Disentangling Timescale of Vibrational Cooling, Solvation and

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Disentangling Timescale of Vibrational Cooling, Solvation and Hydrogen Bond Reorganization Dynamics Using Ultrafast Transient Infrared Spectroscopy of Formylperylene Rajib Ghosh, Aruna K. Mora, and Sukhendu Nath J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01920 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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The Journal of Physical Chemistry

Disentangling Timescale of Vibrational Cooling, Solvation and Hydrogen Bond Reorganization Dynamics using Ultrafast Transient Infrared Spectroscopy of Formylperylene. Rajib Ghosha,b*, Aruna K. Mora,a,b and Sukhendu Natha,b aRadiation

and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India,

b

Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094, India

Abstract Unraveling dynamics of solvation and hydrogen bond (H-bond) reorganization between solute and solvent is crucial to understand the importance of specific and nonspecific interactions in solution phase chemical reaction. Ultrafast time-resolved infrared (TRIR) spectroscopy provides direct opportunity to monitor site-specific intermolecular dynamics in real timescale by probing vibrational marker bands in excited state of a solute. Herein, we report the real time dynamics of vibrational cooling, solvation and hydrogen bond reorganization of formylperylene (FPe) through TRIR spectroscopy of carbonyl (C=O) stretching mode in nonpolar, polar aprotic and polar protic solvents. High sensitivity of C=O stretch frequency (ῡC=O) to photoinduced intramolecular charge transfer processes induced by specific and nonspecific solvent interactions led us to monitor the dynamics of dipolar solvation and site-specific H-bond formation and reorganization processes by TRIR method. In nonpolar cyclohexane, ῡC=O stretch band appears at 1610 cm-1 and exhibits negligible spectral shift over several tens of picoseconds. In acetonitrile, ῡC=O peak shift to 1594 cm-1 and exhibits further temporal red shift of about 5 cm-1 with a characteristic solvation timescale of acetonitrile (τ~0.5 ps). In methanol, ῡC=O exhibits two bands corresponding to free and H-bonded FPe in early timescale. The free FPe population converts to hydrogen bonded population with a lifetime of about 10 ps. Vibrational cooling (τvc ~ 12 to 20 ps) in the excited electronic state of FPe could independently be monitored from the temporal dynamics of the ring vibration mode which is less sensitive to solvation and hydrogen bonding. Present study provides insight to the specific and nonspecific solvation controlled charge transfer dynamics in aprotic and protic solvents using FPe as a probe. 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

1. Introduction Solution phase chemical reactivity relies on solute solvent interaction at molecular level.1,2 Nonspecific dipolar interaction and specific interaction such as hydrogen bonding often play crucial role in the kinetics and outcome of a chemical reaction. Ubiquitous nature of hydrogen bonding in physical, chemical and biological systems has attracted intense spectroscopic investigation for past several decades.3-6 Hydrogen bonding interaction significantly contributes to chemical stability and often plays decisive role in the structure and function of many biological and chemical systems, e. g., electron transfer dynamics, charge separation and stabilization, excited state deactivation, chemical and biological catalysis and so on.7-14 In solution phase, hydrogen bonding interaction between a solute and surrounding environment is known to be dynamic in nature.15-22 With the advent of femtosecond laser spectroscopic techniques, the dynamical features of hydrogen bonding interaction has been possible to interrogate with great details to reveal how hydrogen bond making and breaking processes control the energy flow in a chemical system and how it does affect the chemical reactivity.16-24 Femtosecond resolved fluorescence and transient absorption techniques have popularly been used to address the intermolecular hydrogen bonding dynamics by watching transient spectral evolution measuring energetic stabilization of photoexcited chromophores capable of engaging themselves with hydrogen bond donating solvents. However, these methods extract the dynamic H- bond reorganization event by measuring temporal shift of transient emission or absorption spectra and do not provide molecular level insight to this fascinating interaction. Direct watching the Hbonding reorganization process in real time is desirable as hydrogen bond breaking, making and reorganization can be a sequence of events with site specificity and obtaining molecular level picture of these events would be of broader significance. Ultrafast vibrational spectroscopy has been proven to be a powerful technique as it probes the vibrational frequency changes in real time and the vibrational frequencies are often exquisitely sensitive to the H-bond interaction. In recent years, time-resolved infrared spectroscopy (TRIR) has been explored in great detail, which has advanced the microscopic understanding of static and dynamic feature of H-bonding in equilibrium and non-equilibrium conditions.20-30

