Direct Observation of Cascade of Photoinduced Ultrafast

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Direct Observation of Cascade of Photoinduced Ultrafast Intramolecular Charge Transfer Dynamics in Diphenyl Acetylene Derivatives: Via Solvation and Intramolecular Relaxation Venugopal Karunakaran, and Suresh Das J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05264 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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The Journal of Physical Chemistry

Direct Observation of Cascade of Photoinduced Ultrafast Intramolecular Charge Transfer Dynamics in Diphenyl Acetylene Derivatives: Via Solvation and Intramolecular Relaxation Venugopal Karunakarana,b,*, Suresh Dasa,c,# a

Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-

National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695 019, Kerala, India. bAcademy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India. cJawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India,

#

Present address: Kerala State Council for Science, Technology and Environment,

Thiruvananthapuram, 695 004, Kerala, India. * E-mail Address: [email protected] Telephone No.: 091-471-2515240

1 ACS Paragon Plus Environment


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1.

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Abstract Interaction of light with electron donoracceptor conjugated systems leading to

intramolecular charge transfer (ICT) plays an essential role in transformation of light energy. Here the cascade of photoinduced ICT processes is directly observed by investigating the excited state relaxation dynamics of cyano and mono/di methoxy substituted diphenyl acetylene derivatives using femtosecond pump-probe spectroscopy and nanosecond laser flash photolysis. The femtosecond transient absorption spectra of the chromophores upon ultrafast excitation reveals the dynamic of intermediates involved in transition from initially populated FrankCondon state to local excited state (LE). It also provides the dynamic details of the transition from the LE to the charge transfer state yielding the formation of the radical ions. Finally the charge transfer state decays to the triplet state by geminate charge recombination. The latter dynamics are observed in the nanosecond transient absorption spectra. It is found that excited state relaxation pathways are controlled by different stages of solvation and intramolecular relaxation depending on the solvent polarity. The twisted ICT state is more stabilized (978 ps) in acetonitrile than cyclohexane where major components of transient absorption originate from S1 state.


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2.

Introduction The universal phenomena observed in condensed phase reactions are solvation

dynamics1-3 altering the charge distribution of the solute leading to the formation of charge transfer state and intramolecular relaxation leading to torsional motion or isomerization4-6. The excited state dynamics of the chromophores involving charge transfer processes determine the efficiency of variety of molecular systems including photosynthesis, solar energy conversion and DNA repair7-9. However the dynamics of the charge transfer processes are significantly controlled by solvation dynamics and intramolecular relaxation10-11. A large number of experimental12-20 and theoretical6, 21-26 studies have been devoted to understand the underlying mechanism controlling the electronic pathways, geometrical structure and dynamics of the photoinduced intramolecular charge transfer (ICT) state in the excited state relaxation of the different electron donor-acceptor -conjugated systems upon photoexcitation4-5. Though the characteristics of the ICT state with dual fluorescence are described by different models including planar27, partially twisted or pre twisted28, twisted29, rehybridized30 and multifaceted dynamics31, controversies on the mechanism of ICT are prevalent21, 26, 32-33.

Here we report the excited state structural dynamics and relaxation pathways comprising local excited, ICT and triplet states of the diphenyl acetylene derivatives containing mono methoxy (MA1) or di methoxy (DA1) and cyano as the donor and acceptor groups respectively (Figure 1). These materials showed significant differences in their photophysical properties between the solution and solid states where higher fluorescence quantum yields were exhibited in the solid state (0.8) compared to the solution(0.2)34. Hence these materials could be applied for organic optoelectronic devices. Interestingly they showed highly red shifted emission maxima of 3 ACS Paragon Plus Environment


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DA1 in polar solvent of acetonitrile compared to the non-polar solvent of cyclohexane. Hence the transient absorption spectra were measured using both femtosecond pumpprobe spectroscopy and nanosecond laser flash photolysis in polar and nonpolar solvents to explore the photoinduced charge transfer dynamics. It is found that solvation and intramolecular relaxation dynamics control the charge transfer dynamics of the chromophores based on the polarity of the solvent.

N

OCH3

MA1

N

OCH3

DA1

OCH 3

Figure 1 Chemical structures of the methoxy (MA1) and di methoxy(DA1)cyano diphenylacetylene derivatives.


