Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the

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Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: TimeResolved Visible Pump – IR Probe Spectroscopic Study Rajib Ghosh, Aruna K. Mora, Sukhendu Nath, and Dipak K. Palit J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11293 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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The Journal of Physical Chemistry

Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump – IR Probe Spectroscopic Study Rajib Ghosh, Aruna K. Mora, Sukhendu Nath and Dipak K. Palit* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400094, India.

Abstract: Fluoren-9-one (FL) molecule with a single hydrogen bond accepting site (C=O group) has been used as a probe for investigation of the dynamics of hydrogen bond in its lowest excited singlet (S1) state using subpicosecond time-resolved visible pump – IR probe spectroscopic technique. In hexafluoroisopropanol (HFIP), a strong hydrogen bond donating solvent, formation of FL-alcohol hydrogen bonded complex in the ground electronic (S0) state is nearly complete with negligible concentration of FL molecule remaining free in solution. In addition to the presence of a band due to hydrogen bonded complex in the transient IR spectrum recorded immediately after photoexcitation of FL in HFIP solution, appearance of the absorption band due to free C=O stretch provides the confirmatory evidence regarding ultrafast photodissociation of hydrogen bonds in some of the complexes formed in the S0 state. Peak shift dynamics of the C=O stretch bands reveal two major relaxation pathways, namely, vibrational relaxation in the S1 state of the free FL molecules and the solvent reorganization process in the hydrogen bonded complex. The latter process follows bimodal exponential dynamics involving hydrogen bond making and hydrogen bond reorganization processes. Similar lifetimes of the S1 states of the FL molecules, both free and hydrogen bonded, suggest establishment of a dynamic equilibrium between these two species in the excited state. Investigations in two other weaker hydrogen bond donating solvents, namely, trifluroethanol (TFE) and perdeutarated methanol (CD3OD), however, reveal different features of peak shift dynamics because of prominence of the vibrational relaxation process over the hydrogen bond reorganization process during the early time.

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Introduction Solute - solvent interactions play important roles for non-equilibrium molecular processes in liquid phase chemical reactions.1-2 In particular, intermolecular hydrogen bonding, a site specific interaction, between the solute and solvent molecules significantly affects chemical reactivity of the solute in many chemical and biological systems.3-11 For example, intermolecular photoinduced electron transfer (PET) dynamics between a donor and an acceptor molecule may be strongly affected by hydrogen bond mediated electronic coupling in the excited state.4-7 Similarly, photophysical properties and excited state dynamics of molecules are also sensitive to hydrogen bond donating ability of the protic environment.12-13 Thus, hydrogen bond breaking and making dynamics is of utmost importance to understanding these properties in condensed phase. Both experimental and theoretical investigations on the dynamics of hydrogen bond reorganization are the active areas of research aiming at molecular level understanding of this ubiquitous process.14-30 Dielcetric relaxation or time resolved fluorescence measurement techniques have extensively been used to probe the solvation dynamics of protic solvents which indirectly provides the dynamics of intermolecular hydrogen bond reorganization process.31 - 34 Molecular dynamics simulation has been successfully used to predict the hydrogen bond making, breaking and reorganization times.35 - 37 However, measuring the vibrational frequencies in real time at the hydrogen bonding site can probe the hydrogen bond reorganization event in terms of hydrogen bond breaking and making processes. Han and Zhao published a large volume of theoretical works, which advocated strengthening of hydrogen bonds upon electronic excitation of the hydrogen bonded complex and fast nonradiative deactivation of the excited chromophore via hydrogen bond mediated energy transfer was considered as the direct manifestation of strengthening of the hydrogen bond.17-22 While both the cases, namely, hydrogen bond strengthening and weakening upon electronic excitation of the hydrogen bonded complex may obviously be possible depending primarily on the change of charge density on the donor and / or acceptor groups, many recent experimental results have predicted that hydrogen bond is broken upon electronic excitation of the hydrogen-bonded complex, which is formed between the solute and solvent molecules in the ground electronic (S0) state.23-27 It does not necessarily contradict the predictions made by the theoretical works of Han and Zhao about hydrogen bond strengthening in the excited electronic states.