Hydrogen bonding reorganization and

fluctuation around a solute significantly influence charge transfer, electron transfer and excited state deactivation processes of the solute. Real time measurement of H-bond formation and 2 ACS Paragon Plus Environment

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reorganization provides detailed picture of site specific interaction between solute and solvent molecules and brings mechanistic insight to hydrogen bond induced chemical and photochemical reactivity. Here in, we present ultrafast dynamics of solvation and solute-solvent hydrogen bond reorganization process in the excited state of 3-formylperylene (FPe, Structure shown in scheme 1), a simple chromophore with single hydrogen bond accepting carbonyl site engages in intermolecular hydrogen bonding with protic solvents.31 H

C

O

Scheme 1: Chemical structure of 2-Formylperylene (FPe) used as vibrational probe in present TRIR experiments.

Irrespective of solvent polarity and proticity, the excited state lifetime of FPe is in the range of few nanosecond and triplet yield of this molecule is negligible.31 Hence, early time dynamics in photoexcited FPe is not complicated by its internal conversion or intersystem crossing processes. Previously, employing ultrafast transient fluorescence and absorption spectroscopic techniques in visible spectral region, Prof. Mohammed and coworkers have shown that excited state properties of FPe is quite sensitive to both solvent polarity and intermolecular H-bonding and the charge transfer character of this molecule is enhanced by collective hydrogen bond reorganization process.31,

32

While these previous studies have largely unveiled charge transfer dynamics

facilitated by intermolecular hydrogen bonding, the measurement techniques employed in these studies inherently lack the sensitivity to probe the site-specific hydrogen bonding and hence microscopic details of hydrogen bonding reorganization could not be discerned. A theoretical perspective to molecular level understanding to the hydrogen bonding energetic and kinetics to this system were attempted by quantum chemical calculation33 and molecular dynamics simulation34 and hydrogen bond induced charge transfer enhancement of FPe in alcohols have been supported. In present work, exploiting exquisite sensitivity of the C=O stretch frequency (ῡC=O) to the extent of charge transfer and hydrogen bonding interaction with solvent molecules, a molecular level understanding of hydrogen bond reorganization dynamics and associated 3 ACS Paragon Plus Environment


intramolecular charge transfer kinetics was explored by ultrafast TRIR spectroscopy. Comparison of TRIR spectra and kinetics of FPe in apolar, polar aprotic and polar protic solvents reveal the nonspecific dipolar interaction and site-specific hydrogen bonding dynamics significantly contribute to the enhancement of intramolecular charge transfer in FPe. The time scale of hydrogen bond formation and reorganization in alcohol has been possible to distinguished from the dynamic evolution of ῡC=O stretch frequency. It was also possible to monitor the vibrational cooling kinetics of the Franck-Condon S1 excited state by monitoring the temporal evolution of ring vibration mode which appeared to be less sensitive to solvation or hydrogen bonding event. 2. Experimental Section 3-Formylperylene (FPe) was synthesized by formylation of perylene and was purified by column chromatography. The purified product was characterized by proton NMR spectroscopy [1H-NMR (δ, CDCl3): 10.28 (s, 1H), 9.16 (d, 1H), 8.25 (t, 3H), 7.9 (d, 1H), 7.8-7.48(m, 6H)]. (Deuterated solvents, namely, cyclohexane-d12, acetonitrile-d3 and methanol-d4 were purchased from Armar Chemicals (Switzerland) and used as received. UV-visible absorption spectra and mid-infrared spectra were recorded in a JASCO spectrophotometer (Model No. V670) and FTIR spectrometer (Model No. JASCO FT/IR6300), respectively. Ultrafast time-resolved infrared spectroscopic measurements were carried out by TAPPIR set up (CDP Corporation, Russia) coupled to a femtosecond amplified laser (Amplitude technologies, France) driven optical parametric amplifier - difference frequency generation (OPA-DFG) systems (TOPAS, Light Conversion, Lithuania). Details of experimental set up was reported earlier.35 Sample solution taken in a 0.4 mm thick rotating cell were excited by 100 fs laser pulse at 400 nm and transient vibrational spectra and temporal kinetics in the mid IR spectral region (1500 – 1700 cm-1) were recorded at magic angle polarization condition of visible pump and mid IR probe pulse. The temporal resolution, i.e., instrument response function of the set up is measured to be about 200 fs. The kinetic traces at different frequencies were fitted with a sum of exponentials convoluted with the instrument response function. 3. Result and Discussion Steady state visible absorption spectra of FPe in three different solvents are shown in Figure 1A. The spectrum in cyclohexane exhibits strong vibronic progression, characteristics of perylene moiety. In acetonitrile, the vibronic bands are largely smeared due to dispersive interaction with 4 ACS Paragon Plus Environment