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3.

Experimental Section

General Details:

The synthetic procedure of DA1 and MA1 have already been reported34.

The solvents used in the measurements such as acetonitrile and cyclohexane were of HPLC grade (Merck) and were used as received. Steady State Measurements:

Absorption spectra were recorded using a Shimadzu UV-

2600 UV-Visible spectrophotometer. Steady-state fluorescence experiments were performed with a FluoroLog-322 (Horiba) which was equipped with a 450W Xe arc lamp by using optically dilute solutions. The low temperature emission spectra were also measured using the same instruments at 77K using liquid nitrogen. Nanosecond Transient Absorption Spectra:

Nanosecond

laser

flash

photolysis

experiments were performed by exciting the samples with the third harmonic (355 nm) from an INDI -40-10-HG Quanta Ray Nd: YAG laser and using Applied Photophysics model LKS 60 laser kinetic spectrometer. The probing light source was a 150 W xenon arc lamp. The light of the probe transmitted through a 1 cm sample quartz cuvette was dispersed by a monochromator and detected by a photomultiplier coupled to a digital oscilloscope (Agilent Infiniium DSO8064A, 600 MHz, 4 GSas-1). The analyzing and laser beams were fixed at right angles to each other. The laser power of every laser pulse was registered by using a bypath with a fast silicon photodiode. Solutions for laser flash photolysis studies were deaerated by being purged with argon at least for 20 min before the experiments. All the experiments were conducted at room temperature. Femtosecond Transient Absorption Measurements:

The basic seed laser pulses were

obtained from Ti:sapphire laser (Mai Tai HP, Spectra Physics, USA) pumped by 14 W frequency doubled Nd:YVO4 (532 nm). It was centered at 800 nm (80 MHz repetition rate) with pulse



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width of < 100 fs and an average power of 2.5W. A part of (25%) fundamental pulsed laser beam was amplified at 1 kHz using a Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics) pumped by the second harmonic (527 nm, 30 W) from intercavity-doubled, diode-pumped Q switched Nd:YLF laser (Empower-30 Spectra Physics). The amplified laser output with energy of 4 mJ centered at 800 nm having pulse width of < 120 fs was split (75:25%) into two beams, in which high energy beam is converted to desired excitation wavelengths (pump, 325 nm) by coupling it into a TOPAZ (Prime, Light Conversion). The pump beam was then passed through a computer-controlled optical chopper and focused (3 m) on the sample cell. The another part of amplified beam (200 mW) was focused on a 1 mm thick sapphire plate to generate a white light continuum (350-1000 nm), which finally spilt into two for sample and reference probe beams. The sample cell is a 0.4 mm optical path quartz cylindrical cell placed in a variable speed rotating holder. After passing through the sample cell, the white light continuum is coupled into a 100 μm optical fiber connected to imaging spectrometer. The pump probe spectrophotometer setup was based on an ExciPro spectrometer (CDP Systems Corp). Typically, the time-resolved absorption spectra were acquired by averaging over 2000 excitation pulses at all spectral delay time. The polarization of the pump pulse was set at the magic angle (54.7) relative to the probe pulse to recover the isotropic absorption spectrum. The effective time resolution of the ultrafast spectrometer is determined to be about > 120 fs.


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4.

Results and Discussion Figure 2 shows the absorption and emission spectra of DA1 in acetonitrile (ACN) and

