Solvent effect on multiple emission and ultrafast dynamics of higher excited states

Accepted Manuscript Research paper Solvent effect on multiple emission and ultrafast dynamics of higher lying excited states Dipak Kumar Das, Krishnandu Makhal, Debabrata Goswami PII: DOI: Reference:

S0009-2614(18)30518-9 https://doi.org/10.1016/j.cplett.2018.06.038 CPLETT 35737

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

22 April 2018 8 June 2018 18 June 2018

Please cite this article as: D. Kumar Das, K. Makhal, D. Goswami, Solvent effect on multiple emission and ultrafast dynamics of higher lying excited states, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett. 2018.06.038

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Solvent effect on multiple emission and ultrafast dynamics of higher lying excited states Dipak Kumar Das1, Krishnandu Makhal1, Debabrata Goswami1* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur- 208016, Uttar Pradesh, India E-mail:- [email protected]

Abstract: We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the visible region: S2→S0, and the other at NIR region: S1→S0. We show that exact band-positions, intensities, and fluorescence-decay-timescales (τ) depend strongly on viscosity and polarity of solvents. Interestingly, though the faster component of τ increased for IR144 with increasing viscosity and chain-length of alcohols, the reverse was seen for IR140, indicating the possible formation of ion-pair of IR140 with alcohols.

Introduction: Two tricarbocyanine dyes, namely, IR144 and IR140, are popularly used for research work due to their very high absorption cross-sections at 800 nm and large solvent dependent Stokes shifts[1]. Strong fluorescence emission of these two dyes has made them useful for bio-imaging applications in the biological transparent window (BTW) region[2],

[3], [4]

(650-900 nm) in

comparison to the other NIR dyes as they have lesser photodamage, lesser light scattering and 1

minimum fluorescence background[5]. Furthermore, due to the presence of long extended conjugation, these tricarbocyanine dyes have several other applications, such as, optical data storage[6], nonlinear optics[7],

[8]

optical limiting[9], sensitization[10],

[11]

and light energy

conversion[12], [13]. Strong photo-physical and spectroscopic properties of tricarbocyanine dyes are a subject of intensive investigations in the recent decade[13], [14], [15]. Dual fluorescence of a dye predicts the contravention of Kasha’s rule [16], where the fluorescence is mainly dominated by the excited state with the lowest energy. “Kasha’s rule” was first formulated in 1950 by Michael Kasha, and is a general principle with only a few rare exceptions[17]. Dual fluorescence is the anomalous behavior of a molecule when fluorescence arose from its other excited singlet state and was first experimentally observed and reported by Lippert in 1959[18]. There has been a lot of interest in the synthesis, characterization and the photophysical properties of the NIR dye showing dual fluorescence behavior for applications in bio-imaging due to their long S2 lifetimes[14]. As of now, there are only a few molecules which have dual emissive nature[15], [19]. Molecules showing dual fluorescence can be used as a sensitive marker, such as biosensor[20], ratiometric sensors[21] and multifunctional sensor microarray[22]. The relative intensity of the dual fluorescence can be controlled by changing certain solvent properties, namely, polarity, viscosity, temperature and pH [23], [24], [25], [26], [27], [28]. Inoue et. al. recently presented the major reasons behind dual fluorescence for various kind of molecules[29]. Study the dynamics of the fluorescence coming from S2 state is necessary to get the exact information about the deactivation processes through which the excited molecules come to its ground state. It was reported that the S2 state dynamics of molecules varies from few fs to few ns, depending on the molecular structures and their surrounding solvent environments[30], [31], [32], [33]

. Recently using fluorescence up-conversion technique the S2 state dynamics of several

2

cyanine dyes had been performed by Guarin et. al. with fs time resolution[34]. They have shown that the S2 state dynamics strongly depends on the cyanine structure and conjugation length. As the S2 state fluorescence band position and intensity strongly depends on the surrounding solvent environment so the deactivation timescales will also vary with varying the surrounding environments. In this paper, we first measured the steady state emission and then, by using fluorescence up-conversion technique with femtosecond resolution, we measured the timeresolved emission dynamics. The dynamics of the S2 states were measured by exciting IR144 and IR140 dyes individually with laser pulses at 440 nm in four different primary alcohols with increasing chain lengths, which corresponded to the direct excitation of the ground state to the S2 or higher singlet electronic states (S0 →Sn (n≥2)). We show that the decay of the time-resolved emission of IR144 and IR140 with different solvents is faster for the S2 state, which is in contrast to the relatively slow decay of the S1 state, and this was reported previously [35], [36], [37]. Here, we have also explained the reason for our experimentally observed fast decay of the time-resolved emission from S2 state with varying chain length and polarity and viscosity of the alcohols for both dyes.