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polar solvents. We note that change in absorption spectrum from nonpolar to polar solvent is not due to any kind aggregation effect as confirmed by concentration dependent absorption experiments which shows no dependence in the concentration range of 10 μM to 1 mM. The absorption feature in protic methanol is only slightly shifted as compared to that in acetonitrile as Franck-Condon transition are associated with core C=C vibration of perylene ring which is less perturbed by intermolecular hydrogen bonding interaction at carbonyl group. FTIR spectrum of FPe in the carbonyl stretch frequency region is displayed in figure 1B. As compared to cyclohexane, carbonyl stretch frequency of FPe in acetonitrile is ~12 cm-1 red shifted due to dipolar interaction with polar solvent molecules. In methanol, carbonyl stretch spectrum exhibits two bands. In addition to the dominant higher frequency band at 1694 cm-1, there is a relatively weaker broad shoulder at about 1680 cm-1, attributable to hydrogen bonded C=O stretch. The result is in perfect agreement with previous report of Mohammed et. al.32 We also note that intermolecular H-bond induced red shift of C=O stretch of FPe was qualitatively reproduced by DFT calculation.33 Gaussian deconvolution of the peaks of C=O stretch band in methanol shows that relative ratio of higher and lower energy bands is about 7:3. Considering the fact that transition dipole of C=O stretch vibration does not change significantly upon intermolecular hydrogen bonding,36, 37 we can estimate that at equilibrium, only about 30% of FPe molecule remains hydrogen bonded with methanol. In essence, major fraction of the solute molecules remains free from hydrogen bonding interaction in the ground state of FPe in neat methanol which is in agreement to closely similar visible absorption spectrum in methanol and acetonitrile. A

Cyclohexane-d12 Acetonitrile-d3 Methanol-d4

0.05

0.00 350

400

450

Wavelength /nm

0.05

0.00 1660

500

B

0.10

A bsorbance / OD

0.10

A b so rb an ce /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


5 ACS Paragon Plus Environment

1680

1700

Wavenumber / cm -1

1720


Figure 1: UV-Vis (A) and FTIR (B) absorption spectra of FPe in cyclohexane-d12, acetonitrile-d3 and methanol-d4. TRIR spectra of FPe at a time delay of 100 ps after 400 nm photoexcitation in three different solvents, namely cyclohexane (apolar), acetonitrile (polar aprotic) and methanol (polar protic) are shown in figure 2. At 100 ps delay, the excited state reaches thermal and dynamical equilibrium with the surrounding solvent media. The TRIR spectrum in deuterated cyclohexane is characterized by two positive excited state absorption (ESA) bands at 1540 cm-1 (ESA1) and 1610 cm-1 (ESA2) and a bleach band at 1700 cm-1. In acetonitrile-d3, the ESA1 band appears exactly at same position as in cyclohexane, while the ESA2 band shifts to lower energy to 1583 cm-1. In methanol-d4, a hydrogen bond donating solvent, the ESA2 band further redshifts to 1560 cm-1, while the ESA1 band remains in the same position as in cyclohexane and acetonitrile. The bleach band position in the three solvents corresponds to ground state C=O stretch frequency as shown in figure 1B.

2

A ESA-1

0  A bsorbance / m 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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3

ESA-2

Bleach

B

0 4

C

2 0 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700 Wavenumber / cm -1 Figure 2. TRIR spectra of FPe at 100 ps delay following photo excitation in (A) cyclohexane-d12, (B) acetonitrile-d3 and (C) methanol-d4.