cyclohexane (CHX), where vibrational structure in CHX and broad and structureless feature in ACN were observed34. As the structured emission band observed in non-polar solvent bears a mirror image relationship to the absorption band, it appears to originate from the same excited electronic state. However the observation of significant bathochromic shift along with broadening of emission spectra with increase of solvent polarity could be due to the change of conformation, where excitation of these molecules leads to a weakly emitting locally excited state (LE) retaining nearly the same dipole moments as the ground state, which converts into a more polar emissive state by an intramolecular charge transfer (ICT) process. This was supported by the existence of separated LE (feeble, 351 nm) and ICT (strong, 451 nm) states which were clearly seen in the emission spectra of the compound in ACN. The emission spectra of DA1 in ACN and CHX at low temperature (77 K) were also measured (Grey color in Figure 2). The emission spectra of DA1 in CHX at 77 K become broader and shifted towards red region ~392 nm. This could be due to the increase of solvent polarity with decrease of temperature altering the ICT and LE ratio35. The increase of solvent polarity caused by more ordered orientation of solvent molecules at the vicinity of solute molecule would increase the interactions between the solvent and solute by lowering the temperature36. The observation of larger red shifted fluorescence spectra of DA1 in ACN (~100 nm) compared to the spectra in the CHX reveals that the charge transfer state is more stabilized in the dimethoxy derivative (DA1) compared to the methoxy derivative (MA1) in ACN. The longer fluorescence lifetime (2.03 ns) of DA1 in ACN compared to CHX (0.37 ps) were observed upon exciting at 335 nm using nano second LED source (Figure 2 inset). These observations prompt us to investigate the occurrence and evolution



dynamics of intramolecular charge transfer state by following the excited state relaxation dynamics upon ultrafast photoexcitation. 5

(a) CHX

DA1 10

CHX= 0.37 ns acn= 2.03 ns

4

10

Flu. counts

1.0

0.5

3

10

2

10

1

10 10

0.5

0.0

10 20 Time, ns

30

-1

(b) ACN

8,600cm

exctn= 325 nm

0.0 1.0

0

Fluorescence, normalized

0

Absorbance, normalized

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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300

400

500

600

Wavelength, nm

Figure 2 Normalized absorption and fluorescence spectra of DA1 in CHX (a) and ACN (b) at room temperature. The fluorescence spectra measured at low temperature, 77 K are shown in grey. The insets show the fluorescence decay profiles of DA1 in ACN and CHX. Chemical structure of dimethoxy-cyano diphenylacetylene is also given.

Figure 3 shows the femtosecond time-resolved transient absorption spectra of DA1 in ACN measured upon excitation at 325 nm in room temperature. For sake of clarity, transient absorption spectra at some representative time delays are shown. The panel a shows the spectral evolution from -200 to 150 fs with incremental step of 50 fs. Induced optical density OD > 0 corresponds to photoinduced absorption of excited species and OD < 0 indicates depopulation 8 ACS Paragon Plus Environment

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0.10

490

(a) DA1 in ACN tep

150fs

50 fs s

0.05

-200fs

0.00 0.10

Absorbance

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(b)

0.2 ps

371

0.05

23.5ps

575

0.00

0.04

(c)

28.5ps

X2 476

0.02

1.7ns

0.00 350

400

450

500

Wavelength, nm

550

Figure 3 Femtosecond time resolved transient absorption spectra of DA1 in ACN upon excitation at 325 nm measured at different time delays. The arrows show the direction of the spectral evolution.



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of ground state or the stimulated emission. During the early time scales (panel a), weak stimulated emission (~360 nm) shifted to red and gradually replaced by positive transient absorption bands centered at 371 and 490 nm. The panel b shows the evolution from 200 fs to 23.5 ps where the intensity of the peak around 484 nm was decreased and simultaneously shifted towards blue region. Typically the blue shift of excited state absorption results from the solvent stabilized LE state as solvation dynamics gradually lowering the excited state potential energy surface with delay time. During these spectral time delays there was simultaneous increase of absorption around 371 and 575 nm revealing the formation of the species from the local excited state. The spectral evolutions at longer time scales starting from 28.5 ps to 1.7 ns are shown in the panel c. Though the intensity of overall absorption spectra was decreased, the clear formation of peaks around 476 nm and 410 nm (feeble in intensity) and persistent of absorption maxima at 371 and 575 nm were observed. In order to understand the effect of polarity of solvent on the relaxation dynamics of the excited state, the femtosecond time resolved transient absorption spectra of DA1 in CHX were measured (Figure 4). At the earlier spectral delays (panel a, -200 to 200 fs) there was feeble red shifted stimulated emission attaining the steady state emission maximum (368 nm) and simultaneous increase of excited state absorption around 500 nm as in the case of ACN. The panel b represents the spectral evolution from 0.3 ps to 28.5 ps, where the decrease of absorption intensity was observed. It was found to be a feeble or no blue shift of excited state absorption and an isosbestic point at ~ 436 nm in these spectral delays. At longer time scales (from 37.5 ps to 1.7 ns), decrease of overall spectral intensity along with formation of new peak around 406 nm and persistent of absorption maxima around 483 and 570 nm were observed.