Experimental Section: Materials: We have purchased commercially available organic laser dye molecules, IR144 (SigmaAldrich, USA, S1(a), IR140 (Exciton, USA, Scheme S1(b), Supplementary Materials), and used them without further purification. Both these dyes are polar and as such are insoluble in longchain primary alcohols like 1-pentanol and higher chain length alcohol. For our experiments, sufficient solubility was attainable till n-butanol. In fact, these are not quite soluble in the other isomers of butanol.

High-Performance Liquid Chromatography (HPLC) grade methanol

3

(MeOH), ethanol (EtOH), 1-propanol (PrOH) and 1-butanol (BuOH) were purchased from Sigma-Aldrich (USA inc.) and were used for making the solution for all our experiments. To avoid aggregation of the dye in solution, we kept the concentration at 2×10 -5 M. All solutions were kept in a cool and dark place to avoid photo-degradation before our laser studies. All the experimental data were recorded at room temperature (22oC). Experimental Methods: Steady State Absorption and Emission: Steady-state absorption spectra of IR144 and IR140 in different alcohol solution were recorded by using a UV-Vis-NIR absorption spectrophotometer (JASCO, V-670) using 1 mm thin optical path length quartz cuvette (Hellma, USA, Inc.) with the resolution of 0.2 nm. The excitation and emission spectra were recorded using commercially available fluorometer (Fluoromax 4, HORIBA, Jobin–Yvon) using a 1 cm quartz sample cell (Hellma, USA) with the resolution of 0.2 nm. For our steady-state fluorescence measurements, we used two different laser sources. The first laser system used was a nanosecond Q-switched Nd:YLF laser (EVOLUTION -15, Coherent Inc. USA) with 1 kHz repetition rate, and with the pulse energy of ~10 mJ at the central wavelength of ~527 nm. The second laser source was a home built femtosecond Ti:Sapphire Oscillator (KM Labs), which generated 27 fs pulses at a central wavelength of ~805 nm with the repetition rate of 94MHz. The laser pulses were then focused with a 10 cm focal length lens onto a 1 cm optical path length quartz cuvette (Hellma, USA Inc.) filled with the different dye solutions made in several solvents. The fluorescence was collected at the right angle of the beam propagation using a High-Resolution Miniature Fiber-Optic Spectrometer (HR2000, Ocean Optics USA, Inc,) interfaced with a personal computer. We kept the laser power below the threshold of photo-degradation of the dye molecule, which was further

4

confirmed by taking absorption spectra before and after the experiment. The fluorescence data recorded from our experiments were plotted by using Origin 8.5® program. All the experimental data were recorded at room temperature (220C). Time-resolved fluorescence measurements: For measuring the fluorescence lifetime of the S2 state of IR144 and IR140 under different solvents, with femtosecond time resolution, we used commercially available fluorescence upconversion system (FOG-100, CPD, Russia). The laser system used in this experiment is a commercially available Mai-Tai (Spectra Physics Inc. USA), which operates at 80 MHz, pulse width < 100 fs with the central wavelength 800 nm. This laser system is a tunable one, and it can be tuned from 740 nm-920 nm. The second harmonic, tuned to 440 nm, was used to excite the solutions. The excitation pulses were approximately 45 mW at the second harmonic generation frequency. The full width at half maxima (FWHM) of the instrumental response function (IRF) for the experiments was ~ 180 fs (which was measured by Raman signal of methanol). The timeresolved fluorescence data recorded from our fluorescence upconversion experiments were plotted by using MATLAB® program with tri-exponential expressions. The fitting error is between 3-10% for all lifetime values.

Results and Discussions Steady State Spectroscopy: Representative absorption and fluorescence spectra (excited at 805 nm) of IR144 and IR140 in four different alcohols are shown in Figure 1 and Figure 2 respectively. Figure 1 and Figure 2 shows that the absorption and fluorescence spectra are mirror images of each other, which indicates that ground and excited state energy spacings are almost the same. A very strong absorption band with very large molar extinction coefficient (ε) in the NIR region is attributed to 5

a transition from S0→S1 state[13]. The very small absorption (almost flat) in the visible region (500-550 nm) corresponding to very small molar extinction coefficient (ε) is attributed to the S0→S2 transition. Similarly, a relatively weak absorption in the UV region is attributed to the S0 →Sn (n≥2) electronic transition

[34,38-40]