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The lower energy excited state absorption band (ESA1 band at 1538 cm-1 region), which is less sensitive to solvent polarity and hydrogen bonding is tentatively ascribed to the perylene ring mode vibration. On the other hand, ESA2 band is assigned to the C=O stretch frequency in the S1 state which shifts to lower energy as we increase the solvent polarity and hydrogen bonding. Appearance of ῡC=O stretch at significantly lower frequency in S1 state of FPe as compared to ground state clearly suggest intramolecular charge transfer (ICT) facilitated C=O bond order reduction which is in agreement with TDDFT calculation reported by Yang et. al.33 TDDFT study correctly proposed redshift of ῡC=O (about 50 cm-1) stretch in S1 state, even though failed to estimate it quantitatively (ΔῡC=O = 95 cm-1 in cyclohexane). Our experimental results further reveal that ῡC=O stretch is highly sensitive to solvent characteristics which clearly suggests that intramolecular charge transfer from perylene to carbonyl moiety is facilitated by dipolar stabilization in polar aprotic solvents and is further augmented by hydrogen bonding interaction in polar protic solvents. The enhanced charged transfer results in the decrease in the C=O bond order leading to lowering in the ῡC=O value is in agreement to TDDFT prediction of Yang et. al.33 The increase in differential carbonyl stretch frequency (ΔῡC=O) from ground state to S1 state of FPe, shown in table 1, clearly reflects that solvent polarity and hydrogen bonding increases charge transfer character in FPe resulting in red shift of carbonyl stretch frequency. This observation is in perfect agreement with the conclusion from time-resolved fluorescence and transient absorption experiments by Mohammed et. al. who previously proposed that charge transfer in FPe is enhanced by solvent polarity as well as by collective hydrogen bonding network in protic solvents.31 In the present study, we could directly monitor the effect of dipolar solvation and specific hydrogen bonding interaction at the C=O site due to exquisite sensitivity of the ῡC=O to both of these parameters. On the other hand, the perylene ring mode in the excited state (ESA1 band) is shown to be sensitive to vibrational cooling dynamics due to coupling with the excited low frequency modes.38-39 In the next section, we present early time transient spectral evolution of these transient IR bands which provide the timescale of vibration relaxation, solvation and hydrogen bond reorganization in nonpolar, polar aprotic and protic solvents. Table 1: Carbonyl stretch frequency of FPe in ground (S0) and S1 electronic state in different solvents.

Solvent

ῡC=O (S0) cm-1

ῡC=O (S1) cm-1

ΔῡC=O cm-1

Cyclohexane-d12

1702

1607

95



Acetonitrile-d3

1690

Methanol-d4

1694 1680 (H-bonded)

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1585

105

1562 (H-bonded)

118

Figure 3 presents the TA spectral evolution of the FPe in deuterated cyclohexane. Being apolar in nature, cyclohexane does not impart significant intermolecular interaction on the solute and hence, time-resolved dynamics is expected to represent purely intramolecular relaxation of FPe. TA spectral evolution shows blue shift (about 2.5 cm-1) of ESA1 band in 60 ps timescale, while the ESA2 peak frequency remains unchanged exhibiting nominal band narrowing occurring in this timescale. It can be noted that, as expected, the bleach band corresponding to ground state ῡC=O does not evolve in this timescale as FPe* molecules remain in the excited state for several nanoseconds prior to deactivation to ground state (the lifetime of FPe in cyclohexane is ca. 5 ns31). The band narrowing with significant blue shift of ESA1 band assigned to ring vibration mode reflects vibrational cooling process. The blue shifting of the band originates from the off-diagonal anharmonic coupling of the ring mode with the vibrationally excited several low frequency modes. Relaxation of the low frequency modes results to decrease in the coupling strength and thus a frequency up-shift is observed38-40.


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 A bsorbance / m OD

3

A

Time delay (ps) 0.3, 2 10, 20 40, 60

2 1 0 ESA1

ESA2

Bleach

-1

1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700 1720

wavenumber / cm B

1538

2.0

1538 cm-1  = 18 ps

1.5

-1

C

-1

2.5

 / cm


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


1537

ESA1  = 20 ps

1.0 1536

0.5 0

20

40

60

80

Time / ps

0

100

20

40

60

Time / ps

80

100

Figure 3. TRIR dynamics FPe in cyclohexane-d12 following 400 nm laser excitation. (A) transient spectral evolution, (B) temporal kinetics at 1538 cm-1 (C) dynamic peak shift of ESA-1 band.

