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Ultrafast Fluorescence Photo-Switch Incorporating Diketopyrrolopyrrole and Benzo[1,3]Oxazine Bheerappagari Ramakrishna, Yeduru Venkatesh, Kamatham Narayanaswamy, Venkatesan Munisamy, Haraprasad Mandal, Surya Prakash Singh, and Prakriti Ranjan Bangal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09181 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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The Journal of Physical Chemistry

Ultrafast Fluorescence Photo-switch Incorporating Diketopyrrolopyrrole and Benzo[1,3]oxazine Bheerappagari Ramakrishna, Yeduru Venkatesh§, Kamatham Narayanaswamy§, Venkatesan Munisamy§, Haraprasad Mandal§, Surya Prakash Singh§ and Prakriti Ranjan Bangal§* Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, India-500 007 §

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India-110 001

*Corresponding Author: Fax: (+91)40-27160921, Tel :( +91)40-27191431, E-mail: [email protected]

ABSTRACT: With the objective of developing ultrafast fluorescent switch molecules we have designed and synthesized fluorescence switch molecules incorporating two oxazine photochromes (OX) at the two end of single diketopyrrolopyrrole (DPP) fluorophore giving the shape of the dyad molecule as OX-DPP-OX. In view to precise characterization, steady state photophysical properties, acid-base induced spectroscopic studies and ultrafast transient absorption spectroscopic studies are performed. In acetonitrile (ACN) solution, the benzo[1,3]oxazine ring of studied oxazine derivatives in OX-DPP-OX opens up and reduces the fluorescence intensity of DPP by 66% upon addition of fifty equivalent trifluoroaceticacid (CF3COOH, TFA) and addition of equivalent amount of base, tetrabutylammonium hydroxide ((C4H9)4NOH, TBAOH) closes the oxazine ring reverting the fluorescence intensity of DPP unit back to its original intensity. Likewise, upon 330 nm laser excitation oxazine ring opens up in less than 135 ps in ACN solution reducing the DPP fluorescence by 90%. Both the processes, acidochromic effect and 330 nm laser excitation, generate a 3Hindolium cation, para-nitrophenolate (protonated) and para-nitrophenolate anion respectively. The photogenerated isomer lives for 1.5−1.9 ns in room temperature and reverts to its original conformer with first-order kinetics. This photochromic dyad tolerates thousands of 1 ACS Paragon Plus Environment


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switching cycles with no sign of degradation. However, the Gibbs free energy of the cationic fragment of their photogenerated isomer and DPP fluorophore is exergonic (∆G0 < -0.8) and ultrafast intramolecular electron transfer occurs very fast (in 16 ps time) from DPP moiety to 3H-indolium cation. As a result, the photoinduced transformation of the photochromic component within this dyad results in the effective quenching of the DPP emission. The fluorescence of this photoswitchable compound is modulated on a nanosecond time scale with excellent fatigue resistance under femtosecond (FWHM~100 fs) photo-excitation. Thus, choice of OX as photochromic component and DPP as fluorescence components can ultimately lead to the development of valuable ultrafast photoswitchable fluorescent probes for designing ultrafast switching devices. Such valuable mechanistic insights into their excitation dynamics can guide the design of novel members of this family of photochromic compounds with improved photochemical properties.

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1. INTRODUCTION Ultrafast fluorescent switch materials attract more attention for their possible potential applications in optical memory1-5 and non-destructive optical readout systems,6 which exhibit reversible change in fluorescent intensity. This ultrafast modulation of fluorescence intensity is accompanied by very fast photochromic reaction of the photochromic moieties of the fluorescent probe. In particularly, such type of compounds comprise the fluorescent and photochromic components, which could be integrated within the same molecular skeleton (fluorophore-photochrome type) and the emission of the former, can be switched with the photoinduced interconversion of the latter. For a special class of photo-switch compounds, stimuli-responsive functions can change their properties in a reversible and controllable way in response to stimulus like photo and heat. They have been comprehensively studied as potential candidates in various engineering applications such as biological fluorescent imaging,7,8 detection applications9 and in different molecular device applications, which include fluorescent switches,10 fluorescent sensors11,12 and also other photonic devices.13,14 The fluorescence switching materials allowing to photoinduced interconversion of photochromic units15-20 with distinct absorption spectra under the influence of optical stimulations are exploited to modulate the emission of complementary fluorophores.21-29 In fact, their use as fluorescent probes has already been explored by us with typical examples.30,31 In the wake of these promising results, we designed fluorophore-photochromefluorophore dyad32 (Chart 1) incorporating fluorescent diketopyrrolopyrrole (DPP) component, which emits at 570 nm upon excitation at 500 nm (λON), and a photochromic spiropyran(SP) component, which switches to the corresponding merocyanine (i.e. MC, open conformer) upon irradiation at 330 nm (λOFF). The very fast photoinduced transformation of SP into MC (32 ps) encourages the energy transfer pathways (FRET), which induced effective quenching of the DPP fluorescence. But the photo-generated species MC is 3 ACS Paragon Plus Environment


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Chart 1. Schematic structure of the compounds used. relatively stable at room temperature and reverts to SP in ~30 s with first-order kinetics. Thus, the fluorescence of this particular system can be switched 'off' and 'on' simply by turning 'on' and 'off', respectively an ultraviolet source (150 W xenon lamp). Indeed, a full switching cycle from SP to MC and back can only be completed in few hundred second time domain. However, this relatively slow transformation of MC to SP is hindered for designing ultrafast photo-switch materials. Thus, switching speeds of newly designed fluorophorephotochrome dyad should be improved significantly in view of possible applications in nondestructive optical readout systems, which requires 'to' and 'fro' switching speed at least picoseconds to nanosecond time domain and the ability to tolerate few thousands of switching cycles. In this regard, F. M. Raymo and his group took a lead in developing fast fluorescence switching incorporating single oxazine photochrome into the backbone of selective fluorophores and successfully demonstrated modulation of emission of their synthesized fluorophore-photochrome dyad in microsecond time domain resulting hundreds of switching cycles with high degree of fatigue resistance.33-37 In this work, we report the synthesis of novel dyad with improved performance in terms of fluorescence switching speed in nanosecond time scale resulting for thousands cycles per second and characterization of their photochemical and photophysical properties with a combination of steady-state and 4 ACS Paragon Plus Environment

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time-resolved spectroscopic measurements. However, as numerous organic photochromic compounds show only limited speed of photoconversion because of photodegradation processes and rapid relaxation of the energy-rich photoinduced state proper design of robust photochromic compounds are indispensible. In this regard, novel design of photochrome-fluorophore-photochrome dyad incorporating photochromic moieties like Benzo[1,3]oxazines (Chart 1), a class of fast thermally reversible photochromic molecules33-37 that combine two fused heterocyclic rings, indoline and oxazine,38,39 with an exceptional ability to perform numerous cycles with no signs of photodegradation,39,40 to the skeleton of a suitable fluorophore allows us to have ultrafast switching materials. UV illumination to benzo[1,3]oxazines, uncolored molecules, leads to the cleavage of the C−O bond and consequent opening of the oxazine ring in ~60 ps with formation of a zwiterionic isomer incorporating a 3H-indolium cation and a 4nitrophenolate anion that absorbs strongly centered at 440 nm (Figure 1). In addition to this, it is reported that two enantiomers of the ring closed form of benzo[1,3]oxazines exchange rapidly (nano seconds regime) and exists in thermal equilibrium between ring close and open form and it is substantiated by a resonate single and broad peak of 1H NMR at 1.3−1.6 ppm pertaining to two diastereotopic methyl groups of the indoline ring, adjacent to the chiral C−2 centre.41 This equilibrium can be reversibly moved towards the opened form by the addition of either acid or base. Under acidic conditions (e.g. addition of TFA) the phenolate anion is protonated displacing the equilibrium position towards the formation of the stable opened form producing 3H-indolium cation and protonated 4-nitrophenolate (Figure 1) absorbing at 380 nm.