. Excitation of IR144 and IR140 in different alcohol

solvents with 805 nm femtosecond laser pulses resulted in fluorescence spectra that display a distinct, broad and structureless band with a peak wavelength in the NIR region. The solvent dependent shifts in the absorption spectra are relatively small for IR140 but quite large for IR144. The absorption band is red-shifted as the solvent changes from MeOH to BuOH, which can be explained from the increasing polarizability of the alcohol series from MeOH to BuOH. The solvent dependent fluorescence spectra (excited at 805 nm: corresponding to S 1 fluorescence) of IR144 and IR140 indicate that the fluorescence peak shift of IR144 and IR140 is almost the same across the alcohol solvents (MeOH to BuOH) with the one in BuOH being the most red shifted. Both IR144 and IR140 undergoes negative solvatochromism in a polar medium,11 which results in a decrease in the excited state dipole moment compared to that of the ground state as the solvent varies from MeOH to BuOH. The normalized (with respect to S1) fluorescence spectra of IR144 and IR140 resulting from the excitation at 527 nm nanosecond laser pulses are depicted in Figure 3 and Figure 4 respectively. We observed two distinct emissions peaking in the visible region which is assigned to the S2→S0 transition (S2 fluorescence) and in the NIR region (S1 fluorescence), which is assigned to the S1→S0 transition. We observed that identical fluorescence spectra were generated at 527 nm (CW or mode-locked laser pulses) or 440 nm (bluer laser pulses). We also noticed that the S2 emission intensity varies with increasing chain length of alcohols for both dyes.

6

Both IR144 and IR140 show solvent dependent S2 fluorescence peak shift when excited at 527 nm laser pulse. The S2 fluorescence peak shift is very small (99 cm-1) for IR140, which can be explained as follows; the formation of ion pair occurs for IR140 in MeOH as well as in EtOH, which results in shifting the fluorescence peaks to redder wavelengths It was earlier reported that tricarbocyanine dyes show ion-pair formation in primary alcohol

[41]

. The relatively large (832

cm-1) fluorescence peak shift was recorded for IR144 for going across MeOH to BuOH. All these observed spectroscopic details are tabulated in Table S1. IR144 always shows lower S2 emission intensity as compared to that of S1 emission for the same dye concentration of our alcohol series (Figure 3). But, at the same dye concentration of IR140, the intensity of S2 emission is higher than that of S1 emission in MeOH (see figure 3). The higher S2 emission intensity is due to a large amount of ion pair formation in case of IR140 in MeOH. The rest of the alcohols show that the S 2 emission intensity is always lower than that of the S1 emission for our alcohol series. On the other hand, it is also important to mention that the S2 fluorescence from both IR144 and IR140 lie in the region between the first (NIR region) and second (UV region) absorption transition (Figure S2 and Figure S3, Supplementary Materials). The energy gap, ΔE(S2-S1), between the S1 and S2 electronic singlet states, plays an important role in fluorescence spectroscopy[34]. According to this law, the energy gap ΔE(S2-S1) decreases with increase in the polarizability of the solvent. In our current study, we observed that the energy gap law is valid for IR144. The plot of ΔE (S 2-S1) versus polarizability (f1) for IR144 dye for our alcohol series is shown in Figure S3 (see Supplementary Materials). For IR140, the deviation from energy gap law happens due to ion pair formation in the excited states of IR140

7

in MeOH and EtOH. The relevant photo-physical properties of IR140 and IR144 under different alcohols are summarized in Table S1.

Time resolved fluorescence studies: Direct evidence for S 2 fluorescence To investigate the S2 state dynamics of IR144 and IR140 dyes in our alcohol series, we performed time-resolved fluorescence studies by using fluorescence up-conversion technique. The time-resolved fluorescence studies were performed by using a femtosecond laser (Instrumental response function ~ 180 fs) as an excitation source with a central wavelength at 440 nm. For time-resolved fluorescence studies of both NIR dyes under different solvents, we have collected the fluorescence intensity at S2 peak region. Time-resolved fluorescence traces of IR144 and IR140 in the four different alcohols where these dyes are soluble are shown in Figures 5 and 6. We observed tri-exponential decay dynamics of S2 fluorescence in these alcohols corresponding to three different decay time constants (τ1 τ2 and τ3 respectively). Recently, Guarin et. al.

