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AISRAPS

ISRAPS BulletinA Publication of

Indian Society for Radiation and Photochemical Sciences

Volume 16, Number 3-4 November, 2004

Prof. Bhaskar G. MaiyaMemorial Issue.

Born: June 20, 1956

Died: March 21, 2004

CONTENTS

Editorial 2

2Secretary's Desk

Tetratolylporphyrins as Optical Limiters(D. Narayana Rao*, P. Prem Kiran, N.K.M. Naga Srinivas and S. Venugopal Rao 0) 3

Interaction of porphyrins with plant lectins: Absorption andfluorescence spectroscopic studies(Musti J. Swamy)

Excited State Properties of Certain Ruthenium(lI) Polypyridyl Complexes Bound to DNA:Molecular Light Switches(PoVma Maheswari and M. Palaniandavar*)

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15

Expanded Corrole: synthesis, properties and receptor behaviour(Rajneesh Misra, Rajeev Kumar, Viswanathan PrabhuRaja and Tavarekere K. Chandrashekar*t) 20

'fhe Intrinsic Fluorescence Decay of Folded Cytochrome c: Fluorescene ResonanceEnergy Transfer from the Single Tryptophan to the Heme(Tapan Kanti Das1, Shyamalava Mazumdar*) 25

1

Tetratolylporphyrins as Optical Limiters

D.NarayanaRao*,P.PrellKiran, N.K.M. Naga Srinivas and S. Venugopal Rao a

School of Physics, University of Hyderabad, Hyderabad - 500046*e-mail: dnrsp@Uohyd.emet.in

a Centre for Ion BeamApplications, Department of Physics,National University of Singapore, Singapore 117652

ABSTRACT

Metalloporphyrins are promising materials for optical limiting possessing strong nonlinearabsorptio~ and high third order-optical nonlinearitiescombined with ultrafast response time in thepicosecond and femtosecond time scales. Structural modifications like heavy atom effect andintroduction of donor-acceptor groups are known to vary the excited state properties in these molecules.We discuss the effects of these modifications on the optical limiting properties and excited state

dynamics in phosphorous(V) tetratolylporphyrin and tin(IV) tetratolylporphyrin.

1.Optical Limiting (OL) and Nonlinear Absorption

With the advent of high power laser sources overwide range of wavelengths and pulse widths,protection of sensors and eyes has posed a majorchallenge in the last decade. In this context opticallimiters have received significant attention. Opticallimiters are devices that strongly attenuate opticalbeams at high intensities while exhibiting hightransmittance at low intensities, thus providing safetyto sensors in general and to human eyes in particular.An ideal limiter exhibits a linear transmission below athreshold and a constant above it. Optical limiting(OL)has been achieved by means of various nonlinearoptical mechanisms. Although there are a great varietyof processes leading to optical limiting, most of themcan be divided into two categories. First one is theenergy-spreading type of processes and the other arethe energy-absorbing type of processes. Themechanisms like self-focusing, self-defocusing, inducedscattering, induced-refraction, induced aberration, andphotorefraction result in spreading of the incidentenergy. The processes like excitedsta:te absorption,two-photon absorption, and free-carrier absorptionresult in the absorption of the energy [lJ. Among theabove mentioned processes nonlinear absorption isvery promising for the development of materials for'optical limiting. Nonlinear absorption is defined asan increase / decrease in the absorption coefficient withincreasing intensity. The nonlinear increase inabsorption coefficient is termed as.reverse saturationof absorption (RSA) while the decrease in absorptioncoefticient is termed as saturation of absorption (SA).Recent investigations indicate that materials with RSAbehavior are 9I1eof the potential candidates for optical

limiting applications [2,3] as they give a response thatis closer to that of the ideal opticalliiniter. SA in variousmaterials has been studied for applications in laserpulse compression and laser amplification [4]. Theoptical limiting capability of different materials ischaracterized by the parameter known as limitingthreshold (11/2).Limiting threshold is defined as theinput intensity at which the transmittance of thematerial becomes half of the linear transmittance.