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Figure 1. Reversible ring opening and closing of Benzo[1,3]oxazine ring: optical stimulation and acid-base titration.

3,6-bis(5-bromothiophen-2-yl)-2,5-bis((R)-2-ethylhexyl)-2,5dihydropyrrolo[3,4c]pyrrole-1,4-dione, (DPP(6)) a relatively new class of red pigments shows very high fluorescence quantum yield as well as high thermal stability which tempted us to choose it as a skeleton fluorophore for designing ultrafast fluorescent switch molecule. Because of its unique π- conjugated systems as well as bright shades and outstanding photostability, DPP(6)s are also being widely explored for optoelectronic applications.42-45 Herein, the precursors, OX(5) (photochromic unit) and DPP(6) (fluorescent probe) are coupled via palladium catalyzed Sonogashira cross coupling reaction with benzene unit bridge to afford OX-DPP-OX(7) dyad. The structure of the dyad has been characterized by 1H NMR,

13

C

NMR. Steady state and femtosecond time resolved transient absorption spectroscopic 6 ACS Paragon Plus Environment

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techniques are employed to characterize acid-base fluorescence switching behaviors and photo induced switching behaviors respectively in ACN solution.

2. EXPERIMENTAL SECTION 2.1 Materials. All the Solvents and analytical grade reagents were purchased from commercial sources (Hychem Laboratories, SDFCL, FINAR and MERK) and used without further purification. 2,3,3-trimethyl-3H-indole, 4- formyl benzoic acid, prop-2-yn-1-ol, 2chloromethyl-4-nitrophenol, diethyl succinate, 2-ethylhexylbromide, t-amyl alcohol, sodium metal,

sodium

sulphate,

N-bromosuccinimide,

potassium

carbonate,

TFA,

tetrabutylammonium hydroxide in MeOH, triethylamine and CuI were used as received from commercial suppliers as mentioned. tetrahydrofuran (THF) were distilled over sodium in the presence of benzophenone (Ph2CO) as indicator. CH2Cl2, CHCl3 and DMF were distilled over calcium

hydride.

Spectroscopic

grade

acetonitrile

(ACN),

toluene

(PhMe),

bis(triphenylphosphine)palladium(II), and PPh3 were purchased from Aldrich Chemicals, USA. All reactions were performed under nitrogen atmosphere.

2.2. Physical Measurement and Instrumentation. The 1H NMR and 13C NMR spectra were obtained as solutions in CDCl3 solvent. 1H spectra were obtained on 500 MHz spectrometers with tetramethylsilane (TMS, δ = 0 ppm) as internal standard and

13

C NMR

spectra were obtained on 75 MHz spectrometers with CD3Cl as the internal standard. Chemical shifts (δ) are reported in ppm relative to the residual solvent signal (δ = 7.26 for 1H NMR and δ = 77.0 for 13C NMR). Reactions were monitored by thin-layer chromatography (TLC) using 0.20–0.25 mm silica gel plates. Column chromatography was performed with silica gel (60–120 and 100–200 mesh). FT-IR spectra were recorded using a Thermo Nicolet Nexus-670 Fourier Transform Infrared Spectrometer with reference to KBr.



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UV-vis absorption and steady state emission spectra were recorded at room temperature on a Hitachi U-2910 spectrophotometer and a Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon (USA), respectively. The measurements were conducted in a rectangular quartz cell with a 1 cm optical path. For all the spectroscopic studies of OX-DPP-OX(7) (~6 X 10-5 M) system was carried out spectroscopic-grade ACN from Aldrich Chemicals (USA). Fluorescence quantum yield was calculated using secondary standard method taking Rhodamine B as reference compound whose fluorescence yield was 0.7 in ethanol.46 Femtosecond transient absorption studies were performed with commercial femtosecond Ti:Sapphire regenerative amplifier (Spitfire, Laser Spectra, USA) laser system equipped with a CDP transient absorption spectrometer and automated data acquisition system (CDP System Corporation, ExciPro). The detailed experimental setup of the femtosecond transient absorption (TA) measurements are described elsewhere.47 The regenerative amplifier was seeded with the 100 fs output from the oscillator (Spectra Physics, Maitai). and the amplified 100 fs output pulse of regenerative amplifier was seeded to optical parametric amplifier (TOPAS prime) and the output of TOPAS was used as pump sources at required wavelength and fed into a spectrometer through a synchronized chopper for 1 kHz repetition rate. A lens (f = 200 mm) was used to adjust the pump diameter while an iris and neutral density filter combination were used to adjust the pump energy. A Berek’s variable wave plate was placed in the pump beam and polarization was fixed at the magic angle with respect to the probe pulse. A part of the output of the Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics) with 1 kHz repetition rate and 100 mW average power at 800 nm was fed to the spectrometer. The power of the 800 nm was reduced to 20–25 mW and focused onto a thin rotating (2 mm) CaF2 crystal window to generate a white-light continuum. A fraction of this beam was sent to a photo detector which controls speed and phase of the chopper rotation. The beam of white light was collimated with a parabolic mirror


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(f = 50 mm, 9°). Then this white light was reflected from a beam splitter and mirror into two identical probes and reference beams. Two concave mirrors (f = 150 mm) were used to focus both probe and reference beams to the rotating sample cell. Two lenses (f = 60 mm) created probe and reference images at the entrance surfaces of two optical fibers which are connected to the entrance slit of the imaging spectrometer (CDP2022i). This spectrometer consists of UV-Vis photodiode (Si linear photodiode) arrays and IR Photodiode (GaAs linear photodiode) arrays with spectral response range 200–1000 nm and 900–1700 nm respectively. Quartz cells of 1mm sample path length were used for all studies and IRF was estimated to be ≤ 150 fs. For sub ps dynamic, the signal from the solvent system was recorded under same experimental conditions and placed in respective figures in the text for comparison where overlapping of solvent induced transient with transient signal from the sample is substantial. However, to minimize the solvent signal pump pulse energy was kept below 3 mJ per second and probe pulse energy was from 0.1–0.5 mJ at the sample. For transient absorption spectra the group velocity dispersion compensation of white light continuum (probe beam) was done using studied solvent’s two photon absorption data for few ps delay. All the samples were checked before and after taking the transient absorption to monitor the sample degradation if any.

For Time Correlated Single Photon Counting, femtosecond pulses at required repetition rate were obtained from fractional part of MaiTai output passing through femtosecond Pulse Selector (3980-5S, Spectra Physics, single shot to 8 MHz). The excitation pulses at desired wavelength were generated by frequency doubling with 0.5 mm BBO crystal. This excitation pulses are focused to the sample using our Fluorescence up-conversion set up. The time distribution data of fluorescence intensity were recorded on a SPC-130 TCSPC module (Becker & Hickl).



