δ: 6.72 (s, 2H), 2.35 (s, 6 H). 13C NMR (125 MHz, CDCl3) δ: 182.3, 150.6, 128.1, and 22.47 ppm. HRMS-ESI theoretical m/z [M+H]+ = 141.0369, found, 141.0367, [M+Na]+ = 163.0188, found, 163.0186. 2-(2,6-Dimethyl-4H-thiopyran-4-ylidene)malononitrile (D). A mixture of 0.5 g (3.56 mmol) of 2,6-dimethyl-4Hthiopyran-4-one (C), 0.24 g (3.6 mmol) of malononitrile, and 6 mL of Ac2O was refluxed for 30 min. The reaction mixture was then cooled to room temperature and a precipitate was observed. Filtration gave a purple solid, which after recrystallization with 1:1 hexanes/ethyl acetate afforded 0.33 g of D as brownish solid in 50% yield; mp 165−166 °C. 1H NMR (500 MHz, CDCl3) δ: 7.22 (s, 2H), 2.45 (s, 6H). 13C NMR (125 MHz, CDCl3) δ: 156.9, 150.3, 121.4, 115.2, and B



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Figure 1. Synthetic scheme for the preparation of thiopyranyl 1.

mm, respectively. All other details of this experimental setup were previously reported.38 2.5. Quantum Chemical Calculations. The electronic structure of the spectra of 1 was also analyzed theoretically in the gas phase at the TD-DFT/6-31G* level of theory with various exchange-correlation functionals. Preliminary geometry optimization of the ground state was done using DFT with the same exchange-correlation functional and basis set as for the further TD-DFT calculation. Recently introduced functional M05-1X25 was employed along with the standard M05,39 M052X,40 B3LYP,41 and CAM-B3LYP.42 The fraction of Hartree− Fock exchange (HFX) in the M05-1X25 is 1.25 times higher than that in the original M05. It was shown to perform well in simulation of 1PA and 2PA spectra for fluorene-based organic molecules.43,44 To save computer time, aliphatic side chains (C6H13) in 1 were replaced with methyl groups. This is a reasonable approach because those chains are not in conjugation with the aromatic system and exhibit no noticeable effect on the photophysical properties of π-conjugated structures.45,46 The symmetry was constrained to the Cs point group. The sum-overstate (SOS) expressions for the calculations of 2PA spectra were used in the form presented by Ohta et al.,47,48 with the total number of electronic states equal to 50 and the damping parameter Γ = 0.1 eV for all the states. All quantum-chemical calculations were performed using the Gaussian 2009 rev. A.2 suite of programs.49 Excitation energies and transition dipoles from the ground to excited states are habitually calculated in TD-DFT. The permanent and state-tostate transition dipoles from the SOS expression were obtained using the “dipole approximation”,50 implemented in a locally modified version of the Gaussian’09 code. The “dipole approximation” uses the so-called “unrelaxed” density matrix and is known to produce errors of 5−10% for transition dipoles and of 10−40% for permanent dipoles.51 We consider this accuracy sufficient for transition dipoles and recalculate correct permanent dipoles for ten lowest excited states using the Zvector method, implemented in the original Gaussian 2009 code, which takes into account orbital relaxation.52

of the V-shaped molecule did not decrease the values of the observed anisotropy r(λ),34 that is, r(λ) ≈ r0(λ). The fluorescence quantum yields, Φfl, of 1 were determined by standard methodology34 using Rhodamine 6G in ethanol35 and Cresyl Violet in methanol36 as references. Fluorescence lifetime measurements were performed using a time-correlated, singlephoton-counting system (PicoHarp 300, time resolution ∼80 ps) and linear polarized femtosecond laser excitation (MIRA900, Coherent) oriented by the magic angle. Photochemical stability of 1 was investigated in all employed solvents under UV lamp CW irradiation (LOCTITE 97034, used a wavelength of 436 nm), and the corresponding values of the photodecomposition quantum yields, Φph, were determined by absorption methods previously described.37−41 2.4. 2PA and Femtosecond Transient Absorption Measurements. The investigations of 2PA and ultrafast transient absorption processes in 1 were performed with a dual OPA femtosecond laser system (Coherent, Inc.), described previously in detail.14,38 The output of the femtosecond Ti:sapphire laser (Mira 900-F), pumped by the second harmonic of Nd3+:YAG laser (Verdi-10), was regeneratively amplified with a 1 kHz repetition rate (Legent Elite USP) providing pulse duration, τP ≈ 100 fs (fwhm) and pulse energy of EP ≈ 3.6 mJ. The amplified laser beam with a fundamental wavelength at ∼800 nm was divided into two equal parts and pumped two separate OPAs (OPerA Solo) with a broad tuning range, 0.24 to 20 μ, τP ≈ 100 fs, and EP, up to 200 μJ. The exit beam from the one OPA was used for the determination of 2PA cross sections, δ2PA, in the spectral range 740−1300 nm by the open-aperture Z-scan technique.29 The transient absorption measurements were performed by pump−probe methodology25,26,38 using separate linearly polarized laser beams from two OPAs with Gaussian space profiles and τP ≈ 100 fs. The first beam was used as a pump with EP ≈ 1 μJ and the second one was used as a probe with EP ≤ 2 nJ. The probe pulse was delayed in time by an optical delay line (M-531.DD, PI, Inc.) and overlapped with a pump pulse at a small angle (∼5°) in a 1 mm path length quartz flow cell. The pump and probe pulses were focused into the sample cell to waists of ∼0.5 and ∼0.2 C



