Ultrafast Time-Resolved Emission and Absorption Spectra of meso

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Ultrafast Time Resolved Emission and Absorption Spectra of Meso-Pyridyl Porphyrins Upon Soret Band Excitation Studied by Fluorescence Up-Conversion and Transient Absorption Spectroscopy Prakriti Ranjan Bangal, Yeduru Venkatesh, Venkatesan Munisamy, and Bheerappagari Ramakrishana J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05767 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 8, 2016

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The Journal of Physical Chemistry

Ultrafast Time Resolved Emission and Absorption Spectra of meso-Pyridyl Porphyrins upon Soret band Excitation studied by Fluorescence Upconversion and Transient Absorption Spectroscopy

Yeduru Venkatesh1,2,M Venkatesan1 B. Ramakrishna1, Prakriti Ranjan Bangal1,2*

1

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, 500007, India. 2

Academy of Scientific and Innovative Research, 2-Rafi Marg, New Delhi, 110001, India. * Corresponding author Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract: A comprehensive study of ultrafast molecular relaxation processes of isomeric meso-(pyridyl) Porphyrins (TpyPs) has been carried out by using femtosecond time-resolved emission and absorption spectroscopic techniques upon pumping at 400 nm, Soret band (B band or S2), in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solvent mixture. By combined studies of fluorescence up-conversion, time correlated single photon counting and transient absorption spectroscopic techniques, a complete model with different microscopic rate constants associated with elementary processes involved in electronic manifolds has been reported. Besides, a distinct coherent nuclear wave packet motion in Qy state is observed at low frequency mode ca. 26 cm-1 region. Fluorescence up-conversion studies constitute ultrafast time resolved emission spectra (TRES) over the whole emission range (430 to 710 nm) starting from S2 state to Qx state via Qy state. Careful analysis of time profiles of upconverted signals at different emission wavelengths help to reveal detail molecular dynamics. The observed lifetimes are as indicate: a very fast decay component with 80±20 fs observed at ∼435 nm is assigned to be lifetime of S2 (B) state whereas, being a rise component in between 550 to 710 nm region of emission wavelength pertaining to Qy and Qx states, it is attributed to very fast internal conversion (IC) occurring from B→Qy and B→Qx as well. Two distinct components of Qy emission decay with∼200-300 fs and ∼1-1.5 ps time constants respectively are due to intramolecular vibrational redistribution (IVR) induced by solutesolvent inelastic collisions and vibrational redistribution induced by solute-solvent elastic collision respectively. The weighted average of these two decay components is assigned as the characteristic lifetime of Qy and it ranges in between 0.3 to 0.5 ps. An additional ~20±2 ps rise component is observed in Qx emission and it is assigned to be the formation time of 2 ACS Paragon Plus Environment

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thermally equilibrated Qx state by vibrational cooling/relaxations of excess energy within solvent and this relaxed Qx state decays to ground as well as triplet state by 7-8 ns time scale. The femtosecond transient absorption studies of TpyPs in three different excitations at S2 (400 nm), Qy (515 nm) and Qx (590 nm) along with extensive global and target model analysis of TA data exclusively generate the true spectra of each excited species/state with their respective lifetimes along with microscopic rate constants associated with each state. The following five exponential components with lifetime values 65-70 fs, ∼0.3 to 0.5 ps, ∼20±2 ps, ∼7±1 ns and 1 to 2 µs are observed which are associated with S2, Qy, hot Qx, thermally relaxed Qx and lowest triplet (T1) states respectively when excited at S2, and four (Qy, hot Qx, thermally relaxed Qx and lowest triplet (T1) states) and three (hot Qx, thermally relaxed Qx and lowest triplet (T1) states) states are obtained when excited at 515 nm (Qy) and 590 nm (Qx) respectively, as expected. The TA results are in parallel with fluorescence upconversion studies and both the results not only compliment to each other but also unveil the ultrafast internal conversion from S2 to Qy, S2 to Qx and Qy to Qx for all three isomers in a similar fashion with nearly equal characteristic decay times.



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1. Introduction: Porphyrin, a basic building block of living systems, play a pivotal role in solar energy conversion in natural biological systems such as photosynthetic bacteria and photosystem-II in green plants.1-10 Its efficient but less-well understood role in energy and electron transfer mechanism in leaving light-harvesting systems have led researchers to the studies of synthetic analogues that seek to mimic the sequence of higher quantum yield of charge separation of the photosynthetic organisms. As a results, porphyrins as well as their derivatives and analogues are effectively being attempted to be applied in realizing artificial photodevices such as dye sensitized solar cell in solar energy conversion11-13, molecular switch and photonic devices.14-17 In this contrast the optical properties and photophysical behaviours, specially ultrafast relaxation dynamics of porphyrins and their derivatives continue to attract the interest of chemists.22-26 In this account, free base 5,10,15,20- meso-tetrakis phenylporphyrin (H2TPP), a very simple model porphyrin in porphyrin chemistry, and its metal complex with Zn (Zn-TPP) have been extensively investigated and continue to be more and more investigated.18,19,25,26 Very recent, Kim et al has reported coherent nuclear wave packets in Q States of H2TPP by time-resolved fluorescence studies.26 However, existing information on photophysical properties of H2TPP and Zn-TPP including ultrafast relaxation processes are regarded to be a reference to characterize other phorphyrin derivatives and their metal counter parts. The excited-state relaxation process of porphyrin following Soret band excitation by a very short pulse often occurs in a femtosecond time scale within the manifold of excited states. Owing to their broad energy bandwidth of short pulses in one hand and high extinction coefficient of S2 band of porphyrin, excitation with a short pulse at S2 yields a lot of vibrational eigenstates simultaneously with their coherent superposition. Thus, photogenerated vibrationally coherent states evolves in time and initiates the coherent nuclear wavepacket motion of the molecule which reflects as oscillatory features in time-resolved