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2.3 Transient Data and Global Analysis. To obtain a model-based description in terms species related spectral signature and overall lifetime of the species, the transient data reported in this paper were analyzed using Global and Target analysis.48,49 In view to predict minimum number of components involved in the evolution of transient data, global analysis was performed based on superposition principle of least number of independent exponential components which provided a straightforward description of the data at all measured wavelengths at all time points simultaneously. The numbers of independent components fitted to all data are determined by gradually increasing the number of exponential components until the residuals were effectively zero. A sequential kinetic model, unbranched, unidirectional model, consists of successive monoexponential decays with increasing time constants estimates gross spectral evolution of the data generating evolution associated difference spectra. Finally, data are fitted to a full kinetic model (compartmental scheme), target analysis, which includes possible branching routes and equilibrium between compartments specifying the microscopic rate constants that describe the decay of the compartment as well as transfer of excitation between the compartments. This analysis estimates the real spectra of each compartment (excited species) and is termed as species associated difference spectra (SADS). The whole Global and Target analysis was performed with the R package TIMP and its graphical user interface of Glotaran (for details, see the literature.50-53)

3. SYNTHESIS OF PHOTO-SWITCHABLE DYAD 3.1. Prop-2-yn-1-yl 4-formylbenzoate (2). To a 100 mL round-bottom flask with a magnetic stir bar, 4-formylbenzoic acid (1), (1.07 g, 0.0071 mol, 0.8 equiv.), prop-2-yn-1 ol (0.5 g, 0.0089 mol, 1 equiv.), DMAP (1.09 g, 0.0089 mol, 1 equiv.) and DCC (1.8 g, 0.0089 mol, 1 equiv.) were fully dissolved in 50 mL freshly dried dichloromethane. The mixture was 10 ACS Paragon Plus Environment

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capped by a rubber septum, cooled to 273 K and stirred at that temperature for 10 min. The mixture was allowed to warm up to room temperature and stirred for another 24 h. After that, the white precipitate was filtered off, and the filtrate was evaporated under vacuum. The residue was purified by column chromatography on the silica gel with CH2Cl2/hexanes (v/v = 1/1) as eluent to afford 1.48 g product 2 as a white solid (Yield: 80%), following a literature protocol.54 1H NMR (500 MHz, CDCl3) δ : 10.13 (d, J = 11.5 Hz, 1H), 8.40 – 8.07 (m, 2H), 7.97 (d, J = 8.0 Hz, 2H), 4.97 (d, J = 1.9 Hz, 2H), 2.56 (t, J = 2.1 Hz, 1H).

13

C NMR (75

MHz, CDCl3) δ : 191.5, 164.8, 139.4, 134.3, 130.4, 129.5, 77.8, 76.6, 75.5, 53.0, 33.8, 25.6, 24.92.

3.2. 5a, 6, 6-trimethyl-2-nitro-5a, 6-dihydro-12H-benzo [5,6] [1,3] oxazino [3, 2-a]indole (4). To a 100 mL round-bottom flask with a magnetic stir bar, 2,3,3trimethyl-3H-indole (3), (5.95 g, 0.037 mol, 1.5 equiv.) solution and 2-chloromethyl-4nitrophenol (4.43 g, 0.028 mol, 1 equiv.) in acetonitrile (10 mL) was heated under reflux for 3 days. After cooling to ambient temperature, the solvent was evaporated under reduced pressure and the residue was dissolved in CH2Cl2 (40 mL) washed with KOH (aq) (0.05 M, 25 mL), water (250 mL), dried over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was purified by column chromatography and finally purified by crystallization from dichloromethane–hexanes (v/v = 1/3) to afford [1,3]oxazine, 4 (1.42 g, 0.009 mol) as a white solid, following a literature protocol.42,55

3.3.Prop-2-yn-1-yl(E)-4-(2-(6,6-dimethyl-2-nitro-12H-benzo[5,6] [1,3]oxazino [3,2-a]indol-5a (6H)-yl)vinyl) benzoate, OX(5). To a 50 mL roundbottom flask with a magnetic stir bar, compound 2 (150 mg, 0.79 mmol, 1 equiv.) and 4 (183 mg, 0.592 mmol, 0.75 equiv.) in ACN (10 mL) and 1 mL of TFA was heated under reflux for 7 days. After cooling to ambient temperature, the solvent was evaporated under reduced pressure and the residue was dissolved in CH2Cl2 (40 mL) washed with water (40 mL), dried 11 ACS Paragon Plus Environment


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over anhydrous Na2SO4 and evaporated to dryness under reduced pressure. The residue was purified by column chromatography and finally purified by crystallization from dichloromethane–hexanes (v/v = 1/3) to afford OX(5), (83 mg, 60% yield) as a white solid, following a literature protocol.56

Scheme 1. Synthetic route of the Benzo[1, 3]oxazine,(4) and OX(5).

1

H NMR (500 MHz, CDCl3) δ : 8.08 – 8.01 (m, 3H), 7.49 – 7.44 (d, J = 8.4 Hz, 2H), 7.16 –

7.09 (m, 2H), 6.91 – 6.80 (m, 4H), 6.63 (d, J = 7.8 Hz, 1H), 6.49 (d, J = 16.1 Hz, 1H), 4.99(d, 2H), 4.60 (d, 2H), 2.54 (t, 1H), 1.61 – 1.13 (m, 6H). 13C NMR (75 MHz, CDCl3) δ : 165, 158, 146, 140, 140, 138, 135, 130, 129, 129, 127, 126.8, 124, 123, 122, 120, 119, 117, 116, 108, 103, 77, 76.9, 76, 75, 52, 50, 40, 31, 31, 30, 29, 29, 26, 23.2, 22.7, 18, 14. FT-IR (KBr) νmax = 637, 749, 873, 931, 1057, 1242, 1272, 1340, 1380, 1438, 1486, 1522, 1587, 1612, 1708, 2960, 3400 cm-1.

3.4. Synthesis of OX-DPP-OX(7). Diketopyrrolopyrrole, DPP(6) dye was prepared according to a literature protocol.57 Compound OX(5), (60 mg, 0.107 mmol, 3 equiv.), DPP(6), (23.92 mg, 0.035 mmol, 1 equiv.), Pd(PPh3)2Cl2 (0.005 mol ratio) and CuI (0.01 mol ratio) were added to a 50 mL round bottom flask and subjected to three vacuum/ nitrogen refill cycles. Afterward, distilled THF (20 mL) and degassed Et3N (1.5 mL) were 12 ACS Paragon Plus Environment

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added to the above mixture under N2 atmosphere. The reaction mixture was allowed to stir at 343 K for 12 h. After being cooled to room temperature, the resulting mixture was poured into 50 mL deionised water and extracted with CH2Cl2 (50 mL). Then the combined organic layers were dried over anhydrous Na2SO4. Finally, the solvent was removed by rotary evaporation and the residue was purified by silica column chromatography with petroleum ether/CH2Cl2 as eluent to afford OX-DPP-OX(7) (42 mg, 55%) as a thick blue color solid, following a literature protocol.58 1H NMR (500 MHz, CDCl3) δ : 8.08 – 8.14 (m, 4H), 7.45 – 7.69 (m, 12H), 7.30 (m, 2H), 7.09 –7.06 (m, 4H), 6.80 (m, 2H), 6.64 (d, J = 7.8 Hz, 2H), 6.50 (d, J = 16.1 Hz, 2H), 6.22 (d, J = 14.0 Hz, 2H), 4.99 (s, 4H), 4.56 – 4.66 (m, 4H), 2.75 (m, 4H), 1.92 (m, 2H), 1.86 – 1.20 (m, 4H), 1.19 – 1.35 (m, 12H), 0.99 (t, 6H), 0.88 (t, 6H). 13C NMR (75 MHz, CDCl3) δ : 165, 161, 158, 146, 140, 137, 135, 134, 130, 129, 127, 126, 123, 122, 120, 119, 117, 108, 103, 79, 77, 77, 76, 53, 50, 46, 40, 39, 31, 29, 29, 28, 26, 23, 23, 22, 18, 14, 10, 0.03. FT-IR (KBr) νmax = 750, 823, 949, 1026, 1092, 1177, 1262, 1335, 1434, 1481, 1517, 1609, 1664, 1723, 2856, 2924, 3436 cm-1.