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3. RESULTS AND DISCUSSION The synthetic approach for preparing the V-shape chromophore 1 is outlined in Figure 1. Compound 1 contains a 2thiopyran-4-ylidenemalononitrile derivative as an electronaccepting moiety and diphenylamine as an electron donor. We were particularly interested in 2-(2,6-dimethyl-4Hthiopyran-4-ylidene)malononitrile (D) because it can be envisaged not only as a unique electron acceptor group but also as an analog of 2-(2,6-dimethypyran-4-ylidene)malononitrile, incorporated as an electron acceptor group in a set of our previous chromophores.13 The apparent difference is that thiopyranyl contains a single sulfur atom in a sixmembered heterocyclic ring. The tendency for delocalization involving one of the pairs of periplanar electrons on sulfur in the thiopyranyl system is reflected in the stability associated with the aromatic 6π thiopyrylium structure53 and is the foundation of much of its rich chemistry.54 The synthesis of compound 1 (Figure 1) began with the generation and subsequent addition of two equivalents of 1-propynyllithium to methyl formate, affording diynol A in a good yield (87%) after acidic quench. The oxidation of alcohol A to B was accomplished with BaMnO4, affording compound B in 92% yield. Dialkynyl ketone B was reacted with thiourea to give 2,6dialkyl-4H-thiopyran-4-ones (C) in 70%. The method for converting the ketone group of C the electron-withdrawing substituent was via condensation with malononitrile derivative under the conditions of the Knoevenagel condensation. The desired product (D) was obtained in a 50% yield. Finally, the Vshape chromophore (1) was prepared through Knoevenagel condensation between 2-(2,6-dimethyl-4H-thiopyran-4ylidene)malononitrile (D) and 7-(diphenylamino)-9,9-diethyl9H-fluorene-2-carbaldehydebromoarylaldehyde (E) in 60% yield. 3.1. Linear Photophysical Properties of 1. The linear UV−vis absorption and steady-state fluorescence spectra of symmetrical V-shaped fluorene 1 are shown in Figure 2, while

effects in CHX and ACN were not investigated due to excellent solubility of 1 in these solvents. The excitation anisotropy spectrum (Figure 3a, curve 2) revealed the rather complex nature of the main long wavelength absorption contour of 1 with maximum at ∼500 nm. According to the spectrum, the value of excitation anisotropy r(λ) changed dramatically up to ∼650 nm, which is indicative of the at least two different electronic transitions, S0 → Sn (n = 1, 2, or n), responsible for the main one-photon absorption band of 1. These data are consistent with the assumption of weak electronic interaction between two identical diphenylaminosubstituted fluorene arms, oriented at ∼90° to each other in the symmetrical V-shaped structure of 1. In this case, the 1PA band with the spectral width Δνab ≈ 120 nm (fwhm) can be described as the sum of two spectrally shifted and overlapped contours with Δνab ≈ 60 nm and close maximum intensities.55 The steady-state fluorescence spectra of 1 exhibited one long wavelength band (Figure 2, curves 1′−4′) with a strong solvatochromic behavior (maximum Stokes shift up to 7740 cm−1 in DCM) and specific dependence of the fluorescence quantum yield, Φfl, on solvent polarity. Strong solvatochromism can be explained by large changes in the “x” component of the permanent dipole moment of 1 (see axis orientation in Figure 5) under electronic excitation. In addition to the most intense long wavelength emission band, two additional weak short wavelength fluorescence bands were observed inside the spectral range of the 1PA contour at ∼410 and ∼545 nm (Figure 3b, curves 2′, 3′). Presumably, these short wavelength emission bands originated from a small amount (∼3−5%) of highly fluorescent conformational isomers of 1 existing in the ground electronic state, S0, corresponding to the excitation spectra with maxima at ∼350 and ∼395 nm (Figure 3b, curves 2, 3). The molecular structure of these conformational isomers is characterized by restricted πconjugation (due to a possible nonplanar orientation of the fluorene arms) and a noticeable blue shift of the absorption maxima relative to the main absorption band of 1. It should be mentioned that the excitation spectra presented in Figure 3b are similar to the 1PA bands of unsymmetrical fluorene molecules with diphenylamino end substituents,56−58 which is consistent with the proposed assumption. The values of fluorescence lifetimes of 1, τfl, exhibited two emission components in CHX, THF, and DCM under excitation at ∼400 nm (see Table 1), reflecting a complicated nature of the observed fluorescence spectra that is in good agreement with the proposed model of ground state conformers. A single exponential fluorescence decay was observed in TOL solution of 1 due to relatively high fluorescence quantum yield, Φfl, when the short wavelength emission bands were negligible. In polar ACN, the long wavelength emission band was extremely weak (Φfl < 10−4) and the only one short wavelength fluorescence contour appeared under λex ≈ 400 nm. The values of the photochemical decomposition quantum yields of 1, Φph, were obtained by an absorption method37−41 for air-saturated TOL, THF, and DCM solutions at room temperature (see Table 1) using UV lamp excitation at 436 nm. The absolute values of Φph ≈ 10−6 to 10−5 revealed relatively high photostability of 1 in the solvents studied, which is comparable to the best laser dyes in liquid media.59−61 High photochemical stability is important, suggesting strong potential of 1 for practical applications, including fluorescence bioimaging techniques.