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signals in few ps time window, a vibrational dephasing time in room temperature. However, the role of this coherent wavepacket motion in the relaxation process, especially in internal conversion between excited states of porphyrin is not so well known. 29-31 Isomeric 5,10,15,20-meso-Tetrakis (Pyridyl) Porphyrins (TpyPs) (Chart 1) are the analogue of H2TPP where phenyl ring is replaced by pyridine to the meso-position of the porphyrin. The steady-state spectroscopic features grossly similar to that of the model H2TPP molecule,27,28 but it differs in its electron-accepting capability in singlet excited state in sharp contrast to H2TPP molecule, which is known as a π- electron donor. For instance, in our earlier report,27 we have observed that TpyPs takes part in photoinduced reduction involving proton coupled electron transfer reaction when electron transfer from the phenol to the singlet porphyrin is coupled with the concerted motion of the bound proton to the associated pyridine. Hence, this molecule is found to be a potentially important mimic system to better understand activity in biology, in particular in photosynthesis,3 where tyrosine H-bonded to histidine participates in the PCET process in photosystem-II, which is a porphyrin-based complex. So, at this juncture, understanding of detailed picture of the relaxation dynamics in the S2 and S1 states of these pigments could help to comprehend the long-range electron transfer reaction chain in the photosynthesis reaction center in green plants. Despite the numerous studies on the ultrafast dynamics of different tetrapyrrole compounds exploring the molecular relaxation mechanisms of these pigments in electron transfer events in the reaction center of photosynthesis,32-34 , the characterization of their excited state relaxation processes of TpyPs occurring at femtosecond time scale has not yet been reported. Among the different experimental methods fluorescence up-conversion and femtosecond transient absorption spectroscopy are two distinctly different methods but exploit common ultrafast molecular relaxation processes such as internal conversion, intramolecular vibrational relaxation and moleculer cooling within the solvent. Here we



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report, ultrafast time resolved fluorescence emission spectra (TRES) of TpyPs using fluorescence up-conversion method following S2(Soret or B band) photoexcitation to explore relaxation dynamics between excited S2 and S1 (Qy and Qx states) and spectrotemporal analysis of the femtosecond transient absorption data obtained upon S2 pumping. Kinetic scheme based global and target analysis of the transient absorption estimate species associated difference spectra (SADS), microscopic rate constants ( radiative and nonradiative) and decay times of the respective species/states.

2. Experimental Section 2.1 Sample preparation and steady state measurement. Isomeric 5,10,15,20-meso-Tetrakis (Pyridyl) Porphyrin (TpyPs) were purchased from Porphyrin System, Germany and used as received. Spectroscopic grade dichloromethane (DCM) and tetrahydrofuran (THF) were purchased from Aldrich Chemicals, USA. 4:1 mixture of DCM and THF were used for all experiments. All Uv/vis spectra were recorded using Hitachi U-2910 spectrophotometer before and after laser exposure to the sample to check sample degradation if any. All steady state fluorescence spectra were recorded at room temperature by Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon, USA. 2.2 Femtosecond Laser Apparatus and Procedures. The detail experimental setup of the femtosecond fluorescence up-conversion and transient absorption (TA) measurements are describe elsewhere.35 In brief, fluorescence upconversion study was performed using FOG 100-DX system (CDP System Corp. Moscow, Russian Federation ). The second harmonic of the fundamental beam (~500 mW at 800 nm) of femtosecond oscillator (Mai Tai HP, Spectra Physics , FWHM=100fs) was used as pump and residual of fundamental pulses served as gate beams. The pump beam directed to a rotating sample cell with the help of six BSs and one mirror. A lens (f=40 mm) was used to focus excitation beam into the sample. A neutral density (ND) filter is used to adjust the


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power of the excitation beam. The gate beam was directed by two mirrors to gold-coated retro- reflector mirror connected to 8 ns optical delay line before being focused together with the fluorescence (collected by an achromatic doublet, f=80 mm) on 0.5 mm type-I BBO crystal. The angle of the crystal was adjusted to phase matching conditions at the fluorescence wavelength of interest. The up-converted signal (in the UV range) was focused with a lens (f=60 nm) to an input slit of the monochromator (CDP2022D). The intensity of the up-converted radiation was measured with a photomultiplier tube operating in the photon counting mode. Proper filters were used before the detector to eliminate parasitic light from the up-converted signal if any. The polarization of the excitation pulses was set at magic angle relative to that of the gate pulses using Berek's variable wave plate. The sample solutions were placed in a 0.6 mm or 1 mm rotating cell and absorbance of about ~0.8 at excitation wavelength generally used (yielding a concentration around 100-200 µM). The FWHM of the instrument response function (IRF) in this setup was calculated about 240 fs in the 0.6 mm cell and 260 fs in the 1 mm cell. Given the IRF and the signal-to-noise ratio of the observed data, we could estimate a time resolution of < 100 fs. This time resolution is mainly limited by the optics and the duration of the laser pulses. For data analysis, the fluorescence time profile at a given emission wavelength I(λ,t) was reproduced by the convolution of a Gaussian IRF with a sum of exponential trial function representing the pure sample dynamics S(t). Gaussian term was added to account for fast non-exponential processes if any owing to vibrational or other solvent relaxation process. Using the same set up time resolved emission spectra (TRES) was estimated directly over a broad wavelength range (in between 430-710 nm), without spectral reconstruction in conventional method as the phase matching angle changes for different detection wavelengths and the BBO crystal was rotated for optimum signal, for different time delays from the decay profiles of fluorescence up-conversion signals at different wavelengths