The time constant of the vibrational cooling process is estimated to be about 18 (± 2) ps as obtained from the dynamic peak shift of ESA1 band as well as the temporal kinetics at 1538 cm-1. The time constant of vibrational cooling process measured in present study is consistent with the transient visible spectral evolution of FPe in cyclohexane, reported by Omar F. Mohammed, which showed transient spectral narrowing up to 70 ps.32 As coupling to the low frequency vibrations is modespecific, carbonyl stretch band (ESA2) of FPe in the S1 state seems to be weakly coupled to vibrationally excited low frequency modes and hence exhibits negligible spectral evolution due to vibrational cooling process. Advantageously, less sensitivity of ῡC=O stretch to the vibration cooling does not complicate solvation and hydrogen bond reorganization dynamics probed by dynamic spectral evolution of ῡC=O stretch band. 9 ACS Paragon Plus Environment


TRIR spectral evolution of FPe in acetonitrile is presented in figure 4A. The ring vibration mode (ESA1 band) is shown to undergo similar spectral change in the early 50 ps timescale as observed in cyclohexane. Interestingly, the ESA2 band corresponding to C=O stretch undergo a larger spectral evolution within first couple of picoseconds exhibiting significant red shift with concomitant band narrowing, which is in sharp contrast to minimal spectral evolution of ESA2 band observed in cyclohexane. Spectral evolution of ESA2 implies the effect of solvation in acetonitrile which enhances intramolecular charge transfer process resulting the lowering of the C=O bond order and hence the spectral red shift. The dynamic peak shift of C=O stretch fits exponentially to a lifetime of 0.5 ps, which is in excellent agreement to the solvation time measured by the time-resolved fluorescence stokes shift methods employing standard solvation probes41, as well as with the fluorescence up-conversion measurements of FPe in acetonitrile.31 It can be noted that limited temporal resolution of our TRIR set up (~200 fs) could not resolve the ultrafast inertial component of solvation which gets completed within 100 fs.41 The contribution of ultrafast solvation component is indirectly evident from the fact that at 200 fs, the ῡCO in acetonitrile (1588 cm-1) appears at about 20 cm-1 down frequency as compared to that in cyclohexane (1610 cm-1). This suggests that the inertial component of solvation contributes to major part of solvation response and is accomplished within the instrument response function and could not be captured in our study. We have observed only the slower component of solvation manifested in the dynamic red shift of the C=O stretch band. On the other hand, dynamic blue shift of ESA1 band fits with a time constant of about 19 ps, attributable to the lifetime of vibrational cooling process. This is similar to that of the value obtained in cyclohexane, which suggests, in absence of specific solventsolute interaction, vibrational cooling dynamics appears to be insensitive to solvent polarity. Note that the temporal kinetics of the signal at selected frequencies yield time constants, similar to that obtained from dynamic peak shift analysis (Figure 4B).


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A

Time delay(ps) 0.2, 0.5, 1, 2, 5, 10 20, 30, 50

4  Ab so rb an ce / m OD

2

0 ESA-1

ESA-2

Bleach

-2 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700 Wavenumber / cm -1 1592 C B ESA2 1590 2

 = 0.5 ps

-1

1585 cm 0.5 ps(r), 18 ps(d)

1588 -1

1 0

 / cm


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


4

0

0

1543 cm : 17 ps(r)

1

2

3

4

5

1539

ESA1  = 19 ps

1538

-1

2

1586

1537 0

10

20 30 40 50 Time / ps

0

60

20 40 60 Time / ps

80

Figure 4. (A) TRIR spectral evolution of FPe in acetonitrile-d3 following 400 nm laser excitation. (B) Temporal kinetics of transient signal at two selected frequencies. (C) Dynamic peak shift of ESA1 and ESA2 bands along with exponential fit.