[34]

have reported the dynamics of fluorescence coming from higher

excited states of several cyanine dyes with varying conjugation lengths and size of the end groups in ethanol and ethylene glycol. Analyses of our results for both the dyes in four alcohols are summarized in Table 1. S2 state dynamics of IR144 under different solvents: The S2 fluorescence of IR144 dye in different alcohols (Figure 5) shows a slower S2 fluorescence decay in going from MeOH to BuOH (Table 1). We observed that all the decay time constant values increase on going from methanol to BuOH. In MeOH, τ1 = 226 fs, τ2 = 4.515 ps and τ3 = 52.165 ps and in BuOH τ 1 = 452, τ2 = 33.39 ps and τ3 =199.87 ps, respectively. This monotonically decreasing nature for IR144 followed the energy gap law, 8

which was discussed previously. The S2 fluorescence lifetime increment from MeOH to BuOH can be explained by the increasing polarizability and viscosity of the alcohols. With increasing viscosity, the depletion of molecules from S 0←S2 state becomes very slow, which is reflected in our experimental results (see Table 1). Previous work on the S1 state dynamics of IR144 has shown similar trends, which was explained by the decrease in dipole moments of the S1 excited state in going from MeOH to PrOH [13]. In our current experiments, we also observed the similar trend for the S2 state fluorescence dynamics with increasing alcohol chain length.

S2 state dynamics of IR140 under different solvents: The S2 state time-resolved fluorescence traces of IR140 in the four alcohols (Figure 6) show two distinct sets of dynamics. The first distinct set is that of MeOH and EtOH, where ion pair formation occurs, has been confirmed by the τ1 value of MeOH (332 fs) and EtOH (326fs) (see Table 1). Evidence of ion pair formation has been reported earlier for these two solvents also [36], [37]

in the S1 state. As the solvent molecular size increases, the rate of ion pair formation

decreases, which is reflected in τ1 values of PrOH (257 fs) and BuOH (292 fs). We observed that τ2 and τ3 values keep on increasing on moving from MeOH ( τ2 = 1.99 ps, τ3 = 29.99 ps ) to BuOH ( τ2 = 4.16, τ3= 61.04 ps). The average lifetime values also increase from MeOH to BuOH. The increasing decay timescales, τ2 and τ3 values in BuOH compared to other alcohols can be explained from the larger solvent size and higher viscosity (see table S1) of BuOH (longer chain length compared to that of PrOH) with respect to other alcohols. Comparison of the S2 state dynamics of the two dyes as a function of solvent:

9

Figure 7 shows the variation of the faster component (τ1) of the S2 fluorescence decay time for both the dyes as a function of the number of carbon atoms in our alcohol series. We find that in the case of the IR144 dye, the τ1 values increase monotonically with the increase in the number of carbon atoms of the primary alcohols. The minimum is in the case of MeOH (226 fs) and maximum is in the case of BuOH (452 fs), which is in contrast to the case of the IR140 dye, where τ1 values first decrease from that of MeOH to that of PrOH and then they again increase to the case of BuOH. Such behavior can be explained by using the concept of ion pair formation. The value of τ1 = 332 fs for MeOH is due to the maximum ion pair formation, which decreases for EtOH to 326 fs and finally becomes minimum for PrOH at 257 fs. These results indicate that for PrOH the interaction with dye molecule is minimum (perhaps indicating an absence of ion pair formation). Subsequently, the τ1 value increases in BuOH (292 fs). The average lifetimes of IR140 and IR144 increase on moving from MeOH to BuOH with an increase in the chain length of the alcohols.

Conclusion: In this paper we have presented the investigations on multiple emissions of IR144 and IR140 in primary alcohols as a function of increasing chain length, using steady-state and timeresolved techniques by excitation at a visible wavelength. The S2 excited state dynamics of IR144 and IR140 were investigated by using fluorescence up-conversion technique with femtosecond time resolution ~ 180 fs. The drastic changes of S2 state fluorescence decay time (τ1) of IR140 in different alcohols display the violation of energy gap law which is an indication of ion pair formation in MeOH and EtOH. The slower S 2 fluorescence decay (τ1) confirms the strong evidence of ion pair formation of IR140 in MeOH. IR144 dye strictly follows the energy gap rule in all alcohols that we studied. The ΔE(S2-S1) decreases on going from MeOH to BuOH, 10

by lowering the excited state dipole moment of IR144 dye. The increasing ΔE(S 2-S1) value in MeOH results in the faster decay of the S2 state dynamics. The dual emissive nature of IR144 and IR140 dyes in different alcohols may open up a new window for bio-imaging as until now; only a few dual emissive dyes are available. The events of ion pair formation of IR140 in MeOH and EtOH can be visualized by tuning the excitation wavelengths. The energy gap law followed by IR144 in different alcohols makes it possible to extend the S2 state lifetimes from a few picoseconds to quite larger value ( 83.46 ps, IR144 in BuOH), which may open up a new area for cell imaging and tissue engineering in different alcohols. The higher S2 fluorescence intensity of IR140 in MeOH compared to S1 fluorescence intensity may be used for contrast imaging purposes. The same dye concentrations of IR140 and IR144 in our alcohol series may enable us to further research work on the optical nonlinearities due to their different S2 lifetimes.