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keeping the sample concentration exactly same for all studies. The observed group velocity dispersion at different wavelengths was compensated manually to the time-zero value of solvent Raman signal at 445 nm. However, no correction was made for the spectral response of the detection system for the

reported TRES as well as wavelength dependent up-

conversion efficiency of the used BBO crystal. For transient absorption measurement, the output of optical parametric amplifier (TOPAS prime) was used as pump sources at required wavelength with suitable pump energy and feed into spectrometer through synchronized chopper for 1 kHz repetition rate. A Berek's variable wave plate was used to fix the pump beam at magic angle with respect probe pulse. The amplified 800 nm pulse with 1 KHz repetition rate and 20-25 mW power focused onto a thin rotating (2-mm) CaF2 crystal window to generate a white-light continuum. A fraction of this beam was sent to a photodetector which controls speed and phase of the chopper rotation. The beam of white light was collimated with parabolic mirror (f=50 mm, 90 deg). Then this white light was reflected from a beam splitter and mirror into two identical probe and reference beams. Two concave mirrors (f=150 mm) were used to focus both probe and reference beams to the rotating sample cell. Two lenses (f=60 mm) made probe and reference images at the entrance surfaces of two optical fibers which are connected to the entrance slit of the imaging spectrometer (CDP2022i). This spectrometer consists of UV-Vis photodiode (Si linear photodiode) arrays with spectral response range 200-1000 nm. Quartz cells of 1 mm sample path length were used for all studies and IRF was estimated to be ≤125 fs. Given the IRF and the signal-to-noise ratio of the observed data, we can could estimate a time resolution of at least < 60 fs. To minimize the solvent signal pump pulse energy was kept below 3 µJ per second and probe pulse energy was from 0.1-0.5µJ at the sample. For transient absorption spectra the group velocity dispersion compensation of white light continuum (probe beam) was done using studied solvent's two photon absorption data for few


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ps delay. All the samples were checked before and after taking the transient absorption to monitor the sample degradation if any. 2.3. Transient Data Analysis: To obtain a model-based description in terms of precisely estimated rate constants and species related spectral signature, the transient data reported in this paper were analyzed using a combination of global and target analysis.36,37 Global analysis is performed in two different approaches based on superposition principle of least number of independent exponential components and it provides a straightforward description of the data at all measured wavelengths at all time points simultaneously. The number of independent components fitted to all data are determined by gradually increasing the number of exponential components until the residuals were effectively zero. The most simple description in global analysis uses parallel kinetic model where a number of monoexponentially decaying independent components, each represented by a single rate constant (reciprocal of the lifetime) and an amplitude at each recorded wavelength, yields the decay associated difference spectra (DADS). The DADSs contemplate the rise and decay of the components with their corresponding decay constants, lifetime values. A second sequential kinetic model, unbranched, unidirectional model, consists of successive monoexponential decays with increasing time constants estimates gross spectral evolution of the data generating evolution associated difference spectra (EADS). As for instant, the first EADS represent spectra just after excitation and it decays with first lifetime into second EADS, in turn, second EADS rises with first lifetime and decays with second lifetime, which is longer than first lifetime, into third EADS and so forth. Finally, data are fitted to a full kinetic model (compartmental scheme), target analysis, by combination of parallel and sequential kinetic model of global analysis which includes all possible branching routes and equilibrium between compartments specifying the microscopic rate constants that describe



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the decay of the compartment as well as transfer of excitation between the compartments. This analysis estimates the real spectra of each compartment (excited species) and is termed as species associated difference spectra (SADS). The whole analysis was performed with the R package TIMP and its graphical user interface of Glotaran (for details, see the literature.3841

)

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

TpyP(2)

TpyP(3)

TpyP(4)

Chart 1 Schematic diagram of isomeric 5,10,15,20-meso-Tetrakis (Pyridyl) Porphyrins (TpyPs)


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3. Results

Norm. Fl.Intensity

1.0

(B)

Qx(0,0)

TpyP(2) TpyP(3) TpyP(4)

S0←S2

Qx(0,1)

0.8 0.004

0.6

0.002

Qy(0,1) 0.000 560

0.4

570

580

590

600

x1000

0.2 S0←S1

0.0

1.0

Norm. Abs.