Scheme 2. Synthetic route of OX-DPP-OX(7) dyad using OX(5) and DPP(6).



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4. RESULTS AND DISCUSSION In the light of structural implications in the isomerization mechanism of spiropyran,3032

we realized that the introduction of an appropriate photochrome will offer the opportunity

to implement an ultrafast photochromic transformations. Keeping this in mind, we have designed ultrafast photochromic unit, OX(5) (Figure 1) synthesized from 2,3,3-trimethyl-3Hindole, 4- formyl benzoic acid and 2-chloromethyl-4-nitrophenol compounds in three steps described. To exploit the ultrafast-fluorescent photo-switch materials, we have chosen a photochromic unit OX(5) and a highly fluorescent unit DPP(6), which were coupled with efficient Sonogashira cross coupling reaction with 55% of yield and it was characterized by 1

H NMR,

13

C NMR. Furthermore, steady state absorption and fluorescence spectroscopic

studies along with femtosecond transient absorption studies were performed to reveal the ultrafast photo switching nature in solution.

4.1. Steady-state Absorption Study. The OX-DPP-OX(7) dyad shows two distinct absorption bands system, one is at 276 nm with strong absorption along with a shoulder band at 310 nm and other one is relatively weak and broad at around 470-620 nm region with two peaks at 545 and 590 nm. A weak peak can also be detected at 650 nm. Figure 2A shows the absorption spectra of OX-DPP-OX(7) in ACN. The absorption features of the dyad are essentially identical to those noticed for OX(5) and DPP(6) (See Figure S1). The blue end of absorption spectra of OX-DPP-OX(7) represents the absorption correspond to OX moiety and red end correspond to DPP moiety. Hence, the steady-state absorption spectrum of OX-DPP-OX(7), (Figure 2A, Curve a) is effectively the sum of the absorption spectra of its two constituent components OX(5) and DPP(6) respectively. Note, the absorption peaks of OX-DPP-OX(7) pertaining to DPP show bathochromic shift of about 25 nm than that of the pristine DPP(6) absorption and it could be due to the extended conjugation in the OX-DPP-OX(7) dyad upon coupling of OX(5) unit to DPP(6) moiety. 14 ACS Paragon Plus Environment

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Figure 2. (A) Shows absorption spectra of OX-DPP-OX(7), (~6 X 10-5 M) in ACN solution (a), OX-DPPOX(7) with the addition of TFA (AC) in ACN (b), and further subsequent addition of (C4H9)4NOH (BS)at room temperature (c). (B) Shows absorption spectral change of OX-DPP-OX(7) in ACN upon successive addition of TFA (0, 5, 10, 20, 25, 30, 40, 50 equiv.).

However, upon addition of TFA (diluted in ACN) to the ACN solution of OX-DPP-OX(7) dyad, the brown colour ACN solution of dyad is changed to a pale red colour solution. The significant change in absorption spectrum of the protonated form of OX-DPP-OX(7) is recorded. The 276 nm absorption peak disappears and 310 nm shoulder peak becomes prominent. Most importantly, a new absorption peak at 383 nm is emerged but the peak position and shape of the main absorption peaks corresponding to DPP unit remain unchanged. A noticeable amount increase in absorbance at 560 nm region is observed (Figure 2A, Curve b) which could be due to the overlapping of the tail of 383 nm band. The addition of equivalent amount of TBAOH (diluted in MeOH) to acid mediated OX-DPP-OX(7) dyad, the characteristic absorption spectra of neutral OX-DPP-OX(7) with original brown colour of the solution reverts back (Figure 2A, inset). To have controlled conversion, a delicate spectroscopic titration of OX-DPP-OX(7) dyad was performed with dilute TFA in ACN and it is shown in Figure 2B. Absorption peak at 276 pertaining to OX moiety and weak 650 nm band of OX-DPP-OX(7) decrease and a new distinct peak at 383 nm increases upon 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

successive addition of TFA with clear isosbestic points at 300 and 600 nm respectively. Titration of OX(5) with TFA shows similar spectral change (See Figure S2) with distinct peak at 383 nm region. These observations on OX-DPP-OX(7) dyad lead to the fact that OX(5) retains59 its acidochromic reversible transformation properties in OX-DPP-OX(7) dyad upon acid base titrations. Upon addition of TFA(0→50 equiv.) to ACN solution of OXDPP-OX(7) dyad, apparent formation of new band at 380-400 nm, decrease of 276 nm band and enhancement of absorbance in 475 to 650 nm spectral range are attributed to the sensitive cleavage of C–O bond in OX of the OX-DPP-OX(7) dyad that eventually leads to the planarization of the two originally orthogonal heteroycles in OX to form the ring-opened conformation which enhances the π-conjugation in the system as well as gives rise the formation of p-nitrophenolate (protonated) and 3H-indolinium cation. The formation of new absorption band corresponds at 380-400 nm is attributed to p-nitrophenolate (protonated) chromophore (Figure 1).41

4.2. Steady-state Fluorescence Study. The fluorescence feature of OX-DPPOX(7) is similar to that of DPP(6), which indicates that the fluorescence emission of OXDPP-OX(7) is from DPP moiety. The DPP(6) has been an interesting fluorescent probe due to its higher thermal stability, solubility and high quantum yield.58,60 It shows very strong fluorescence in ACN solution upon excitation at 500 nm (See Figure S3) and fluorescence quantum yield of DPP(6) is calculated to be 0.64 in ACN solvent. The singlet lifetime of this was measured to be 6 ns. The fluorescence intensity and fluorescence quantum yield of DPP(6) along with singlet lifetime remain unaltered in presence of TFA (See Figure S4A). However, here we are able to modulate the fluorescence intensity of DPP moiety of OXDPP-OX(7) relentlessly by acid-base induced transformations of OX moiety. The OX-DPPOX(7) dyad showed a broad emission profile in the range of 550-700 nm with two main emission peaks at around 605 nm and a shoulder at 660 nm (Figure 3A, Curve a), featuring 16 ACS Paragon Plus Environment