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1.0

MeOH EtOH PrOH BuOH 0.8

0.8

0.6

700 720 740 760 780 800 820 840 860

0.6

Wavelength (nm)

0.4

0.4

0.2

0.2

0.0

Normalized fluorescence (a.u.)

Normalized absorbance (a.u.)

1.0

0.0 200

300

400

500

600

700

800

900 1000

Wavelength (nm)

Figure 1: Representative normalized absorption (solid line) and S1 fluorescence (dashed line, excited at 805 nm) spectra of IR144 in methanol (black), ethanol (red), 1-propanol (green) and 1-butanol (blue), the inset shows the zoomed part of the absorption and fluorescence spectra.

13

Normalized absorbance (a.u.)

0.8

0.6

780

800

820

840

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0.6

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0.4

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Normalized fluorescence (a.u.)

MeOH 1.0 EtOH PrOH BuOH 0.8

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Figure 2: Representative normalized absorption (solid line) and S1 fluorescence (dashed line, excited at 805 nm) spectra of IR140 in methanol (black), ethanol (red), 1-propanol (green) and 1-butanol (blue). The inset shows the zoomed part of the absorption and fluorescence spectra.

14

Normalized fluorescence

MeOH EtOH PrOH BuOH

500

600

700

800

900

1000

Wavelength (nm)

Figure 3: Normalized fluorescence (both S1 and S2) spectra of IR144 in different alcohols excited at 527 nm nanosecond laser pulse.

15

Normalized Fluorescence

MeOH EtOH PrOH BuOH

500

600

700

800

900

1000

Wavelength (nm) Figure 4: Normalized fluorescence (both S1 and S2) spectra of IR140 in different alcohols excited at 527 nm nanosecond laser pulse.

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1.0

MeOH EtOH PrOH BuOH

Normalized intensity

0.8 0.6 0.4 0.2 0.0 0

50

100

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Delay (ps) Figure 5: S2 state time-resolved fluorescence dynamics of IR144 in different alcohol by the excitation with 440 nm laser pulse.

17

1.0

MeOH EtOH PrOH BuOH

Normalized intensity

0.8 0.6 0.4 0.2 0.0 0

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Delay (ps) Figure 6: S2 state time-resolved fluorescence dynamics of IR140 in different alcohols by the excitation with 440 nm laser pulse.

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340 IR140 IR144 t1(fs) of IR140 dye

320

400

300

350 300

280

t1(fs) of IR144 dye

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250 260 1

2

3

4

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Figure 7: Variation of τ1 for IR144 and IR140 with the number of carbon atoms of primary alcohols. (see text for details).

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Table 1: S2 state time-resolved fluorescence decay times of IR144 and IR140 in different alcohols.

Name of dyes IR144

IR140

Name of solvent MeOH EtOH PrOH BuOH MeOH EtOH PrOH BuOH

߬ଵ (fs)

226±30 295±38 352±45 452±60 332±45 326±39 257±30 292±30

߬ଶ (ps)

ܽଵ

0.34 0.36 0.36 0.35 0.71 0.73 0.64 0.66

4.52 ± 20 6.3±0.25 20.36±0.4 33.39±0.5 1.98±0.2 3.28±0.25 3.61±0.35 4.16±0.38

20

ܽଶ

0.28 0.29 0.31 0.28 0.22 0.20 0.26 0.24

߬ଷ (ps)

52.12±3.8 116.60±5.2 182.43±8.6 199.87±9.5 29.99±2.0 38.34±3.2 46.35±3.8 61.04±4.7

ܽଷ

0.38 0.35 0.33 0.37 0.07 0.06 0.10 0.10

Highlights: v Tricarbocyanine dyes are showing dual emissive properties with the excitation at a visible wavelength. v The S2 state fluorescence band shape and peak wavelength maxima strongly depend on the chain length of the primary alcohol. v The S2 state fluorescence dynamics observed are in ultrafast time scales. v The S2 state fluorescence dynamics strongly depends on the chain length of the primary alcohols and the decay time constants are slower with increasing chain length.

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