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


420

(A)

480

540 600 660 Wavelength (nm)

S2←S0

0.8

Soret /B band

0.8 TpyP(2) TpyP(3) TpyP(4)

0.6

B(0,0)

0.0

400

0.06

0.2 0.0

400

450

0.00

420

440

Qy(1,0)

(ii)

Qy(0,0) Qx(1,0) Qx(0,0)

0.02

Q band S1←S0

(i)

0.4

0.04

0.4

720

500

550


600

650

650

Figure 1. The normalized ground-state absorption (A) and steady state fluorescence emission spectra (B) of TpyPs in DCM/THF mixture (4:1) at micro molar concentration of the samples. The fluorescence emission spectra were collected upon 400 nm excitation, shoulder band. Inset (i) and (ii) in (A) are the enlarged form of Soret and Q band absorption respectively and Inset in (B) is the enlarged part of Qy fluorescence in the range of 560 to 600 nm. 3.1.Preliminary: Substitution on phenyl ring of tetraphenylporphyrins often leads to well-defined effects to the ground-sate and excited-state properties based on the extent of resonative interactions between porphyrin macrocycle and peripheral substitutions 41,42 Isomeric TpyPs are a class of tetraphenylporphyrins based molecule whose sensitive and specific responses to 11 ACS Paragon Plus Environment


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peripheral hydrogen-bonding to the N-atom of pyridine prior to the N-atom of inner core pyrrole to make it useful for different studies including our interest exploring the role of Hbonding in electron transfer event. Understanding the femtosecond dynamic processes in isomeric TpyPs requires a clear picture of the steady state spectroscopic properties of the systems in mind and it was describe elsewhere.28 However, for the sake of completeness we summarize here in brief. Figure 1A shows comparative detail picture of ground-state absorption and fluorescence emission spectra of three isomeric TpyPs in 4:1 DCM and THF mixture. The absorption spectra of all three isomers consist of a Soret band or B band, conventionally identified as S0→S2 transition, at around 416±2 nm with a shoulder at 400±2 nm and two components Q band (S0→S1 transition), Qx and Qy, with four distinct peaks at 512±2, 545±2, 588±2 and 641-654 nm respectively. In comparison to the prior assignment of absorption peaks of tetraphenyl porphyrin (TPP) within the frame work of Gouterman’s well-described “four orbital” model 43-46, we safely assign 416 nm band as B (0←0) and remaining all peaks in Q band as y-polarized Qy(1←0), Qy(0←0), and x-polarized Qx(1←0), and Qx(0←0) respectively, where numbers in parenthesis represents number of quanta in the active vibrational modes in the upper and lower electronic states of the transitions in FranckCondon principle. As is observed in Figure 1 (A) a noticeable bathochromic shift of 13 nm in the peak position correspond to Qx(0,0) transition on going from TpyP(4) to TpyP(2) and rationalised as increased resonative interaction of negative charge density around the N atom of pyridine with porphyrin macrocycle on going from TpyP(4) to TpyP(2). Figure 1B portraits the normalized steady state fluorescence emission spectra of all three isomeric TpyPs in 4:1 DCM and THF solvent mixture upon 400 nm excitation. Likewise in absorption, the Qx (0→0) fluorescence peak moves from 646 to 658 nm on going from TpyP(4) to TpyP(2), whereas no distinct change in peak position is seen for a lower 12 ACS Paragon Plus Environment

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energy Qx (0→1) peak. The relative fluorescence quantum yields (Φ) of TpyP(2) isomers are found to be bit lower (0.08) than that of other two isomers (0.1). The Qx fluorescence emission lifetime of TpyP(2) is measured to be 7.0 ns and it is little less than that of the other two isomers which is observed to be 7.6 ns. A careful analysis of fluorescence emission spectra could ensure the appearance of Qy(0→1) fluorescence peak at ~590 nm which is apparently 300 times lower in intensity than that of Qy(0→0) fluorescence could be predicted at ~540 nm and its intensity is 1000 times lower than the intensity of Qx(0→0) emission band at ~640 nm and these observations are in parallel to the Qy fluorescence as observed for TPP.25 A very weak S2 emission (Soret fluorescence) is observed below the Qy band, apparently peaking at ~425 nm for TpyP(2) and TpyP(3) and 433 nm for TpyP(4) with a solvent Raman peak at 445 nm. The intensity of this S2 fluorescence is 3-order lower than the S1 emission and it is in accordance to the previously observed S2 fluorescence of different porphyrins by several groups.19,20,36,47,48 Hence, the fluorescence quantum yield ( Φ S2) of S2 emission is approximated to be in the range of 8x10-5 to 1x10-4 order. Hence, total fluorescence yield (fluorescence yield of S2, Qy and Qx ) upon 400 nm excitation tends to be 10 percent. Therefore processes following excitation to S2 are governed essentially by the dependence of internal conversion (IC) to Qy and Qx (S1) followed by intersystem transitions (ISC) to lowest triplet state T1 and the lifetime of the T1 state is generally expected to be of millisecond order as in other tetrapyrrole systems, like TPP.49 3.2 Fluorescence up-conversion studies: In order to have better understanding of very short time relaxation processes of TpyPs which are involved in electronically higher excited states followed by S2 excitation at 400 nm, the ultrafast time-resolved emission studies were performed in the whole emission range among the B (Soret) state to Qx via Qy of S1 states from 430 to 710 nm. The observed 13 ACS Paragon Plus Environment


fluorescence up-converted (FU) signals have been collected individually at different time domains/windows in the emission range of 430 to 710 nm

Expt. data Fit, S2 Fit, Qy (0,0) Fit, Qy (0,1)