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the similar emission profile of DPP(6). A systematic bathochromic shift of around 25 nm, in commensurate to the absorption peak shift, can be estimated for both the emission peaks with respect to its parent DPP(6) emission peak position. This bathochromic shift of emission peak of DPP moiety in OX-DPP-OX(7) can be combined to the fact of extended conjugation of OX-DPP-OX(7). In no surprise, due to bulkiness of OX-DPP-OX(7) the fluorescence quantum yield of the OX-DPP-OX(7) is measured to be slight lower (~0.3) than that of DPP(6). Accordingly, singlet lifetime of OX-DPP-OX(7) reduced to 4.5 ns. However, upon addition of fifty equivalent TFA (in ACN), the fluorescent intensity of OX-DPP-OX(7) quenches 66% of its initial fluorescence. A systematic titration of OX-DPP-OX(7) with TFA (diluted in ACN) shows the proportionate decrease of DPP(6) fluorescence without losing its structural identity (Figure 3B). However, upon addition of tetrabutylammonium hydroxide (in MeOH) in the solution the fluorescence intensity of OX-DPP-OX(7) reverts back almost to its initial intensity with no loss of emission profile features (Figure 3A, Curve c). This acidbase induced fluorescence modulation ("off" and "on") of DPP moiety can be repeated several times without much loss of the molecules. As shown in Figure 1 and discussed above, upon addition of acid OX moiety of OX-DPP-OX(7) opens up and leads to form pnitrophenolate (protonated form) and 3H-indolium cation chromophore with strong absorption at 383 nm and slight enhancement of absorbance at 560 nm region. However, no clear spectral overlap, absorption of ring open form to that emission spectra of DPP(6), satisfying the fluorescence resonance energy transfer (FRET) could be visible. Hence, FRET induced decrease of DPP fluorescence due to acidochromic effect can safely be ruled out and decrease of DPP fluorescence could be rationalized by electron transfer from excited DPP to 3H-Indolium cation. Indeed, photoinduced electron transfer process in this case is highly exoergonic and Gibbs free energy (∆G0) broadly estimated to be ~-0.8 eV considering



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reduction potential of 3H-indolium (~0.8 eV34), oxidation potential and excited singlet state energy of DPP(6) (~0.8 eV61, and 2.4 eV (605 nm) respectively ).

Figure 3. Presents the switching reproduction of OX-DPP-OX(7) fluorescence at 600 nm in ACN solution. The fluorescent intensity was 66% quenched and fully restored by the acid base titrations, and the fluorescent intensity did not decay obviously after six switching cycles. The exceptional fluorescence photo-switching performance of OX-DPP-OX(7) indicates to potential candidate for fluorescence read out materials.

In order to understand the nature of acid induce quenching of fluorescence of DPP moiety in OX-DPP-OX(7), whether it is static or dynamic quenching, we have measured the fluorescence lifetime of OX-DPP-OX(7) in ACN as a function of TFA concentration. As shown in Figure 4, OX-DPP-OX(7) shows single exponential decay with 4.5 ns time constant along with a minor rise component with nearly 200 ps time constant. Upon addition of TFA, this rise component vanishes and the time profile follows bi-exponential decay which consists of very short component with time constant less than response function of our instrument (200 ps) and a along component with time constant 4.5 ns, natural lifetime of OXDPP-OX(7). However, it is evident from the Figure 4 that the amplitude of short component gradually increases on increasing TFA concentration upto 100 equivalent. Keeping the time constant of the short component to be a constraint of fit at 100 ps, we calculate the amplitude of both the components and time constant of the slower component which remains invariant as 4.5 ns and they are shown in Table 1. As shown in Table 1, upon increasing TFA 18 ACS Paragon Plus Environment

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concentration amplitude of short component increases and concomitant decreases of slower component. It is worthy to mention that in presence of TFA opening of OX ring in OX-DPPOX(7) is in equilibrium and it is a pseudo first order reaction in ground state where a stationary state is reached between ring closed form of OX-DPP-OX(7) and ring open form of OX-DPP-OX(7) for any given TFA concentration.

Table 1. Fitting parameters of fluorescence time profile measured at 610 nm consisting decay constants (τ1, τ2) and corresponding amplitudes(a1, a2) of DPP(6) (60 µM) and OX-DPP-OX(7) (~60 µM) in ACN in absence and presence of different TFA concentration.

Molecule [TFA]

DPP(6)

OX-DPP-OX(7)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1st Component 2nd Component

in equiv.a τ1(ps)

a1(%)

τ2(ps)

a2(%)

0

200

-36b

6±0.5

100

500

200

-37b

6±0.5

100

0

200

-30b

4.5±0.5

100

10

200

-17b

4.5±0.5

100

20

200

-7.3b

4.5±0.5

100

30

200

-1b

4.5±0.5

100

40

100

21

4.5±0.5

84

50

100

53

4.5±0.5

36

60

100

68

4.5±0.5

40

200

100

80

4.5±0.5

20

500

100

90

4.5±0.5

16

a

equiv. stands for equivalent concentartion of TFA w.r.t DPP(6) or OX-DPP-OX(7) concentration, indicates rise component.


b

negative amplitude


[TFA] in equiv. 0 40

10000

30

50

60

200

500

Fit

10000

Counts

Counts

IRF 1000

1000 100 0

2

4

6

8

Time(ns)

Residual

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 0 -100 -200 0

10

20

30

Time(ns) Figure 4. Fluorescence decay profiles of OX-DPP-OX(7) in absence and in presence of different equivalent concentration of TFA in ACN.

Upon photo excitation in acidic condition, DPP moieties of both the conformers get excited. In ring open conformer photoinduced intra-molecular electron transfer reaction occurs from excited DPP moiety to 3H-indolium cation resulting very fast quenching of fluorescence lifetime where as DPP moiety in closed conformer shows natural fluorescence of OX-DPP-OX(7) with same time constant of 4.5 ns. As a results, a radiometric change of amplitudes of respective components are observed (Table 1). Even though ring opening reaction is a ground state phenomena, quenching of fluorescence lifetime clearly suggest that quenching of steady state fluorescence is a dynamic process leading to electron transfer from excited DPP to 3H-Indolium cation. Likewise acid-base induced reversible fluorescence switching of OX-DPP-OX(7), thermally reversible photochromic properties of OX with very high switching speed between closed and open form (Figure 1) is explored in OX-DPP20 ACS Paragon Plus Environment

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OX(7) in ACN solution. Photoinduced reversible fluorescence switching is another possible and very fast mode of fluorescence switching of this system. In this studied system photoinduced transformation, closed to open form of OX(5), is ultrafast in nature and followed by very fast thermal reversible transformation. As a result simple steady state fluorescence studies are highly inefficient to monitor the fluorescence on/off states of this system. Even if, nanosecond flash photolysis does not reveal proper dynamic. Hence, we have performed femtosecond time resolved transient absorption (TA) studies of OX-DPPOX(7) along with constituents OX(5) and DPP(6) as control to unveil ring open and close dynamic and therefore fluorescence switching speed of DPP in OX-DPP-OX(7) dyad.