Figure 2. Normalized linear absorption (1−5) and steady-state fluorescence (1′−4′) spectra of 1 in HEX (2, 1′), TOL (3, 2′), THF (4, 3′), DCM (5, 4′), and ACN (1).

its main photophysical parameters are summarized in Table 1. The linear absorption spectra exhibited well-defined 1PA peaks at ∼300, 390, and 500 nm and a weak dependence on solvent polarity, Δf. No aggregation effects in the linear absorption spectra of 1 were observed in TOL, THF, and DCM solutions with dye concentrations up to C ≈ 10−2 M. The possible aggregation D



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max Table 1. Linear Photophysical and Photochemical Parameters of 1: Absorption λmax Maxima, Stokes ab and Fluorescence λfl Shifts, Maximum Extinction Coefficients εmax, Fluorescence Quantum Yields Φfl, Lifetimes τfl, Photodecomposition Quantum Yields, Φph

N/N Δf λmax ab , nm λmax fl , nm Stokes shift, nm (cm−1) εmax·10−3, M−1·cm−1 Φfl, % τfl,b ns (Ai)c Φph × 106 a

CHX −4

3 × 10 496 ± 1 563 ± 1 67 ± 2 (≈ 2400) 3.0 ± 0.6 0.2 (0.96) 0.78 (0.04)

TOL

THF

DCM

0.0135 502 ± 1 630 ± 1 128 ± 2 (≈ 4050) 71 ± 3 24 ± 3

0.209 496 ± 1 760 ± 1 264 ± 2 (≈ 7000) 69 ± 3 0.8 ± 0.3 0.57 (0.94) 3.73 (0.06) 1 ± 0.5

0.217 511 ± 1 845 ± 1 334 ± 1 (≈ 7740) 67 ± 3 1.5 ± 0.5 0.35 (0.96) 3.36 (0.04) 2.4 ± 1

0.47 (1.00) 20 ± 5

ACN 0.305 487 ± 1

0) from the vibrationally excited S1 or S2 Franck−Condon electronic states34 and gain processes (ΔD < 0) observed in polar DCM for λpr ≥ 740 nm. It should be mentioned that S1 and S2 electronic states of 1 have close energies and determine the shape of the main long wavelength 1PA contour (see Sections 3.1 and 3.3) and therefore can nearly equally participate in the ultrafast ESA. The influence of SA processes was also detected (negative ΔD in Figure 9, λpr = G



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S1 state. (See Table 1.) The transient absorption kinetic data were obtained over a broad spectral range of λpr and allowed the reconstruction of the time-resolved spectral dependences ΔD = f(λpr) presented in Figure 12. The dominant broadband ESA spectra with several maxima were obtained in nonpolar TOL solution of 1 for λpr ≥ 560 nm (Figure 12a). No evidence of noticeable spectral-shape temporal evolution was observed, and no gain was detected, even in the spectral range of potential amplification (i.e., in the main fluorescence band 600−800 nm). In contrast, in DCM, 1 exhibited a relatively intense, well-defined ESA band with a maximum at ∼620 nm and efficient gain processes for λpr ≥ 760 nm (Figure 12b). The maximum of the gain band at ∼820 nm roughly corresponded to the maximum of the fluorescence spectrum of 1 in DCM (Figure 2, curve 4′). The observed gain spectrum appeared at ∼400 fs after excitation and substantially increased in intensity in the next 4−6 ps. These gained properties of 1 in DCM potentially can be used for light amplification under femtosecond pumping. Nevertheless, no superfluorescence or lasing effects were observed for 1 in polar DCM, presumably due to relatively low fluorescence quantum yield in this medium and corresponding high laser threshold.