Figure 5A shows the TRIR spectral evolution of FPe in deuterated methanol. Immediately after photoexcitation (i.e. at 200 fs), ESA2 band appears very broad due to coexistence of both free and H-bonded C=O population, similar to that in ground state. However, in ground state, free and H-bonded FPe exist in equilibrium with the dominance of the former. In contrast, we note that even at 200 fs, the peak intensity corresponding to H-bonded C=O is higher than that of free C=O 11 ACS Paragon Plus Environment


stretch. This indicates that a significant fraction of free FPe molecules gets hydrogen bonded within 200 fs of photoexcitation to S1 electronic state. Possibly, the solvent molecules which are slightly misoriented in the ground state, manage to form hydrogen bond by small reorganization of bond length and bond angles. Thus, the inertial solvation in methanol has a contribution from hydrogen bond reformation in the first solvation shell. This situation in present case is contrasting to the behavior of Coumarin102 (C102)25 or Fluoren-9-one (9-FL)35 in alcoholic solutions where ultrafast cleavage of solute-solvent hydrogen bond is reported to take place following photoexcitation of the solutes to S1 state. Ultrafast breaking of hydrogen bond in C102 or 9-FL in S1 state was attributed to the decreased electron density at the hydrogen bonding site upon electronic excitation.25, 35 In present case, the electronic excitation is dominated by the charge transfer from the perylene ring to the carbonyl group resulting significant electron density augmentation favorable for stronger H-bonding interaction.33 However, a significant fraction of FPe* population remains free from hydrogen bonding at 200 fs and hydrogen bond reformation takes place on a slower timescale of few tens of picoseconds through diffusive motion of the solvent molecules to appropriately orient at the carbonyl site to form strong hydrogen bond. This is captured from spectral evolution of ESA2 band in 30 ps timescale, which exhibits decay of ῡC=O at 1600 cm-1 region corresponding to depopulation of free solutes and concomitant growth in the 1560 cm-1 region, corresponding to population of H-bonded complexes. Thus TRIR spectral evolution in methanol directly captures the hydrogen bonding event in the excited state of FPe*. Temporal kinetics at these wavelengths measure the hydrogen bond formation time of about 10 (± 1) ps at the carbonyl site (Figure 5B). We note that this time constant of hydrogen bond formation of a methanol molecule with the carbonyl of FPe matches with recombination lifetime of methanol following hydrogen bond cleavage in methanol oligomer.42


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5

Time delay (ps) 0.2 10 2 20 5 30

(A)

3 2 1 0 ESA-1

-1 1520

1540

1560

1580

Bleach 1600

-1

3 (B) 2

-1 1540 cm :12 ps

1

1620

1640

wavenumber / cm -1 1575 (C)  / cm

1500

ESA-2

1570

1660

1680

1700

ESA-2  (ps) = 2.1, 16

1565 1560

1590 cm -1 : 11 ps (d) 1560 cm -1 : 9 ps (r)

1

-1

0 1539

ESA-1  = 12 ps

 / cm


4


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


1538

0

0

10

20

30 40 Time / ps

0

50

10

20 30 Time / ps

40

50

Figure 5. (A) TRIR spectral evolution of FPe in deuterated methanol following 400 nm laser excitation. (B) temporal kinetics at selected frequencies encompassing ESA1 and ESA2 bands. (C) Dynamic peak shift of ESA1 and ESA2 band.

On the other hand, orientational relaxation of methanol molecules is reported to take place with a lifetime of about 1.7 ps and 17 ps.43 Hydrogen bonding reorganization by orientational relaxation of methanol molecules around FPe in excited state facilitates intramolecular charge transfer which manifest as dynamic red shift of ESA2 peak from 1570 cm-1 to 1560 cm-1, as well as significant band narrowing up to 50 ps. Dynamic peak shift of ESA2 band (i.e., carbonyl stretch band) fits 13 ACS Paragon Plus Environment


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biexponentially with lifetime of about 1.9 ps and 16 ps (Figure 5C) in agreement to orientation relaxation time of methanol.43 Thus, hydrogen bond reorganization process facilitating attainment of equilibrium conformation of hydrogen bonded network of the solvent molecules in the first and second solvation shell around photoexcited FPe has been captured. On the other hand, ESA1 peak shows small blue shift representing the vibrational relaxation and the time constant of this process is determined to be about 12 ps from the exponential fit of the transient signal at 1540 cm-1 as well as dynamic peak shift kinetics (Figure 5B and 5C). It is interesting to note that the vibrational cooling time in methanol (~12 ps) appears to be significantly faster than that in cyclohexane and acetonitrile (~18-20 ps), attributable to intermolecular hydrogen bonding interaction, which provides a faster channel for solute to solvent energy transfer. This is consistent with the previous reports where faster vibrational cooling is noted in protic solvents. 44-46 Thus two transient IR bands of FPe probe two independent relaxation process, namely, vibrational cooling of the photoexcited chromophore and the solvent reorganization dynamics. The timescale of these processes in different kind of solvents are summarized in Table 2. Table 2: Summary of the time constants of different processes elucidated from TRIR experiments.