4.3. Ultrafast Transient Absorption Studies. In view to have detail insight of switching dynamic of studied OX(5) we have extended our investigation to femtosecond time resolved TA studies along with model based data analysis approach comprising Global and Target analysis for transient data sets. Figure 5 shows the profile picture of TA studies, which consists of heat map of TA data along with selected TA spectra at different delay times and time profiles at selected wavelengths of OX(5) in ACN solution upon 330 nm excitation. The profile picture summaries switching cycle of OX(5) in ACN solution. A spontaneous rise of positive signal, within 100 fs of excitation, at around 610 nm region (red to green area in heat map, Figure 5(a), spectra Figure 5(b), time profile Figure 5(c)), are observed which decays completely within few ps and this band is assigned to Sn←S1 transition of OX(5). However, this rapid decay of this 610 nm band gives rise to another positive photoinduced absorption band peaking at 435 nm (yellowish green area Figure 5(a), spectra Figure 5(b), time profile Figure 5(c)) which again completely decays within in few ns time scale. In comparison to the peak position we assign this new band to be the ground state absorption of p-nitrophenolate anion.58,60 To have exact value of dynamic parameters and species associated spectra we analyse the TA data matrix globally using sequential target kinetic model Scheme 3. At least 21 ACS Paragon Plus Environment


four sequential components with gradually increased time constants are necessary to fit the data satisfactorily. Vibrationally hot S1, thermally relaxed S1 of ring closed form along with thermally hot ring open form and thermally relaxed ring open form of OX(5) have been considered in four states model. Upon 330 nm excitation some excess energy is deposited to the molecules which undergo very fast to relaxed S1 state and the cleavage of C–O bond occurs in S1 state in 30 ps time giving birth of un-relaxed ring of S0 of ring open form which gets relaxed in 60 ps time and finally relaxed ring open form thermally revert to ring closed from by 2 ns. Time (ps) 40 500 1000

∆OD

2500

0.04

0.04 0.03 0.02 0.01 0.00

(c)

0.03

436 nm 610 nm

0.02 0.01 0.004

0.2

100 500 940

-0.006

2000

610

600

600

(a)

550

2 ns 500 ps 100 ps 40 ps 9 ps 0.2 ps

500

500 450

450 436

550

Wavelength (nm)

∆ OD

1 5

Wavelength(nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(b) 400

400 1 5

40 500 1000 Time(ps)

2500

0.00

0.02 ∆OD

0.04

Figure 5. ∆A heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for the isolated OX(5) (a) in ACN; excitation at 330 nm. As indicated in the color map, the zero level is colored in light blue, red to green indicates positive signals (i.e., photoinduced absorption), and blue denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state bleaching). TA spectra at selected delay time (b) and time traces at selected probe wavelengths (c). Vertical lines in heat map indicate the position selected delay time whereas horizontal lines indicates position selected wavelength for which TA spectra and time profiles are plotted in (b) and (c) respectively.


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The species associated difference spectra (SADS) and estimated population time profiles of each state are shown in Figure 6 (A&B). The SADS1 can be recognized as hot S1 of OX(5) with a strong peak at 615 nm and a broad peak around 475 nm and lifetime of this state ca 1.3 ps. The SADS2 represents the relaxed S1 of with relatively narrow and blue shifted of peak position to 610 nm along with complete disappearance of 475 nm band with lifetime of 30 ps. The SADS3 corresponds to un-relaxed ring open form which shows a new absorption at 435 nm along with ~ 50% of 610 nm absorption. The lifetime of this state is estimated to be 60 ps. This result indicates that in this state, molecules retain both ring open and ring closed identities. Finally SADS4 represent the ground state absorption spectra of ring open form and lifetime of this state is found to 1.9 ns. This result confirms that complete cleavage of C−O bond in OX(5), forming ring opened form of OX(5), occurs in 60 ps and making back of C−O bond, formation of ring closed form from ring opened form, takes place in 1.9 ns. In fact, a full switching cycle, from closed to open form and revert back, can be completed in

hot S1 , τ1=0.9(1.3) ps S1,τ2=41 (30) ps

Ring open conformer

SADS1 SADS2

Ring closed conformer

~1.9 ns.

330 nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SADS3

unrelaxed open OX S0 , τ3=135 (60) ps

SADS4

relaxed open OX S0 , τ4= 1.5 (1.9)ns

S0 (ring closed form) Scheme 3. Kinetic scheme used for Global and Target analysis of the TA data upon 330 nm for both isolated OX(5) and OX-DPP-OX(7). The dynamics was modeled with four sequential decays. The estimated time constants, lifetime of the respective states, are indicated in the figure. The value in the parenthesis are meant for isolated OX(5) in the respective states.



Hot S1, Close OX,

SADS1, Hot S1,Close OX

0.04

Unrelax Open OX,

SADS2, S1, Close OX

S1, Close OX

1.00

Relaxed Open OX

0.75

SADS3, Unrelax Open OX

0.03

(B)

SADS4, Relaxed Open OX

(A)

0.50

0.02

0.25

0.01 0.00 0.03

SADS1, Hot S1,Close OX

Hot S1, Close OX,

(C)

Unrelax Open OX,

SADS2, S1, Close OX

0.02

S1, Close OX Relaxed Open OX

(D)

0.00 1.0 0.8

SADS3, Unrelax Open OX SADS4, Relaxed Open OX

Population

SADS (OD)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.01

0.4

0.00

0.2 -0.01 400

450

500

550

600

0.1

Wavelength (nm)

1

10

100 1000

0.0

Time (ps)

Figure 6. The comparative population profiles and estimated SADS arising out of Global and Target analysis using scheme 3 for isolated OX(5) (A,B) and OX-DPP-OX(7) (C,D) respectively. No much change in dynamic and SADS are observed between isolated OX(5) and OX-DPP-OX(7) upon 330 nm excitation.

To understand the transient behaviour of DPP(6), we have recorded transient absorption spectra of reference (isolated) DPP(6) upon 330 nm excitation, tale of S3 band of DPP(6). The transient absorption spectra is mostly dominated by negative signal due to ground state bleaching (GSB) and stimulated emission (SE) in the 500−600 nm spectral window ( See Figure S5 for TA spectra and decay profiles) along with a broad positive signal below 475 nm region peaking around 395 nm. The spectral profile and dynamic closely resemblance to the reported TA spectra observed for DPP(6) derivative.61 The dynamic of this positive signal exactly follows the dynamics of GSB and SE. Hence this positive band could be assigned to excited state absorption corresponds to Sn←S2 and Sn←S1 transition. Global and Target analysis of observed transient data show minimum of 5 major components with 0.15, 15, 238 and 8000 ps respectively along with a microsecond components (See 24 ACS Paragon Plus Environment