Although the hydrogen bond(s) breaks immediately after electronic 2

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excitation due to major changes in the distribution of electronic charge in the excited state of the solute, hydrogen bond(s) may subsequently be reformed via reorganization of the solvent molecules around the excited state solute to form a more stable hydrogen bonded complex as compared to that in the ground state. An organic chromophore with a single hydrogen bonding site, such as a carbonyl group, is an ideal probe to monitor hydrogen bond dynamics by directly probing the stretching frequencies of the C=O group in real time following photoexcitation. Elsaesser and coworkers, for the first time reported hydrogen bond dissociation upon electronic excitation of Coumarin 102-chloroform hydrogen bonded complex employing femtosecond resolved IR spectroscopic measurement.23-25 Palit et al. reported hydrogen bond breaking followed by reorganization of hydrogen bond in the excited state of the hydrogen bonded complex of Coumarin 102 and aniline.26 Palit et. al. also investigated the excited state dynamics of Fluoren-9-one (FL) in alcohols using visible pump – visible probe spectroscopy and suggested hydrogen bond breaking in the excited state of FL – alcohol hydrogen bonded complex and hydrogen bond reorganization taking place in the similar time scale of average solvation time of the alcohol.15 Tominaga and coworkers reinvestigated the hydrogen bond dynamics in the excited state of FL in CD3OD using sub-picosecond resolved visible pump IR probe spectroscopy to show hydrogen bond breaking and reorganization processes.27-28 Han et al, however, advocated for hydrogen bond strengthening upon electronic excitation of these chemical systems from the results of TDDFT calculations.19-20 FL has a single hydrogen bond accepting site (C=O group) and forms 1:1 and/ or 1:2 hydrogen bonding complexes with the hydrogen bond donating solvents through association with either or both of the available lone pair of electrons on the oxygen atom of the carbonyl group, respectively (Scheme 1).15,

27

Occurrence of hydrogen bonding interaction can be

probed by monitoring C=O stretch frequencies both in the ground (S0) as well as in the lowest (first) excited singlet electronic (S1) states.



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R

R

O H

O

a

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O

O H

O

b

R

H

O

c

Scheme 1: Chemical structure of Fluoren-9-one (FL) and hydrogen bonded complexes with

alcohols: Non-hydrogen-bonded FL with free C=O (a); 1: 1 hydrogen – bonded complex (b) and 1:2 hydrogen bonded complex (c). R stands for CH3- (for methanol), CF3-CH2- (for trifluoroethanol (TFE) and (CF3)2-CH- for hexafluoroisopropanol (HFIP).

In this work, we unequivocally demonstrate breaking of intermolecular hydrogen bond between FL and a hydrogen bonding solvent in the electronic excited state of FL by employing sub-picosecond time-resolved visible pump – IR probe spectroscopic technique. For this purpose, we have deliberately selected a strong hydrogen bond donating solvent, namely 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), in which formation of hydrogen bonded complex with FL is almost complete in its S0 state and concentration of remaining free FL molecules in solution is nearly negligible. Therefore, appearance of significant population of the excited singlet (S1) state of free FL molecules following photoexcitation, provides the confirmatory evidence regarding photodissociation of the hydrogen bonded complex. Further, we demonstrate that a significant fraction of the population of FL molecules in the S1 state remains free without being engaged in formation of intermolecular hydrogen bond with the solvent. For the first time, we could resolve bimodal hydrogen bond reorganization in the hydrogen bonded complex of the S1 state of FL in real time. In addition, similar lifetime of the S1 state of free FL molecule and those forming hydrogen bonded complex suggests a dynamic equilibrium between them and hence the lifetimes of hydrogen bond making and breaking processes are faster than the lifetime of the S1 state of FL. 2. Experimental Section 4 ACS Paragon Plus Environment