800

Fit,Qx (0,0)

Qx(0,0)

Qy(0,0)

FUS Counts

1000

(B) Qy(0,1)

1200

Resultant fit

600

S2

400 200 450

500

550

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650

700

Wavelength (nm)

FUS Counts

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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750

(A)

500

S 2

0 ps Qx(0,0) 0.1 ps 0.3 ps 0.5 ps 1 ps Qy(0,1)

TpyP(4) Qy(0,0)

250 0 1050 700

S2

Qy(0,0)

TpyP(3)

Qx(0,0) Qx(0,1)

Qy(0,1)

350 0 900 600

Qx(0,1)

Qx(0,0) S2

TpyP(2)

Qy(0,0)

Qy(0,1)

500

550

600

Qx(0,1)

300 0

450

650

700

Wavelength (nm) Figure 2. (A) Time resolved emission spectra (TRES) of three isomeric TpyPs in a 4:1 DCM and THF mixture. Fluorescence up-converted signals were collected in the whole emission region from 430 to 710 nm as function of delay time upon excitation at Soret (S2),400 nm and converted into emission spectra directly. TRFS were shown in upto 1.0 ps delay after excitation. (B) representative multi-peak fit of TRES of TpyP(3) obtained at 100 fs delay after excitation resolving S2, Qy, and Qx emission bands.


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and the fluorescence transients of three TpyPs were plotted to obtain time-resolved emission spectra (TRES) from 0 to 1 ps and they are shown in Figure 2. As shown in Figure 2, TRES of three TpyPs are consist of three bands, the Soret band (S2) at ~435 nm, Qy band with vibrationally resolved 0→0 transition at ~550 nm and 0→1 transition at ~600 nm and finally well resolved Qx (0→0) band in the range of 640 to 660 nm on going from TpyP(4) to TpyP (2) in commensurate to their steady state counterpart. The instrument limitation hindered the well resolved identity of Qx (0→1) fluorescence band of all TpyPs. Due to large difference in fluorescence quantum yield between S2 and Qx fluorescence as well as Qy and Qx fluorescence observation of steady state fluorescence emission from higher energy states are generally overlooked.25 However, ultrafast TRES clearly show that in early time (time zero) fluorescence intensity of all bands are not only quite comparable but also confirm the very fast simultaneous internal conversion from S2→Qy and S2→Qx. The fluorescence intensity corresponding to S2 and Qy bands attained maxima less than 100 fs after excitation followed by complete decay within 2-3 ps time window and the decay of S2 fluorescence is observed to be faster than that of Qy for all three TpyPs. However, the fluorescence intensity corresponding to Qx band continued to be increasing not only in 2-3 ps time window but also over a few tens of ps time window from its inception at time zero. A careful observation on TRES, especially on fluorescence emission peak of Qx band, clearly shows a bathochromic shift of 5-10 nm on delay time of 1-2 ps and it can be attributed as a dynamic Stokes shift. Furthermore, an isoemmisive point can be assumed at 630-640 nm range for all three isomers indicating an equilibrium between Qy and Qx states. These observations suggest the possibility of internal conversion occurring from Qy to Qx and Kim et.al reported similar kind of IC in case of H2TPP.26 The increase of Qx fluorescence beyond 2 ps time



window can be attributed to the formation time of thermally relaxed Qx state via intramolecular vibrational energy redistribution (IVR) and solute-solvent intermolecular vibrational relaxation (VR).19 Table 1. Component lifetime values and relative amplitudes of fits of fluorescence upconversion signals at different wavelengths upon 400 nm excitation of TpyP isomers in 4:1 DCM and THF mixture.

λEM (nm)

State

435-500 550 580 600 615 630 640 650 710 435-490 550 580 600 615 630 640 660 710 430-500 550 580 600 615 630 645 660 710

S2

TpyPs TpyP(2) TpyP(3) TpyP(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

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Qy

Qx

S2 Qy

Qx

S2 Qy

Qx

τ1=(80±20)fs A1(%)

100 -100 -100 -100 -100 -90 -70 -96 -95 100 -100 -100 -100 -100 -85 -75 -85 -95 100 -100 -100 -100 -100 -90 -75 -95 -96

τ2= (200300) ps A2(%) --89 80 75 60 ----------70 65 60 50 ----------80 77 70 60 ---------

τ3=(1-1.5) ps, A3(%),

∙ ∙ b

--11, (0.29 ps) 20, (0.36ps) 25, (0.40 ps) 40, (0.52 ps) 30 10 4 0 --30, (0.44 ps) 35, (0.48 ps) 40, (0.52 ps) 50, (0.60 ps) 15 7 ------20, (0.36 ps) 23, (0.38 ps) 30, (0.44 ps) 40, (0.52) ps 15 10 0 ---

a

τ4=(20±2) ps τ5≡ (7±1) ns a A4(%) A5(%) ---------

---------

-10 -30 -4 -5 -----------15 -25 -15 -5 -----------10 -25 -5 -4

70 90 96 100 ----------85 93 100 100 ----------85 90 100 100

lifetime value was constraint of fits, b average lifetime values for 2nd and third component, ≡ value was the constraint of fit , A negative amplitude stands for rise component instead of decay component.