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Figure S6). The species associated different spectra and concentration profile of each component are shown Figure S7. Upon 330 nm S3 excitation, large amount of energy deposited to S3 state which rapidly relaxed to S2 in 150 fs. SADS1 represents S3 state. SADS2 corresponds to S2 which gets relaxed in 15 ps generating vibrationally hot S1 state. Hot S1 state (SADS3) of DPP(6) decays to thermally relaxed or solvent induced conformational relaxed S1 by 238 ps (SADS4) state. A minor (15% of total decay component) rise component of around 200 ps was also detected in fluorescence decay measured by TCSPC. It is important to mention here that time constants of above two states are presumably higher with respect to their assigned state. Similar kinds of time constants were also observed for other derivatives of DPP and were discussed in termed of solvent relaxation.62 Considering the relatively amplitude corresponding to these two time constants, assignment SADS2 and SADS3 are tentative in nature. To unveil the exact nature of these component an excitation dependent transient absorption studies in different solvents of varying polarity are necessary and it is out of the scope of this article. However, the depopulation of S1 state occurs in two channels, it decays to ground state via emissive pathway and populate lowest triplet state by intersystem crossing, in 8 ns time scale. Finally, SADS5 represents Tn←T1 transition with lifetime in microsecond time scale. The global analysis of transient data in 4 ns domain estimates the lifetime of S1 to be 8 ns which bit overestimated with respect to lifetime measured by TCSPC (6 ns) due to presence of long live Tn←T1 transition in TA data. In order to explore the photoinduced switching dynamics of OX in OX-DPP-OX(7) and modulation of fluorescence dynamic of DPP moiety in OX-DPP-OX(7), the transient behaviours of OX-DPP-OX(7), we measured TA spectra of OX-DPP-OX(7) upon 330 nm excitation at which the OX moiety absorbs predominantly. In no surprise, the observed results are grossly parallel to that of isolated OX(5), in term of spectral shape and their dynamics. Figure 7 illustrate the profile picture of transient absorption studies, which consists 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of heat map of TA data along with selected TA spectra at different delay time and time profiles at selected wavelength, of OX-DPP-OX(7) in ACN solution upon 330 nm excitation. The profile picture summarizes switching dynamic of OX-DPP-OX(7). Likewise in OX(5), Sn←S1 excited absorption arises within 100 fs of excitation, at around 610 nm region (reddish yellow to green area in heat map, Figure 7(a), spectra Figure 7(b), time profile Figure 7(c)), and it decays completely within few ps. Note, a negative signal consists of ground state bleaching and stimulated emission is also appeared corresponding to the ground state absorption and fluorescence emission region of DPP moiety of OX-DPP-OX(7) and it becomes more prominent upon decay of Sn←S1 signal of OX(5) moiety. This result clearly reveals that pumping at 330 nm not only excites OX moiety of OX-DPP-OX(7) but also weakly excite DPP(6) moiety of OX-DPP-OX(7). However, likewise isolated OX(5) , the rapid decay of this 610 nm band gives rise to the ground state absorption of p-nitrophenolate anion band peaking at 435 nm (yellowish green area Figure 7(a), spectra Figure 7(b), time profile Figure 7(c)) which again completely decays within in 1−2 ns time scale. It is very important to mention here that, the appeared negative signal (GSB/SE) at 530−600 nm region is also completely disappeared in the same time scale of 435 nm band. Global and Target analysis of TA data of OX-DPP-OX(7) is exactly replicated to that of isolated OX(5) (Scheme 3).


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0 2

5

0.01

∆OD

2500

620 nm, 545 nm, 409 nm

(c)

0.02 ∆ OD

Time (ps) 8 100 500 1000

0.03

595 nm, 436 nm,

0.02 0.007

0.00

-0.003

-0.01

0.2

5

20 400 1000

-0.01

2500 620

600

600

595 545

(b)

500

550

500

450

450

Wavelength (nm)

550

5 ps, 1 ns,

Wavelength (nm)

(a) 0.2 ps, 400 ps,

436 409

400 0 2

5

8 100 500 1000

2500

Time (ps)

20 ps 2.5 ns

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

0.00 0.02 ∆OD

Figure 7. ∆A heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for OX-DPPOX(7) (a) in ACN; excitation at 330 nm. As indicated in the color map, the zero level is colored in light blue, red to green indicates positive signals (i.e., photoinduced absorption), and blue denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state bleaching). TA spectra at selected delay time (b) and time traces at selected probe wavelengths (c). Vertical lines in heat map indicate the position selected delay time whereas horizontal lines indicates position selected wavelength for which TA spectra and time profiles are plotted in (b) and (c) respectively.

Likewise OX(5), four sequential components with gradually increased time constants are good enough to fit the data satisfactorily. Vibrationally hot S1, thermally relaxed S1 of ring closed form along with thermally hot ring open form and thermally relaxed ring open form of OX-DPP-OX(7) are the necessary states. The comparative concentration profiles and species associated spectra of respective states of OX-DPP-OX(7) with respect to isolated OX(5) are shown in Figure 6 (C,D). Hot S1 states of OX-DPP-OX(7) relaxed to S1 state in 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.9 ps and in S1 state cleavage of C–O bond occurs in 41 ps time giving birth of un-relaxed S0 of ring open form which gets relaxed in 135 ps time and finally relaxed ring open form thermally revert to ring closed from by 1.5 ns. In fact, a full switching cycle, from closed to open form and revert back, can be completed in ~1.5 ns. In isolated DPP(6), GSB recovery and SE decay of DPP(6) occurs in 6 ns, and these results predict the fluorescence lifetime of DPP(6) is to be 6 ns.61 Whereas, GSB recovery and SE decay of DPP in OX-DPP-OX(7) occurs in 1.5 ns which indicates that fluorescence lifetime of DPP in OX-DPP-OX(7) is quenched by ~80%. This reduction of fluorescence lifetime of DPP in OX-DPP-OX(7) is combined to dynamic fluorescence quenching and can be rationalized by electron transfer reaction from DPP to 3H-indolium cation. Hence, this TA study confirms that fluorescence intensity of DPP moiety of OX-DPP-OX(7) is modulated by photoinduced switching of OX moiety between ring closed and ring open form upon 330 nm excitation. In fact, a full switching cycle, from closed to open form and revert back, is completed in ~1.5 ns. Furthermore, since this photoisomerization of closed to open form does not require any sensitization of singlet oxygen, this photochromic system can be switched back and forth between its two states for few thousands of times without any decomposition, even in the presence of molecular oxygen. Thus, the fluorescence of DPP in OX-DPP-OX(7) can be switched on and off for few thousands cycles with no sign of degradation by turning 330 nm light "on" and "off".


Page 28 of 41

0

20

Time (ps) 40 150 800

1500

405 nm 460 nm 610 nm

40

40 20 0 -10

0 0.2 10

90

SADS1

40

SADS3

SADS2

60

(C)

20

60

m∆ ∆OD

(D)

20 0

1000 400

610

500

500

(B)

460

450

450

550

1.0

0.8

SADS1

0.6

SADS2

0.4

SADS3 (E)

0.8 0.6 0.4 0.2 0.0

0.2

0

10 20 30 40 50 60 Time (ps)

0.0

405 40 150 800 Time (ps)

1500

m∆ ∆OD

60

20

40

0

20

400

600

1.0

Population

(A)

550

Wavelength(nm)

550

500

Wavelength (nm)

600

0.2 ps 10 ps 90 ps 1 ns

Wavelength (nm)

600

450

Population

m∆ ∆OD

60

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m∆ ∆OD

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400

0

500

1000

1500

Time (ps)

Figure 8. ∆OD heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for OX(5) (~60 µM) in ACN in presence of 100 equivalent TFA (A) excitation at 330 nm. As indicated in the color map, the zero level is colored in light-blue, green to red indicates positive signals (i.e., exited state absorption, ESA), and blue denote negative signals (i.e., decrease in absorption due to stimulated emission (SE) and/or ground-state bleaching, GSB). TA spectra at selected delay times (B) and time traces at selected probe wavelengths (C). Vertical lines in heat map indicate the position of selected delay time whereas horizontal lines indicates position of selected wavelength for which TA spectra and time profiles are plotted in (B&C) respectively. The estimated SADSs (D) and corresponding population profile (E) arising out of global fitting of TA data with three state kinetic model. Inset in (E) shows the population profile in very short time scale.