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FL was purchased from Sigma-Aldrich and was recrystalized from ethanol before use. Coumarin 153 (C153) dye was purchased from Lambdachrome, Germany, and was used as received. Spectroscopic grade solvents, 2,2,2-trifluoroethanol (TFE) and HFIP were purchased from Sigma-Aldrich. Per-deuterated solvents, namely, acetonitrile-d3 (CD3CN) and methanol-d4 (CD3OD), with more than 99% isotopic purity were purchased from Armar Chemicals, Switzerland. These solvents were used as received without further purification UV-visible and FTIR spectra were recorded using JASCO spectrometers (model V670 and FT/IR6300, respectively). Time-resolved infrared (TRIR) spectroscopic measurements were performed using a visible (400 nm) pump – IR probe spectrometer (TAPPIR, CDP Corporation, Russia), which is described in brief as follows: The pump pulses at 400 nm were the second harmonic of the output of a laser system based on Ti:sapphire oscillator and a chirped pulse amplifier consisting of a pulse stretcher, a regenerative amplifier, a multipass amplifier and a pulse compressor, producing laser pulses of 40 fs duration and energy of 3 mJ/pulse at 800 nm at the repetition rate of 1 kHz (Amplitude Technologies, France). Tunable IR pulses in the 1400 - 1700 cm-1 region were generated by difference frequency mixing of the signal and idler outputs of an optical parametric amplifier (TOPAS-C from Light Conversion, Lithuania). The probe and reference pulses were obtained using the reflection of a CaF2 wedged window. Probe pulses ( FL*-CD3OD. This inference is in perfect agreement with the energy of the hydrogen bonds between the carbonyl group of an excited probe molecule and those strong hydrogen bond donating solvents as estimated earlier.55, 57 Rate of nonradiative energy relaxation in hydrogen bonded complex also depends on the strength of the hydrogen bond.19,20, 58,59 Shortest lifetime of the S1 state of 24 ACS Paragon Plus Environment

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the FL*-HFIP (80 ps) also justifies the formation of the strongest hydrogen bond in the S1 state of FL.

Conclusion: We experimentally demonstrate the photoinduced cleavage of intermolecular hydrogen bond in FL-alcohol hydrogen bonded complex. We further demonstrate that major population of the FL molecules remain free from intermolecular hydrogen bonding during the excited state lifetime. This is in stark contrast to the fact that in the ground electronic state, nearly all the FL molecules remain hydrogen bonded with strong hydrogen bond donating solvents. Vibrational cooling of the hot FL molecules in the excited state and hydrogen bond reorganization in the hydrogen bonded complex of the excited FL molecules are the major relaxation processes observed following photoexcitation of the hydrogen bonded complex. Similar decay lifetimes of free and hydrogen bonded FL molecules in the excited state possibly suggest a dynamic equilibrium between them because of very fast hydrogen bond breaking, making and reorganization times. We propose that partial n* character due to mixing of nπ* and * kinds of the excited states possibly responsible for hydrogen bond breaking in the FL-alcohol system and the relaxation behaviour described above. Stronger hydrogen bonded complex is formed in the S1 state of FL in the stronger hydrogen bond donating solvent and hence shorter the lifetime of the S1 state because of faster nonradiative rate. ASSOCIATED INFORMATION SUPPORTING INFORMATION Deconvolution of the FTIR spectra assuming Lorentzian shapes of the IR absorption bands, C=O stretch frequencies (in cm-1) and relative intensities of the peak corresponding to free C=O or two kinds of hydrogen bonded complexes, Deconvolution of the TRIR spectrum of FL in cyclohexane recorded at 0.5ps delay time, Deconvolution of the TRIR spectra of FL in acetonitrile at different delay times, TRIR spectra recorded following photoexcitation of FL in HFIP to show the real changes of absorbance during evolution of the transient spectra, Deconvolution of the TRIR spectra of FL in HFIP at 0.5 ps and at 50 ps delay times, Deconvolution of TRIR spectra of FL in TFE at three delay times, TRIR spectra recorded following photoexciytation of FL in CD3OD, Deconvolution of TRIR spectra of FL in 25 ACS Paragon Plus Environment


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CD3OD at 0.5 ps, 4 ps and 50 ps, Peak shift dynamics and temporal dynamics following photoexcitation of FL in CD3OD, Correlation of C=O frequencies in the S0 and S1 states and solvent parameters.

This information is available free of charge via internet at http://pubs.acs.org. Author Information Corresponding author :*E-mail: [email protected], Phone: (+91)-22-25595091. SN and DKP are also associated with Homi Bhabha National Institute, Mumbai.

Acknowledgements Authors gratefully acknowledge full financial support from Department of Atomic Energy, India. DKP acknowledges the J. C. Bose National Fellowship Award of Department of Science and Technology (DST), India.

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Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the

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