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(C)

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490 nm

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IRF

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0.6 0.8 Time (ps)

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Figure 3. Temporal profiles of fluorescence up-converted signals (FUSs) of TpyP(3) following Soret band, S2 excitation at 400 nm in 4:1 DCM and THF solvent mixture. Temporal profiles of Soret (S2 or B) band at 435 nm, 445 nm, 455 nm, 470 nm and 490 nm (A), representative normalized time profile of Soret, Qy and Qx emission (B), Emission wavelength dependent time profiles of Qy and Qx emission in 2 ps time window (C), 15 ps time window (D) and 200 ps time window (E). Open circles are observed data points, solid lines are the corresponding exponential fits and also a signal of solvent response (IRF) is shown at 445 nm with a Gaussian fit whose FWHM is 240 fs.



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To extract the complete dynamic parameters of the molecular relaxation processes of the relevant fluorescence transients of all three TpyPs, we have individually monitored and analysed the selected temporal profiles of fluorescence up-converted signals (FUS) at different emission wavelengths of S2, Qy and Qx fluorescence for different time windows following excitation on Soret band at 400 nm in (4:1) DCM:THF solvent mixture. A typical set of fluorescence decay profiles of TpyP(3) comprising different time windows and at different emission wavelengths is shown in Figure 3, (for other isomers see ESI Figure S1 & S2). The temporal profiles of the FUS of TpyPs from 430 nm to 490 nm spectral window show a little asymmetric feature (Figure 3A) and follows mono-exponential decay and the characteristic decay constant is found to be 80 ± 20 fs and it can be attributed to S2 lifetime. Baskin et al. predicted the lifetime of S2 fluorescence of H2TPP is less than 100 fs 19 and Kim et al reported the same to be precisely 63 fs.26 Very recent our group also observed the S2 lifetime of H2F20TPP, an electron deficient derivatives of H2TPP, is around 160 fs.35 A complex temporal profile of FUS is observed in the region of Qy fluorescence emission, in between 550 and 615 nm spectral window (Figure 3B& 3C). The overlapping of S2 emission with Qy (0→0) transition and Qx emission with Qy (0→1) transition made the temporal profile of Qy emission very complex (Figure 2B). The contribution of S2 fluorescence decay on the time profile at the blue edge of Qy emission and contribution of Qx florescence on the time profile at the red edge of Qy cannot be ignored. As a result a multiexponential fit is essential to extract the relative contributions of different florescent transient species. It is important to mention here that multiexponential fit for different orders of decay constants for such kind of data yields broad limit of the kinetic parameters and the values obtained from this type of fits may not be precise indeed. However, the temporal profiles of 2 ps data in this spectral window can be fit by three-exponential function with a rise component of ~100 fs time constant and two decay components with 200-300 fs and 1-1.5 ps time constants


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respectively. The characteristic lifetime of Qy state can realised by weighted average of these two decay components and it ranges between 0.3 to 0.5 ps. The fit results are listed in Table 1. The time profile of Qx emission, above 615 nm, are quite interesting and it shows different dynamics in different time scales (Figure 3C-3D). To have complete kinetic picture temporal profiles of three different time scales of 2 ps , 15 ps and 200 ps are analysed collectively at different emission wavelengths. At least four-exponential components are necessary to fit the data satisfactorily, of which two are rise components with ~100 fs and 15-25 ps while rest two are decay components with 1-1.5 ps and 6-8 ns. The long live decay component is the measured lifetime of Qx state of TpyPs by TCSPC method. The amplitude values obtained from four-exponential fits are summarised in Table-1 and solid lines in the figure demonstrate the fitting curve. Femtosecond studies eventually could directly provide the insight into the dynamics of coherent wave packet motions originating from the superposition of photoexcited vibrational states coupled to the resonant electronic transition of molecules in the condensed phase. The motion of wave packet evolves in time leading to oscillatory features in the excited state electronic transitions, in particular time-resolves signals in few ps time domain.50-57 A closer look to the temporal profiles of FUS (2-3 ps time window) in Qy fluorescence emission region of TpyP isomers, 500 to 630 nm spectral window, showed the presence of very weak oscillatory features superimposed on Qy population decay. A typical modus operandi to recover the frequency domain spectra from the time-domain signals is shown in Figure 4, for the FUS profile at 580 nm (Qy(0→0) transition) upon 400 nm excitation. The residuals obtained after subtraction of fit curve arising out of threeexponential contribution in electronic transition from FUS data shows characteristic oscillatory features of the wave packet motions (Figure 4B) and fast Fourier-transform (FFT) of the residuals rebuilds the frequency spectra ( Figure 4A, Inset). As shown in figure 4A, a 19 ACS Paragon Plus Environment


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prominent vibrational peak is observed at low frequency mode ca. 26 cm-1 region, and a prominent shoulder at 47 cm-1 and these values are bit lower than that observed for H2TPP.26 Exactly similar coherent wave packet motion is observed for other two isomers (Figure S3). These results clearly suggest that replacement of C atom by relatively heavier N atom reduces the frequency of the coherent wave packet motion, whereas position of N atom in the phenyl ring does not affect to the frequency of the coherent wave packet motion. Similar type of coherent wave packet motion is also observed at 600 nm, peak position of Qy (0→1) transition for all three isomers. (see Figure S4). A coherent wave packet motion could be observed at 640 nm, peak position of Qx (0→0) transition, although its amplitude is too weak to calculate vibrational peak convincingly for all three isomers. The reason behind the appearance weak coherent motion in Qx state over Qy state could be the lone lifetime of Qx state and strong signal strength of electronic transition over large time window, which could overhaul the oscillatory fluctuation of FUS. However, this observation of coherent wave packet motion upon S2 band excitation immediately suggests that IC from S2 to Qy and Qx is very fast, simultaneous and impulsive in nature.