In order to understand the dynamic of intra-molecular electron transfer between DPP and 3H-indolium cation we have also studied OX(5) and OX-DPP-OX(7) in ACN in presence of 100 equivalent TFA. Figure 8 portrays the summary of the studies of OX(5) in presence of TFA which includes heat map of ∆OD (Figure 8A), transient absorption spectra at selected delay time (Figure 8B), time profile at selected wavelengths(Figure 8C), species associated difference spectra (Figure 8D) and concentration profile of respected SADSs (Figure 8E) In 100 equivalent acidic condition, OX exists predominantly in open ring conformer and pumping at 330 nm excites both 3H-indolium cation and p-nitrophenolate (protonated) moiety as both has absorbance at this wavelength. Immediately after excitation TA spectra appears with two distinct positive signal peaking at 470 and 610 nm along with negative signal below 410 nm associated with ground absorption corresponding to p29 ACS Paragon Plus Environment


nitrophenolate. In 10 ps time domain 460 nm band reduces significantly where as 610 nm band remain unchanged. In longer delay 610 nm band also disappears and a long live a broad band peaking around 440 nm appears. A minimum of three components are required to fit the data globally. Three states sequential target model yields three species associated difference spectra (SADSs) (Figure 8D) along with population profile of the respective state (Figure 8E). SADS1 can be represented as hot S1 of ring open conformer of OX(5) with ESA at 470 and 610 nm and GSB below 420 nm and lifetime of this state is calculated to be 4 ps. SADS2 can be recognize as relaxed S1 state with slight blue shift of the both bands observed in hot S1 state and lifetime of the state is found to be very first which is 55 ps. Finally, a long live SADS3 can be assigned as T1 state of ring open conformer of OX(5), with a ESA band peaking at 450 nm, which decay insignificantly in 6 ns time window.

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Figure 9. ∆OD heat map as a function of probe wavelength (vertical) and probe delay (horizontal) for OXDPP-OX(7) (6x10-5 M) in ACN in presence of 100 equivalent TFA (A) excitation at 330 nm. As indicated in the color map, the zero level is colored green, green to red indicates positive signals (i.e., photoinduced absorption), and blue denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state bleaching). TA spectra at selected delay times (B) and time traces at selected probe wavelengths (C). Vertical lines in heat map indicate the position of selected delay time whereas horizontal lines indicates position of selected wavelength for which TA spectra and time profiles are plotted in (B&C) respectively. The estimated SADSs (D) and corresponding population profile (E) arising out of global fitting of TA data with three state kinetic model. Inset (D) shows the normalized SADSs and inset in (E) shows the population profile in very short time scale.


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Figure 9 shows the pictorial review of the transient absorption studies of OX-DPPOX(7) in ACN in presence of TFA (100 equivalent) following excitation of all three moieties, DPP and 3H-indolium cation and -p-nitrophenol, which includes heat map of ∆OD (Figure 9A), transient absorption spectra at selected delay time (Figure 9B), time profile at selected wavelengths(Figure 9C), species associated difference spectra (Figure 9D) and concentration profile of respected SADSs (Figure 9E) Immediately after excitation TA spectra appears with a distinct positive signal(ESA) centered at 480 nm along with negative signal peaking at 540 and 590 nm associated with ground absorption corresponding of DPP moiety. In first few ps, along with blue shift of ESA band significant change in size and shape is observed and all TA spectra appeared to decay within few tens of ns. Global and Target analysis was performed on TA data matrices using four compartmental/states model (See Figure S8) and obtained species associated difference spectra and corresponding population profiles are shown in Figure 9(D&E). The first SADS with ESA at 455 nm and 1.5 ps time constant can be interpreted as hot S1 of ring open conformer of OX-DPP-OX(7). The shape of SADS2 is quite similar to that of pure DPP(6) but lifetime of this state found to be very smaller that of DPP and it is estimated to be 16 ps. Hence, SADS2 can be recognize predominantly as S1 state of DPP in ring open form of OXDPP-OX(7) and quenching of S1 state can rationalized by intra-molecular electron transfer from DPP to 3H-indolium cation and the time constant of this state can grossly be treated as the formation of charge separated state. The spectral shape and size of SADS3 is quite different than that of SADS2. SADS3 becomes narrow with peak position red shifted and amplitude of absorbance reduces significantly and lifetime of this state can be estimated to be 660 ps. Based on the magnitude of the time constant, shape and size of SADS3, this state can be considered as charge separated (CS) state and the time constant of this state can be assigned as charge recombination (CR) time. Finally, long live SADS4 can be identified as T1 31 ACS Paragon Plus Environment


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state of ring opened OX-DPP-OX(7) which shows lifetime of few tens of ns. Overall, these results clearly describe the phenomena of intra-molecular electron transfer and probably dynamic of CS state.

5. CONCLUSION We have designed and synthesized a new photochrome-fluorophore-photochrome dyad molecule OX-DPP-OX(7) attaching two OX(5) moieties with the skeleton of a fluorophore, a halo functionalized diketopyrrolopyrrole (DPP(6)) by Sonoghashira cross coupling. The steady state photophysical properties, ultrafast switching dynamic, acidochromic and photochromic properties of this dyad have been studied. The OX(5) moieties preserve its photochemical properties regardless of the presence of DPP(6) adjunct. The as prepared dyad exhibit reversible acidochromic behavior allowing switching of oxazine ring between closed and open forms producing the 3H-indolium cation with extended conjugation and the protonated 4-nitrophenolate anion chromophores. Upon addition of TFA the emission of DPP quenched (“off” state) significantly and very fast intramolecular electron transfer occurs from DPP to 3H-indolium cation with 16 ps time constant. Addition of base (TBAOH) allowing ring open form of Oxazine to revert back to ring closed form leading to regain fluorescence emission (“on”) state of DPP. The femtosecond TA studies upon 330 nm excitation reveal almost similar photochromic relaxation dynamic of OX(5) and OX-DPP-OX(7) indicating the retention of photophysical properties of OX in OX-DPP-OX(7) despite the presence DPP appendage. Upon ultraviolet excitation the [1,3]oxazine ring opens to generate a zwitterionic isomer in less than 135 ps (60 ps in case of isolated OX(5)) and in both the instances, the photogenerated species reverts spontaneously back to the original one in 1.5−2 ns. The state of the photochromic moiety within the OX-DPP-OX(7) dyad completely regulates the 32 ACS Paragon Plus Environment

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relaxation dynamics of the fluorescent fragment. The S1 lifetime of DPP in OX-DPP-OX(7) reduces from 6 ns (fluorescence lifetime of isolated DPP(6)) to 1.9 ns, the thermally survival time of ring open form of oxazine, indicating 70% quenching of fluorescence of DPP moiety. Indeed, the fluorescence of DPP in OX-DPP-OX(7) can be modulated with nanosecond speed by photo and thermal induced the interconversion of OX. Furthermore, this as-prepared photo-switchable dyad shows no sign of degradation for few thousands of switching cycles. Hence, this fluorescence photo-switching dyad with ultrafast speeds and very high fatigue resistance may have implication for potential application in designing ultrafast switching devices.

ACKNOWLEDGEMENT B.R and K.N thanks to UGC and Y.V thanks to CSIR for providing fellowship and P.R.B acknowledge the support from CSIR Network project INTELCOAT, CSC-0114.

ASSOCIATED CONTENT Supporting Information Steady-state absorbance spectra of OX(5) and DPP(6), Acid titration of OX(5), 1H NMR, 13C NMR spectra of OX-DPP-OX(7), Fluorescence decay profiles of DPP(6), Profile picture of TA data of DPP(6), Target model SADSs and concentration profile of each state of DPP(6) in ACN and OX-DPP-OX in presence of TFA upon 330 nm excitation in ACN solution.

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