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(a) DA1 in CHX

500

200fs

st ep

0.06

50 fs

0.03

-200fs

0.00

Absorbance

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0.06

0.3ps

(b)

28.5ps

0.03 436 0.00 367 0.06

(c) 37.5ps 1.7ns

0.03 406

483

0.00 350

400

450

500

550

Wavelength, nm

Figure 4 Femtosecond time resolved transient absorption spectra of DA1 in CHX upon excitation at 325 nm recorded at different time delays as shown in Figure 3.



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Similarly the transient absorption spectra of the mono substituted alkoxy compound (MA1) were measured in ACN and CHX to examine the effect of alkoxy on the excited state dynamics. The transient absorption spectra of MA1 in ACN and CHX are given in the supporting information as Figure S2 and S3. Over all it is found to be nearly the same spectral behavior with increase of time delay as in the case of the dimethoxy derivative (DA1). In ACN, immediately after excitation strong excited state absorption signal around 488 nm raised and decayed rapidly to transform into excited state absorption maxima around 370 and 590 nm. Compared to the DA1 in ACN, there was a clear appearance of two isosbestic points (430 and 570 nm) which signaled the interconversion of two participating states.

In order to understand the complete dynamics of long lived components ( viz. triplet state and radical ions) seen in the femtosecond transient absorption spectra, the nanosecond transient absorption spectra of DA1 and MA1 derivatives were measured by laser flash photolysis in Ar and O2-saturated ACN and CHX, using a 355 nm nanosecond laser (9ns) as the excitation source. The transient absorption spectra of DA1 in ACN (Figure 5a) exhibited absorption maxima at around 410 and 480 nm along with feeble shoulder around 370 and 570 nm. The negative change in absorbance, the bleach at around 330 nm is due to the ground state absorption of the molecule. With increase of spectral time, the overall decrease of absorbance was observed. The Figure 5a inset compares the transient absorption spectra of DA1 in argon (black) and oxygen (red) atmospheres at 1 μs after the laser irradiation. In the presence of oxygen, the transient at 480 nm was quenched completely whereas the peak at 410 nm along with shoulder around 370 nm persisted.


Absorbance

(a)

410

ACN

480

0.2

0.2

0.0

-0.2 300

400

500

600

700

Wavelength, nm

0.0 0.5 s , 5.0 s,

0.05

0.00

3.0 s,

exctn= 355 nm

330 400

(b) 0.10

1.5 s, 10.0 s

CHX

500

700

490

0.5 s , 1.0 s, 2.0 s, 3.0 s

0.5

0.0

400

570

400

400

600

1.0

Absorbance

-0.2 300

Absorbance

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


Absorbance

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500

500

600

700

Wavelength, nm

600

700

Wavelength, nm Figure 5 Nanosecond transient absorption spectra of DA1 in Ar saturated ACN (a) and CHX (b) obtained by laser flash photolysis upon excitation at 355 nm. The spectra at different time scales are given in the figure. Transient absorption spectra measured in the argon (Black) and oxygen (Red) -saturated conditions at 1 μs after the laser pulse are shown in the insets for comparison. 13 ACS Paragon Plus Environment


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To explore the effect of solvent polarity on the formation and decay of triplet state and radical ions of the chromophores, nanosecond laser flash photolysis has also been carried out in nonpolar solvent, CHX. The Figure 5b shows the nanosecond transient absorption spectra of DA1 in CHX at different time delays. It showed a transient peak at 490 nm with clear shoulder around 400 and 570 nm. It is interesting note that the intensity of 490 nm band is stronger in CHX compared to the ACN. This may reflect the direct formation of triplet from the local excited state of the molecule. The insets Figure 5b showed the comparison of transient spectra of DA1 in CHX in the Ar and oxygen medium at 1 s. In the presence of oxygen the peak at 490 nm disappeared completely and feeble absorption at around 400 and 570 nm were seen. Similarly nanosecond transient absorption spectra of MA1 in ACN were measured and given in the Supporting Information as Figure S4. As seen in the DA1, it showed the absorption maximum at around 410 and 490 nm along with bleach around 320 nm. Figure S4b compares the transient absorption spectra of MA1 in argon and oxygen atmospheres at 0.5 μs after the laser irradiation. In the presence of oxygen, the peak at 490 nm disappeared completely whereas the peak at 410 nm was persisted with noticeable absorbance. The analysis of the femtosecond transient absorption spectra consisting of a three dimensional dataset (wavelength, time and change in absorbance) was performed with the global analysis program GLOTARAN37. The intramolecular charge transfer dynamics of the chromophores cannot be described by a mono exponential time constant, as it could be overlapped with other simultaneous process of solvation dynamics, vibrational relaxation and intramolecular vibrational redistribution which caused the spectral relaxation leading to the complex dynamics31. Five and four exponential components were optimally obtained to fully describe the relaxation dynamics of DA1 in ACN and CHX respectively.