Figure 8. (1) Calculated 2PA spectrum for 1a obtained at the TDM05-1X25/6-31G* level of theory using the SOS expression with 50 states and (2) experimental 2PA spectrum for 1 in DCM. Vertical bars represent contributions to the 2PA cross section (δ2PA) from each excited state. The bars are marked with A′ and A″ notations corresponding to symmetric and asymmetric excited states, respectively. The solid theoretical curve (1) is a convolution of oscillator strength bars with Lorentz shape-factors normalized to unit at the maxima and line widths equal to 0.1 eV. The dashed-dotted curve (3) is plotted for the four-states model, where only the four lowest singlet excited states are included in the SOS expression from Table 2.

4. CONCLUSIONS The synthesis, linear photophysical and photochemical characterization, 2PA, and ultrafast excited-state dynamics of a new sulfur-containing V-shaped symmetrical fluorene derivative 1 were reported. The steady-state, nonlinear optical, and transient absorption properties of 1 were investigated in solvents with different polarities. Linear absorption spectra were nearly independent of solvent polarity and exhibited a complex nature of the main long wavelength 1PA band, determined by the weak interaction of the two identical conjugated fluorenebased chromophores in the molecular structure of 1. Steadystate fluorescence spectra revealed a strong solvatochromic effect with a maximum Stokes shift up to ∼7740 cm−1 in polar DCM. Additional weak fluorescence bands were observed in

11b) provide evidence of at least two relaxation components with opposite signs, which is consistent with the proposed mechanism of ESA from closely overlapped S1 and S2 vibronic bands. The second type of observed kinetic process revealed 3−6 ps characteristic times and, presumably, corresponds to a solvate relaxation phenomenon.34,67−69 After these fast relaxations, the values of photoinduced optical density ΔD relax back to zero in accordance with the much longer fluorescence lifetimes of the

Figure 9. Transient absorption dependences ΔD = f(τD) for 1 in TOL at selective λpr. H



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Figure 10. Transient absorption dependences ΔD = f(τD) for 1 in DCM at selective λpr.

Figure 11. Transient absorption dependences ΔD = f(τD) for 1 in TOL: λpr = 580 nm (a), 640 nm (b), and 840 nm (c) and in DCM: λpr = 640 nm (d), 860 nm (e), and 940 nm (f) for λpm = 400 nm (1) and 500 nm (2).

Figure 12. Spectral dependences ΔD = f(λpr) for 1 in TOL (a): τD = 250 fs (1), 500 fs (2), 1.5 ps (3); and for 1 in DCM (b): τD = 200 fs (1), 400 fs (2), and 2 ps (3).

decomposition quantum yield of Φph ≈ 10−6 to 10−5 under UV irradiation. The 2PA spectra of 1 were obtained over a broad spectra range, and only one well-defined band with a maximum δ2PA ≈ 600−700 GM was observed in the main one-photonallowed absorption band. Quantum chemical analysis of 1PA

the 1PA contour of 1, which can be attributed to a small number of highly fluorescent conformational isomers in the ground state S0. The advantageously high photochemical stability of 1 was determined quantitatively with a corresponding photochemical I



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and 2PA spectra of 1 was performed based on a DFT approach, and the best agreement of the theory and experimental data was obtained with the new M05-1.25X functional. Ultrafast excited-state dynamics of the V-shaped fluorene 1 was investigated in TOL and DCM solutions over a broad spectral range, under S0 → S1 and S0 → Sn excitations. Two different types of relaxation processes were observed in the excited states of 1 on time scales of 400−800 fs and 3−6 ps, which can be explained by fast-transient ESA and solvate reorganization phenomena, respectively. No noticeable dependence on excitation wavelength was observed under the experimental conditions employed. An efficient gain band arising in ∼2 ps was detected for 1 in DCM in the spectral range of the main fluorescence contour. Ultrafast dynamic processes observed for 1 under femtosecond excitation, along with its high photostability and 2PA efficiency, provide the basis for exploring this intriguing new compound in a number of nonlinear optical applications, including 2PFM and STED techniques.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We wish to acknowledge the National Science Foundation (ECCS-0925712, CHE-0840431, and CHE-0832622), the U.S. National Academy of Sciences (PGA-P210877), and the National Academy of Sciences of the Ukraine (grants 1.4.1.B/ 153 and VC/157).

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