An intense laser pulse redis tributes the molecularpopulation between the ground and excited statescausing a transient modification in the opticalproperties of the material. The dependence oftransmission in organic molecules on light intensitycan yield considerable information on the molecularlevel system that can lead to variety of nonlinear opticalprocesses like limiting, switching and bistability.Organic materials with delocalized electrons are of agreat deal of importance because of their large nonlinearoptical absorption coefficients, architectural flexibility,and ease of fabrication. Metalloporphyrins form animportant class of electronic materials because of thelarge n-electron conjugation over two-dimensionalmolecular structure [5]. In organic molecules, RSAarises mainly due to two different processes namedexcited state absorption (ESA) and two-photonabsorption (TPA). Since the mechanism of limiting insuch materials depends on the absorption of excitedmolecules, it is important to characterize the excitedstate dynamics and evaluate parameters like excitedstate absorption cross-sections, lifetimes etc. so as tooptimize their properties for the realization of afunctional device [6,7]. Structural modifications to theporphyrin ring can be expected to result in molecules

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with diverse photophysical and ph.otochemicalproperties that will in turn affect their opticalnonlinearity [8]. Porphyrins, owing to strong RSAarising from the E5A from both singletand triplet states,are among the most effective optical limiters in thevisible region [9,10J. In our search for better opticallimiting materials, we have improvised the presentporphyrin systems using various structuralmodifications as introduction of heavy-atom, chargetransfer (CT)states. These modifications would leadto a desirable change in a variety of excited stateprocesses including enhanced internal conversion andintersystem crossing (I5C), ion-association, excitationenergy transfer (EET),photoinduced electron transfer(PET). 5uch effects can be conveniently harnessed toenl1ance the third order nonlinearity, and hence to.develop promising materials for optical limiting. Oneof the key structural modifications in this regard, is theintroduction of heavy atoms in the porphyrin ring,which in turn, due to faster ISCreduces the fluorescenceyield, increase triplet formation and hence furtherenhancing absorption from Tl ~ Tn' PET and EET arethe other phenomena that come into play due to thepresence of additional donor/acceptor units in themqlecule. This paper explains the effect of structuralmodifications on the limiting behavior of Phosphorus(V) tetratolylporphyrins and hybrid porphyrin arraysbased on tiri (IV) tetratolylporphyrin scaffold.

2.Porphyrin media for OL and Nonlinear Absorption

5trong nonlinearity and fast response times arethe desired criteria for making porphyrins useful forphotonic devices. Because of the two-dimensionaldelocalization of the n-electrons throughout themacrocycle ring, porphyrins are interestingchromophores for second and third order nonlinearoptical (NLO) properties. Porphyrins have been studiedextensively for their second order NLO properties usingvarious techniques like HRS, EFI5HG etc. Third-orderNLO properties are also studied in variety of porphyrinmolecules, like tetra phenyl porphyrins (TPP),tetrabenzoporphyrins (TBP), tetrakispentadecyl-phenyl porphyrins, octaethylporphyrins (OEP),OctaphenyHetraazaporphyrin (OPTAP), baskethandleporphyrins, conjugate polymers, tetratolylporphyrins(TIP) to name a few. These molecules are among themost effective third-order NLO materials possessinghigh second hyperpolarizabilities. Porphyrins were~lso studied extensively for the optical limiting andnonlinear absorption properties. After the first reportof R5A in porphyrins by Blau et. al. [9], the interest invarious porphyrins to enhance the RSA has increasedgreatly. Porphyrins, owing to strong E5A from both

singlet and triplet states, are among the most effectiveoptical limiters in the visible region [11].

.' The nonlinear absorption behavior in theporphyrin molecules can be explained using ageneralized five-level energy diagram (Jablonskidiagram) containing three singlet states (50,51and 5)and two triplet states(TI and Tn)' Laser light excitesmolecules from the ground state 50into the vibrational-rotational states in the first excited singlet state via50~ 51with a cross-section of 0'0'which relaxes veryrapidly (approximately picoseconds) to the lowervibronic levels of this state. This level relaxes either byboth radiative and nonradiative decay within the singletsystem or to the first triplet state via ISc. E5A can occurfrom the singlet state 51up to a higher singlet state 5nwith cross-section SIand from the triplet state up to ahigher triplet state Tl'~ Tn with a cross-section of 0"2'!he ratio of the absorption crqss-sections 0"/0'0 and0'/0'0 indicate the material's capability as an opticallimiter and are known as material figure of merit. Forpicosecond pulse excitation, triplet level contributionto thp nonlinear absorption can be neglected becauseof the slower I5C rate and the singlet levels 50,51and5n playa dominant role, whereas with nanosecondpulses the E5A from the triplet levels associated by theI5C from 51to Tl play an important role in the opticallimiting behavior. Thougl1 some of the porphyrinsshow good RSA with ps pulse excitation, at higherinput intensities saturation of absorption takes overthus reducing the optical limiting behavior. Our attemptis to have materials that work as good optical limitersin both ns and ps timescales.