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-3

(B)

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FFT intensity (arb.unit)

1.0

(A) 0.8

Normalized FU Counts

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Residual (x10 )

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0.6

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-1

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300

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1.5 TIme (ps)

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3.0

Figure 4. Temporal profile of fluorescence up-conversion signal of TpyP(3) at 580 nm upon excitation at 400 nm with 100 fs pulses (open circles) and three-exponential fit curve (solid line) assuming Gaussian instrumental response function (lower panel). Residual data (open circles ) and solid line shows damped sinusoidal motion, a guide to the eye, after subtraction of fitted three-exponential curve from raw data and fitting curve (solid line) (upper panel). Fast Fourier-transformed power spectrum of residual data of upper panel (inset).



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3.3 Femtosecond Transient Spectroscopic studies:

Figure 5. 3D surface plot of TA spectra of TpyP(3) in 4:1 DCM and THF mixture on 400 nm

excitation, X axis represents time, Y-axis represents wavelength and Z-axis represent ∆OD, bottom X-Y surface shows the heat map of ∆OD of TA spectra, and as indicated in the color map, the zero level is colored in light blue, dark red indicates positive signals (i.e.,photoinduced absorption), and blue/violet denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state photobleaching)(A) Two representative TA decay profiles at 435 and 475 nm are shown in Z-X plane and two representative TA spectra at 1and 185 ps are shown on Z-Y plane (A). Temporal evolution of ∆OD at different wavelengths and red solid lines are fits obtained from global target analysis (B). Transient absorption spectra at different time and solid red lines are results of global and target analysis (C).


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τ1=(k1+k2+k3)-1 S2 Lifetime (ττ1)

τ2=(k4+k5)-1 k3

k2

Qy Lifetime (ττ2)

Excitation

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


τ3=(k6+k7)-1 k5

Qx (hot) Lifetime (ττ3)

k7 τ4=(k8+k9)-1 Qx k9 τ5=(k10)-1

Lifetime (ττ4)

k1

k4

k6

k8

T1 Lifetime (ττ5)

k10

S0 Figure 6. Schematic kinetic model used for target analysis of TA data matrices upon 400 nm excitation. τ1 , τ2, τ3,τ4, and τ5 are global lifetimes of the respective states. k1, k2, k3, k4, k5, k6, k7, k8, k9, and k10 are microscopic rate constants of the respective transitions. Femtosecond transient absorption technique is an important spectroscopic tool to directly explore both the ultrafast radiative and nonradiative relaxation processes with relatively better time resolution than the fluorescence lifetime measurements technique. To further understand the process of deactivation of the S2 state and described complex fluorescence time profiles of the Qy and Qx emission, femtosecond transient absorption measurements were performed on TpyPs isomers. As shown in Figure 1, TpyPs have strong Soret band at around 420 nm which has almost 10 times stronger molar extinction coefficient than the rest of absorption bands spread over whole detected spectral window, 425-670 nm. As a result, negative signal pertaining to ground state bleaching at 420 nm dominates the spectral feature of transient absorption spectra and it has been judicially excluded from the studied spectral window putting more importance to the positive signal of TA spectra. The



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samples have been excited to their S2 state (λexc=400 nm) and data was collected in between 425 to 670 nm in different time windows from 2 ps to 6 ns. A typical heat map of transient absorption difference spectra of TpyP(3) in 170 ps time window in DCM:DMF (4:1) mixture is shown in Figure. 5A (for more see Figure S5-S8). The time evolution at different detection wavelengths and spectral evolution on different times are shown in Figure.5B and Figure 5C. The temporal evolution of the observed TA of TpyP(3) have different profiles at different wavelengths because several processes, such as ground state bleaching (GSB), stimulated emission (SE) from S2, Qy and Qx states, excited state absorption from S2 →Sn , Qy→Sn , Qx→Sn and T1→Tn, take place concomitantly after photoexcitation with spectral signature overlapping to each other and it is difficult to analyze by single wavelength kinetic profile fitting. For this reason, to interpret the femtosecond transient absorption 3D data matrices were used for analysis. A global and compartmental based target model was used combining the sequential and parallel kinetic scheme (Figure S9 &S10), in which, transition from S2 state to ground state (S0), internal conversion (IC) from S2→Qy→Qx(hot) and S2→Qx(hot), transition from Qy→ S0, intersystem crossing (ISC) from Qx→T1, transition from Qx→ S0 along with Qx→T1 are considered to be the major processes following excitation. Spectral evolution due to transition from Qx(hot)→S0 and Qx(hot)→Qx are also taken into account in order to include intramolecular vibrational redistribution (IVR) and vibrational relaxation (VR) as a consequence of very fast IC from S2→Qx(hot). The TA data were collected upto 6 ns time window, the maximum delay limit between pump and probe beams of our existing setup, the exact lifetime of Qx as well as T1 state cannot be estimated precisely from this data set. Hence, in global and target analysis lifetime of Qx was kept as constraint to the value obtained by TCSPC method and lifetime of T1 was kept fixed in 1-2 µs. Based on this model, the data of all three isomers are fitted convincingly well and the spectra of the components as well as the rates of the fast processes are thus determined and listed in Table 1. The goodness 24 ACS Paragon Plus Environment