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Those were 1 = 58 fs, 2 = 539 fs, 3 = 3.28 ps, 4 = 978.36 ps, and 5 ≥ 1.95 ns for ACN and 1 = 1.12 ps, 2 = 8.48 ps, 3 = 361.74 ps and 4 ≥ 1.2 ns for CHX. [Due to the limited range of investigated delay times (maximum 1.7 ns), the longest time constant would have erroneousness]. The corresponding decay associated spectra of DA1 in ACN and CHX are shown for comparison in Supporting Information as Figure S5. In the case of DA1 in ACN the decay associated spectra of the lowest time constant 1 (58 fs) reflects the formation of excited state absorption and stimulated emission immediately after the Franck Condon state upon photo excitation. This can be due to the initial solvation relaxation dynamics dominated by coupled libration and translation motion which are highfrequency modes of the solvent with small amplitude38-40. During the second time constant (539 fs), the stimulated emission evolving towards red and excited state absorption evolving towards blue represents the solvation dynamics contributed by damped rotational motion and / or diffusional rotational and translational motion of solvents38-41. Thus the chromophores relaxed to the local excited state. Alternatively as the rate of the process is so fast, the chromophores may consider to be vibrationally hot upon excitation and the time constant would reflect the intramolecular vibrational relaxation10-11. The third time constant reveals the formation of intramolecular charge transfer state as the 3 (3.28 ps) is longer than the dielectric relaxation. Now the excited state absorption maximum shifted to 460 nm. It is consistent with the previous report that the charge transfer state of MA1 appeared with time less than 10 ps in ACN and the extent of charge separation depended on the polarity of the solvent, though the time resolution of the experiments was about 10 ps 42. As mentioned earlier the observations of relatively large Stokes shift, broad and unstructured fluorescence spectral profiles in polar solvents compared to the non-polar solvents



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suggests that compounds undergo a significant structural change upon photoexcitation in polar solvents. Thus the conformation of the ICT state could be quite different from the FranckCondon ground state. The crystal structure of DA1 showed the dihedral angle of 27.22 between benzonitrile and dimethoxy benzene moiety34. It could be possible that the ICT state would undergo more twisted relaxation, resemblance to the twisted ICT model. Hence in such a high polarity, additional twisting relaxation involving both the solvent and vibrational coordinates could lead to twisted intramolecular charge transfer

(TICT) state (4 = 978 ps)

with

instantaneous and large change of dipole moment (13.74D experimentally determined34). Thus the charges are well separated by twisting relaxation leading to the formation of the cation and anion radical species. From the charge separated state it decays to the triplet state by the geminate charge recombination, well known and main decay channel of ICT43-44. Hence 5 (1.95 ns) could reveal the formation time constant of triplet state of the chromophores. In the case of DA1 in CHX the solvation processes (1= 1.12 ps and 2 = 8.48 ps) were slowed down compared to the polar solvent. The decay time constant of the peak at 500 nm and the third time constant 3 obtained by global analysis were found to be about 361 ps which was similar to the fluorescence lifetime (370 ps, Figure 2 insets) of DA1 in CHX. Therefore this band can be assigned to the Sn  S1 transition of DA1. In addition the close resemblance of decay time constant at 500 nm and formation time constant of the peak at 406 nm (See Figure S6), and existence of isosbestic point at 436 nm reflecting that the transient at 406 nm was formed from the state responsible for the transient at 500 nm. With increase of spectral delay time, the broad band of 500 nm was gradually transformed to the peak at 480 nm to which Tn T1 transition was ascribed (videinfra).