3. Experimental tecluliques

The processes E5A and 1'PA leading to opticallimiting behavior are studied in the nanosecond andpicosecond regime. Frequency doubled Nd: YAG lasersat 532 nm with 25 ps and 6 ns pulse widths, 10 Hzrepetition rate are used for the experiments. We haveemployed two different experimental techniquesknown as optical limiting test bed (figure 1) and openaperture Z-scan [12] and using f/5and f/30 opticalgeometries respectively, and the transmitted light is.collected with a fast photodiode, which was given todata acquisition system. The peak intensities used inthe Z-scan experiments with 25 ps pulses areapproximately in the range of 500 MWcm-2 to 53GWcm-2. The input fluence (energy density) is variedin the range of 30 J.l.Jcm-2to 70.Jcm-2in the ns regime.The linear transmission is approximately 70-75% forI-rom path length of the sample. The optical limitingand Z-scan studies are performed at the same

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Porphyrinsolution

Iin

~lout

(a)

(b) ;;0>-<

Iin

Fig. 1: (a) Schematic of optical limiting setup and (b) responseafidealopticallimiter.

concentration of ~ 10-4 M ensuring identicalexperimental conditions for both ps and ns regimes.

4.Effect of Structural modification on OLproperties

Our group has been involved in studies on a classof porphyrins, known as tetratolylporphyrins, forachieving higher optical limiting performance andnonlinear absorption. The tetratolylporphyrinssynthesized have higher third order nonlinearities [10]and efforts are being made to attain large excited stateabsorption cross-sections through structuralmodifications. We have taken two molecular systemsbased on P(V)TTP and Sn(IV)TTP scaffold as thestarting molecules for structural modifications.

The first modification is the introduction of heavyatom in the P(V)TTPmacrocycle. With introduction ofheavier chlorine in place of OH in the axial position of5,10,15,20-(tetratolyl) porphyrinato phosphorous (V)dihydroxide [(TTP)PV(OH)2]+' we have observedstronger RSA in both the ps and ns regimes. With thenanosecond excitation, the ISChas increased by 10timesleading to the enhancement in the limiting behavior byapproximately 30 times and in the picosecondtimescales the limiting behavior is enhanced byapproximately 3 times [13]. Among the various factorsthat can be invoked to explain the improvement of theoptical limiting performance due to heavy atom effectcoming because of the axial chloride substitution ([(TTP)PVCl2]+)in the ns time regime, faster intersystemcrossing rate is the most important as it plays asignificant role by enhancing the population of thetripletstate. The long lifetime of the T1state comparedto the ns pulse duration, leads to larger effective ESAfrom T1-7 Tn' leading to reduction in optical limiting

0 ((TJ1'J'(VXOlI,f

" ((TJ1'J'(VXJ\ZI)f

x ((TII'J'M<AZN),f

1.6 <> ICnp~'VXA7B)l

~ . l('ITI'J'(V)C1,f<.> 0"Cl.2 0"

g <> "0 xOg 08 0 <> .6 0 Xp.:; . x

15 00 .6~<9B< AO

<5 0.4 0 .11.11

1112345"

Input Fluente (J em')

Fig. 2: Optical limiting curves of modified PhospllOrous(V)

te tratol y lporphyrins.

threshold. With picosecond pulse excitation, we haveobserved saturation of absorption (SA) at higher inputfluences (> 0.5 Jcm-2)after the initial RSA at lower inputfluences. Even though such behavior is not useful foroptical limiting purposes, the extent of saturation givesan insight into the excited state dynamics of themolecule. Based on the rate equation model explainingthe various processes taking place in the molecule, wefound that the saturation at higher intensities is a resultof the competition between ESA from Sl to Snand thelifetime of the excited singlet state Sn[13].

In our attempt to reduce SA at higher intensitieswith ps pulse excitation we have taken up the moleculeshaVing azoarene subunits axially linked to' the centralPhosphorous of the P(V)TTP. In these systems, the axialazoarene subunits act as electron donors leading toPET from axial subunits to the central porphyrin. Inthese porphyrins, donor / acceptor levels and a chargetransfer (Cf) state is intrOduced due to the introductionof azoarene (AZT =4-hydroxy, 4'-methyl azobenzene,AZB = 4-hydroxy azobenzene, AZN = 4-hydroxy, 4'-nitro azobenzene) subunits in place of H in axial (OH)-in 5,10,15,20-(tetratolyl) porphyrinato phosphorous (V)dihydroxide [(TTP)PV(OH)2]+without any charge-transfer states. An intra-molecular PET from the axialazoarene donors to the singlet-excited state of the basalphosphorus (V)porphyrin is responSible for quenchingof fluorescence in these complexes. An additionalenergy level known as charge-transfer (CT) state isintroduced leading to more delocalization of the singletle~els. With nanosecond excitation, limiting thresholdhas lowered by a factor of 2 due to enhanced ISc. Withps pulse excitation, at lower intensities only RSA isobserved, but at higher intensities we observed SA

.within RSA due to saturation of higher excited states.But the observed saturation has reduced greatlycompared to that observed in [(TTP)P(V)C~]+as shownin figure 3. Table 1 summarizes the excited stateparameters obtained for modified phosphorus(V)

/

5

... .2 0 27JZ,

Fig.3: OpenapertureZ-scancurvesof (a)[(TTP)P(V)CI)+and (b) [(TTP)P(V)(AZB)2)+with 25 ps pulse excitation.