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of the fits are shown in Figure 5B and 5C. The estimated species associated difference spectra and population time profiles are shown in Figures 7A and 7B, respectively for TpyP(3) (for other two TpyPs, Figure S11 &S12 ). The first SADS1 can be identified as the S2 state of the TpyPs, with a GSB at 425 nm and a distinct SE peak at 440 nm corresponding to S2 emission and it is clearly different than that of other stats and the lifetime(τ1) of this state is found to be 70, 65 and 69 fs for TpyP(2), TpyP(3) and TpyP(4) respectively. From the S2 state two different species/states are populated: the Qy and hot Qx of TpyPs. The SADS2 and SADS3 represent Qy and hot Qx state with excited state absorption (ESA) peak at ~435 nm and GSB and SE corresponding to the ground state absorption peaks and Qy and Qx emission. The lifetime (τ2) of Qy state is calculated to be 0.4 to 0.5 ps (Table 2) and partial population of hot Qx state occurs via IC process from Qy. The lifetime (τ3) of hot Qx is 19-21 ps for TpyPs (Table 2) and it decays to thermally relaxed Qx by VR. SADS4 represents the thermally relaxed Qx state and it undergoes a tiny blue shift to 644 nm GSB and SE signal with respect to SADS3. Finally Qx state decays to ground state by radiative pathway as well as to lowest triplet state T1 via ISC, a long-live component. SADS5 can be recognized as T1 state with ESA peak at 437 nm and it becomes narrower than SADSs associated with singlet states with almost no positive signal above 500 nm.



SADS (OD)

SADS2, (Qy)

(A)

0.10

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0.2

SADS1, S2 0.0 SADS2, Qy 0.0 SADS3, Qx, hot 0.8 SADS4, Qx 0.6 SADS5, T1 0.4 IRF

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(2)

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2 3 4 Time (ns)

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Figure 7. (A) Estimated species associated spectra (SADS) of TpyP(3) arising out of target analysis. Ground state absorption spectrum (dashed line curve) has been included to distinguish excited state absorption and ground state bleaching. (B)The population profiles of respective states/ species, enlarge view of population profiles in 2 ps time window (inset 1) and population profiles in 6 ns time window.

Discussions: Fluorescence up-conversion technique and femtosecond transient absorption technique are complementary methods to each other and focus to estimate common molecular relaxation processes, such as internal conversion, intramolecular vibrational relaxation, and molecular cooling within the solvent, by detecting different observables. Fluorescence up26 ACS Paragon Plus Environment

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conversion technique monitors the temporal evolution of the relative concentrations of “transient fluorescent species” only and provide the relaxation dynamic of the system while, femtosecond transient absorption technique measures the temporal evolution of the ''fluorescent and nonfluorescent transient species" and offers towards complete picture of relaxation processes. The above described results obtained both from fluorescence upconversion and transient absorption studies, reveal that, relaxation dynamics are almost identical for all three isomers and the successive relaxation processes of excited TpyPs occur from Soret band (S2 excited state) to Qx (thermally stable S1 excited state) in four steps followed by the population of triplet-stateT1. Out of these four steps, the initial steps occurs within 500 fs time scale i.e. the transfer of population from S2 state to Q state (both Qy and Qx) occurs very rapidly. Time resolved emission spectra (TRES) of TpyPs make obvious evidence not only for different decay dynamics correspond to three fluorescence regions, S2, Qy and Qx, but also provide evidences of an IC from S2 to Qy and Qx by different fluorescence rise time. The TpyPs were excited at 400 nm which is ~950 cm-1 excess vibrational energy of S2 absorption peak, and temporal profiles across S2 fluorescence band are well fit by monoexponential component of 80±20 fs lifetime values (τ1) with no prominent initial rise component. This result clearly builds the fact that vibrational relaxation in S2 state is very fast, beyond the detection limit of our system, and it could be well below than 80 fs, the assigned characteristic lifetime of S2 state. The temporal profile of up-conversion signals in between 500 and 615 nm are bit critical and decay of the FUS completes in 2 ps time domain. These signals are consist of one ultrafast rise component of ~100 fs (τ1) time constant followed by two decay components with 0.2-0.3 (τ2) and 1-1.5 ps (τ3) time constant respectively. Interestingly, the amplitude of 0.2-0.3 ps component decreases with concomitant increase of the amplitude of 1-1.5 ps component on going from 550 to 615 nm (Table 1). The rise component again confirms the IC from S2 to Qy and weighted average of 27 ACS Paragon Plus Environment


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two decay components can be attributed as the characteristic lifetime of Qy and it ranges between 0.3 to 0.5 ps (Table 1). However, the origin of two decay components can be rationalized based on the magnitude of the time constant. The component with 0.2-0.3 ps time constant could be assigned to IVR while the slower one can be attributed to intermolecular vibrational energy distribution (VR) induced by solute−solvent inelastic collisions.19 It is important to note here that the rate of IVR for in general organic molecules in solution phase depends on molecular size and occurs in a 5-8 ps time scale for medium size molecules (atoms

Ultrafast Time-Resolved Emission and Absorption Spectra of meso

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