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The global analysis of the transient absorption spectra of MA1 in ACN and CHX resulted the time constants of 1 = 64 fs, 2 = 444 fs, 3 = 2.16 ps, 4 = 659 ps, and 5 ≥ 1.45 ns for ACN and 1 = 1.14 ps, 2 = 6.78 ps, 3 = 344 ps and 4 ≥ 1.04 ns for CHX. It was found that transient components were more stabilized in DA1 compared to MA1. The pattern of the latter transient absorption spectra obtained from the femtosecond pump-probe spectroscopy resembled to the nanosecond transient absorption spectra at earlier time scales in both solvents. The results of analysis of nanosecond transient absorption spectra of DA1 and MA1 in argon and O2 equilibrated ACN and CHX by fitting with either mono or bi exponential decay are given in the Supporting Information as Table S1. The kinetic decays of DA1 in ACN at 480 and 410 nm in Ar and O2 saturated medium are shown in Supporting Information as Figure S7 a and b respectively. The decay at 480 nm fitted with two exponential decay provided time constants of 2.3 s (0.31) and 9.7 s (0.69) in argon-saturated solutions. Similarly for 410 nm, the lifetimes were determined to be 2.1s (0.71) and 8.9 s (0.29) in deaerated solution. The Figure S8a and b show the kinetic traces of DA1 in CHX at 490 and 400 nm in the presence of Argon and Oxygen. The decay at 490 nm were fitted with two exponential decay in the Argon medium and provided the time constant of 0.67 s (0.26) and 1.94 s (0.74). For 400 nm, the fitted results provided the time constant of 0.96 s (0.62) and 2.02 s (0.38). It is found that nanosecond transient spectra obtained in CHX recovered faster compared to that in ACN. The kinetics at 490 nm for MA1 in ACN was fitted with mono exponential provided the time constant of 0.75 s (Figure S9a). Whereas the kinetics at 410 nm were fitted with two exponential decay providing the time constant of 0.85 s (0.27) and 5.3 s (0.73) (Figure S9b). The transient absorption maximum observed around 480 nm in both solvents

was

assigned to the Tn  T1 absorption as it was perceived during the laser pulse (~10 ns), 17 ACS Paragon Plus Environment


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diminshing of its time constant by molecular oxygen and similar homologues of DA1 showed triplet state in this region which was senstized by -carotene34. It is well known that radical anions will quench effectively by molecular oxygen, whereas radical cations are relatively unaffected. From the spectral behavior and data analysis in argon and oxygen medium, it is inferred that the peak around ~410 and 570 nm due to the absorption of the anion and cation radical species of the chromophores respectively. This is supported by the fact that the transient absorption at 410 nm closely resembles the absorption spectra of benzonitrile anion45-47 and 570 nm could be due to the alkoxy benzene cation radical48. It is to be noted here that though the charges are completely separated, the Columbic interaction and solvent induced polarization can shift or broaden the absorption spectra of radical species.46 It is observed that triplet state and anion radical of benzonitrile are stabilized in DA1 in ACN compared to CHX. To correlate the experimental findings, theoretical optimization of the two derivatives with accurate characterization of the potential-energy profiles using the time-dependent density functional theory (TDDFT) is required and presently in progress.

The proposed excited state relaxation dynamics of the chromophores upon ultrafast photoexcitation are shown in the Scheme 1 based on the spectral behavior and kinetic analysis. In polar ACN, after the laser irradiation, the chromophore is excited to the Frank-Condon (FC) state where a dipole is induced and the solvent configurations are nearly the same as in ground state molecules (GS). As there is an interaction between the electric field and permanent dipoles of solvents, the solvent molecules begin to reorient themselves to reach their new equilibrium position. The initial dominant solvation relaxation (Hot) due to the coupled libration and translation motion of solvent followed by combined the intermediate and final part of solvation


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Scheme 1 Schematic diagram illustrating excited state deactivation dynamics involving the solvation and intramolecular charge transfer relaxation after laser excitation. Earlier dynamics involves mostly the solvent rearrangement and later dynamics relaxes along theintramolecular coordinate.