Table 1: Figures of merit for optical limiting, Lifetimesof higher excited state and ISC times for themodified phosphorus (V) tetratolylporphyrinmolecules estimated from Z-scan curves with "25

ps pulses and from b6 ns pulses. Values for[(TTP)PV(OH)2)+ are also given for comparison.

Porphyrin 1112 "cr/cro ""tso bcr/cro b"t;sc

(J cm-Z) (fsec) (psec)

tetratolylporphyrins., Excited state parameters of thestarting porphyrin are also given for comparison. Both

I theheavy atom effect and the PET played an important. role in enhancing the optical limiting properties with

the nanosecond excitation [14]. PET has resulted inI the reduction of saturation at higher intensities withI ps excitation.

In order to control the excited state dynamics toachieve better optical limiting properties, we extended

,our studies to porphyrinoligomers. We studied theI 'axial-bonding' type hybrid porphyrin trimer andI hexamerarrays based on tin(IV) tetratolylporphyrin

scaffold. The architecture of the trimer arrays [15] isI such thatSnIV complex of meso-5,10,15,20-I (tetratolyl)porphyrin forms the basal scaffolding unit,I the free-base, Nil(porphyrins occupy the two axial si tesI ::!a an aryloxy bridge. The architecture of hexamersI [16] is based on a covalently linked SnIv porphyrinI dimer, with eaoo of the two SnIv porphyrins centersI trans-axially ligated to two free-base, zinc(II)

porphyrins. The effect of different central metal atonlS

0.75

Q)u

~0.50's~'"~ 0.25

0 Sn ""'Klmer

. Sn.(H,),lrimu

. Sn,-m,',hex"",,,0.00IE-3 0.01 0.1 I 10 100

Input Fluence (1 cm.2)

Fig. 4: Nonlinear transmittance curves of hybrid arrays basedon Sn([V)TTP scaffold.

substituted adjacent to the tin(IV) porphyrin in theoligomer structure is 'studied. In these systems inaddition to the PET from the axialty linked porphyrinsto the central Sn(IV)TTP, excitation energy transfer(EET)from the central Sn(IV)TTP to the axial porphyrinsis operative. Both the PET and EET lead to furtherdelocalization of the singlet states.

With 6 ns pulse excitation very goodenhancement in the optical limiting properties due toRSAis observed. OLcurves for monomer SnTTP, trimer

Sn2-(H2)2(TIP)3 ~d hexamerSHz-(H2)4 (TTP)6at ~75%linear transmission at 532nm shown in Fig.4 clearlyindicates the enhanced limiting behavior as one movesto higher homologues.With the introduction of heaviermetalloporphyrin in the axial position in place of free-base porphyrin, in trimer and hexamer donor-acceptorhomologues, lead to slightly reduced limiting response.Though the limiting threshold has not varied greatly,the onset of limiting varies certainly, wi th lower valuesfor the oligomers. Throughput fl1iences fr9m thesehybrids are as low as 35 -52 mJcm-2 with input fluencesin the range of ~ 26-74Jcm-2making these arrays verygood optical limiters at higher intensities. Table 2summarizes the material figure of merits and the ISCtimes.

With ps pulse excitation the nonlinear absorptionbehavior is totally different from that of observed withns excitation. At lower intensities for ps pulses thesematerials'show only RSA. With increasing intensityan interesting changeover from RSA to SA and thenback to RSA is observed. At higher intensities the ESAsaturatesa...d SA followed by RSA is observed. At stillhigher intensities the nonlinear absorption shows asudden transition to RSA at higher intensities withinSAbehavior (Fig. 5(a».