Time constant (ps)a

Solvent Vibration relaxation

Solvation and /or H-bond reorganization

Cyclohexane-d12

19 (±1)

-------

Acetonitrile-d3

18 (±1)

0.5 (±0.1)

Methanol-d4

12 ((±1)

2.0 ((±0.1), 16 (±1) 10 (± 1)b

aUltrafast

component (< 200 fs) of solvation or hydrogen bond reorganization could not be measured due to the limitation of temporal resolution of TRIR spectrometer. bTime constant of hydrogen bond reformation process.

It is important to note here that the present system offer the opportunity to monitor hydrogen bond formation and reorganization around the FPe* due to existence of both H-bonded and free FPe molecules in the ground state equilibrium, even in neat methanol, while in excited state, entire population of FPe solutes dynamically reaches hydrogen bonded configuration due to thermodynamic stabilization of charge transfer state augmented by intermolecular H- bonding. Previous studies on dynamic peak shift of transient fluorescence or transient absorption in visible 14 ACS Paragon Plus Environment

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region of FPe measured total solute-solvent interaction energy and timescale of the process, without detailing microscopic interaction mechanism. Further, in time-resolved optical spectroscopy, optically prepared Franck-Condon state undergoes both vibrational relaxation and solvation in similar timescale and often makes it difficult to distinguish the two processes, as dynamic stokes shift measures the cumulative effect of solvation and vibrational relaxation process. Present work disentangles vibration cooling dynamics from dipolar solvation as well as hydrogen bond formation and reorganization timescales owing to difference response of two vibrational marker bands in excited state to vibrational relaxation and solvent reorganization induced charge transfer event. On the other hand, hydrogen bond reformation and reorganization event timescales were separated due to exquisite sensitivity of carbonyl stretch to hydrogen bond induced charge transfer phenomenon. Temporal dynamics of hydrogen bond formation and reorganization process in schematically presented in Scheme 2. H 3C O

CH 3

H H

C

H

O

S1

C

O

H 3C

H

O H

O

O CH 3

O H

~10 ps

~16 ps

H-bond formation

H-bond reorganization

Excitation

CH 3

H H

C

O

O CH 3 H

H

S0

C

H

O

C

O CH 3

O H

+

Scheme 2. Schematic representation of hydrogen bond making and hydrogen bond reorganization following photoexcitation of FPe in methanol. The ultrafast component (< 1 ps) of solvation originated from reorganization of preformed hydrogen bonded population is not shown.



4. Conclusion Photoinduced charge transfer associated dynamics of solvation, hydrogen bond reorganization and vibrational relaxation of FPe in protic and aprotic solvents were probed in real time by monitoring carbonyl stretch and ring mode in the excited state. Temporal dynamics of perylene ring vibration mode measures vibrational cooling dynamics which is shown to be accelerated in hydrogen bonding solvent. Nonspecific polar solvation dynamics of acetonitrile is exhibited by charge transfer induced red shift of carbonyl stretch. Site specific H-bond formation and reorganization in the excited state of FPe in methanol could be directly monitored. The timescale of hydrogen bond formation and hydrogen bond reorganization was measured to be about 10 ps and 17 ps, respectively. Molecular level detailing of solvation and hydrogen bond reorganization process by monitoring vibrational marker band in photoexcited state in real time is expected to be useful for mechanistic understanding of hydrogen bond induced charge separation and catalytic processes.

AUTHOR INFORMATION *Corresponding author: Dr. Rajib Ghosh, E-mail: [email protected], [email protected] Note: Authors declare no competing financial interest.

ACKNOWLEGEMENT Generous funding from Department of Atomic Energy, India is gratefully acknowledged. Authors thank Dr. H. Pal and Dr. P. D. Naik of RPC Division for their support and encouragement. REFERENCES: 1. Kumpulainen, T.;

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TOC graphic


Disentangling Timescale of Vibrational Cooling, Solvation and

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