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dynamics involving damped rotation, and diffusive rotational and translational motion (LE, Sol) were observed38-40. These time constants are in good agreement with solvation dynamics of coumarin153 in ACN

38

. Thus the excited chromophore reaches to local excited state (LE) by

reorganization of solvents. As DA1/MA1 derivatives are push–pull chromophores, the formation of intramolecular charge transfer state (ICT) is more stabilized in polar solvent (emitting at 451 nm) compared to the CHX (emitting at 351 nm)34. As the third time constant is longer than the dielectric relaxation and based on the literatures31, the time constant of 3.25 ps is proposed to the initial charge transfer processes leading to planar intramolecular charge transfer (PICT) state where the chormophores would be in planar or feebly twisted in configuration.

PICT state is

further relaxed through twisting between benzonitrile and methoxy benzene moieties, similar to the twisted ICT model31, having higher dipole moment (13.74D) than the ground state (6.72D). It was reported that the excited state dipole moment of MA1 were determined by anisotropy decays, found to be 17 D for ACN and linearly dependent on the solvent polarity42. Thus the charges are well separated leading to the formation of radical cation and anion species which finally recombined to form the triplet states. In the case of CHX, it is well known that in non-polar solvents, LE is the lowest excited state and emission comes mainly from the LE state19. As the intensity of 490 nm band is stronger in CHX (Figure 5) compared to the ACN and the fraction of ICT state decreases with decrease of polarity of the solvent18, there is not much involvement of TICT state, by twisting relaxation along the intramolecular nuclear co-ordinate. Hence the major path ways for the formation of triplet state was from the local excited singlet state and PICT.


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5.

Conclusions The main findings of this work can be summarized as follows. 1. As the transient

absorption spectra at later timescale measured in the femtosecond pump-probe spectroscopy is consistent with the transient absorption spectra at earlier time obtained by the nanosecond laser flash photolysis, it is feasible for the complete assignment of intermediates involved in the excited state relaxation dynamics of chromophores after the photoexcitation. 2. The solvation time constants obtained for the compounds in ACN are consistent with the literatures and 1 and 2 are assigned to the different stages of solvation of the solute38. 3. The transient absorption spectra peaked around 500 nm at early dynamics can be assigned to the Sn S1 transition. 4. It is found that the formation of PICT state is about 2 order of magnitude faster in ACN (3.25ps) compared to CHX (361ps) revealing the solvent polarity dependence of the ICT state formation barrier49. 5. The decay associated spectra of 978 ps obtained for DA1 in ACN reflects the absorption spectra of twisted intramolecular charge separated state where 420 and 575 nm are assigned to benzonitrile anion and alkoxy benzene cation radical species respectively. 6. The band around 480 nm in the nanosecond transient absorption spectra is ascribed to the TnT1 transition and stabilized in ACN compared to the CHX. Thus in ACN, excited state dynamics is controlled by intramolecular charge transfer dynamics along with ultrafast solvation processes. Whereas in the nonpolar solvent, relaxation dynamics is primarily controlled by the solvation dynamics. The overall results allowed us to realize the simultaneous roles of the solvation and the intramolecular charge transfer process in the excited state relaxation dynamics depending the polarity of the solvents.



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Associated Content Supporting Information The details of the experiments and instrumentation, the femtosecond and nanosecond transient absorption spectra of MA1 in ACN and CHX, decay associated spectra of DA1 and nanosecond transient kinetics in Argon and Oxygen atmospheres and are given in the supporting information. This information is available free of charge via the Internet at http://pubs.acs.org

Author Information Corresponding Author *E-mail: [email protected]. Phone: 091-471-2515240 Notes The authors declare no competing financial interest.

Acknowledgment V.K. gratefully acknowledges the DST-SERB Extra Mural Research Funding (EMR/2014/001116) Government of India for financial support. This work is supported by Council of Scientific and Industrial Research (CSIR) under the Project NWP 55. The authors acknowledge DST Indo-European project (OISC-LARGECELL) for setting up femtosecond pump-probe spectrometer.


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Table of Content


Direct Observation of Cascade of Photoinduced Ultrafast

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