In the case of trimers and hexamers with

Sn(IV)TTP surrounded by freebase porphyrin (H2TTP),

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[(TTP)PV(OH)2]+ 3.0 2.67 450 2.086 1000

[(TTP)pvClz]+ 0.1 3.56 600 12.15 100

[(TTP)PV(AZB)2]+2.02 2.93 440 4.76 750

[(TTP)PV(AZT)2]+1.52 2.88 415 8.44 620

[(TTP)PV(AZN)2]+1.30 3.07 375 9.28 500

Table 2: Excited state parameters 0)aO' Tsnand TPA coefficient ~with 25 ps pulse excitation and a/all' 'rIsewithnspu1~~.",':I"~;~IC:. -

n~

n..,,;zu,<m),

04

i "'lC.~ I",... Ij(.

] 0

,

4 ""'.Im"p .

§ 01Z

., -I 0

71Z,.

Fig. 5: Opell aperture Z-scan curves of fa) 5n2-Z114 hexamer

and (b) SI1-Ni2 trimer with 25 ps pulse excitatioll.

at lower input intensities RSA is observed and withincreasing intensity, saturation of the higher exCitedstates due to the S] -7 S" transition is observed. Whilereplacing H2TIP in axial position with NiTIP lead toa total reversal in the nonlinear absorptiorL behavior.For Sn-Ni2(TTP)3'at lower input intensities saturationof absorption alone is observed. With gradual increasein the input intensity RSA followed by initial SA isobserved. As the intensity is increased further, RSAstarted to take over al1d at higher intensities RSAcompletelydominates due to TPA (Fig.5(b)). However,Sn2-Zn4(TTP)6has shown an interesting nonlinearabsorption behavior with varying input intensities. Atlower intensities (- 29 GWcm-Z), saturation ofabsorption due to excited states is observed (similar toFig 3a) with transmittance at focus going up to 0.84and the curves appear slightly broader indicating thecontribution of ESA. As the intensity increased to -53GWcm-zRSAis.,9bserved after saturation of absorptiondue to TPA (similar to that shown in Fig. 5(a)). Such acrossover from RSA to SA and then back to RSA with

25 ps pulses can be utilized to make these arraysdesirable materials for nonlinear optical switchingpurposes. Though the ESA, TPA and the lifetimes ofthe higher excited states appears to play an importantrole in the nonlinear absorption of all these oligomers,TPA is more predominant in oligomers having metal-metal interactions (Sn-Ni, Sn-Sn, Sn-Zn) and is lowerin molecules having metal-freebase interaction. Inmetalloporphyrins, Nand L bands exist at higherenergies (317 nm) in addition to the low energy BandQ bands. The Nand L bands are more prominent inSnTIP compared to NiTIP and ZnTIP, whereas inHzTTP, Nand L bands are not prominent. At higherexcitation intensities, the possibility of two-photontransition to the N, Lbands from the lower singlet statesincreases[17].

5. Summary

To summarize we have studied the effect of

introduction of halogen atom leading to increased ISc,donor-acceptor units leading to PET, and the hybridarrays with both the heavy atom, PET and additionalEET on the optical limiting properties. With thesestructural modifications we are able to identify thematerials that show.RSA at both the nanosecond andpicosecond pulse regimes.

Acknowledgements

Authors thank financial support from DAE-BRNS and DRDO. Authors are grateful to Late Prof.Bhaskar G. Maiya, for his invaluable discussions andthe porphyrins.

6. References

1. L. W. Tutt and TF. Boggess, Prog.Quant. Elce/ron. 17, 299(1993).

2. R. Sutherland, R. Pachter, P. Hood, D. Hagan, K. Lewis,j.W. Perry, eds., Materials for Optical limiting II, VoJ.479 ofMater. Res. Soc. Proc. (MRS, PHtsburrgh).

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Porphyrin iV2 °a/ao

(J cm-2)

Sn(TIP) 12.30 18.32

Sn-(Hz)Z(TIP)3 2.91 3.13

Sn-NiPTP)3 3.46 35.7

Sn2-(Hz)4(TIP)6 0.46 1.93

Sn2-Zn4(TIP)6 1.16 2.39

Tsn bcr/cro 'r.IS<

(psed (em/GW) (psec)

60 0.45 3.24 1000

500. - 7.26 380

50 1.45 6.21 550

500 - 19.45 220

15 0.9 6.73 600

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and K J. McEwan, Adv; Miller. 13,652 (20Ot). -11. N.K.M.N. Srinivas, S.V.Rao,D.V.GL.N. Rao, B.K.Kimball,

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17. P.P. Kiran, D.R. Reddy, B.G.Maiya, A.K. Dharmadhikari,GR. Kumar and D.N. Rao, Manuscript submitted to Opt.Commun.

About the Author

D. Narayana Rao is a Professor in the School of Physics at University ofHyderabad. He obtained his Ph. D. in 1979.from IIT Kanpur. His areas ofinterest are experimental nonlinear optics and spectroscopy.

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