TWO-PROTON PHOTOTAUTOMERIZATION DYNAMICS OF 7-AZAINDOLE COMPLEXES -Tapas Chakraborty Indian Association for the Cultivation of Science Calcutta, India TK02, OSU ICMS 2013

Tautomeric forms of 7-azaindole S0S0 S1S1 4.2 eV 2.8 eV Normal Tautomer 0.65 eV (~ 15 kcal/mol )

Double proton transfer phenomenon UV light  300 nm Taylor, El-Bayoumi and Kasha, PNAS, 63 (1969) 253

Taylor, El-Bayoumi and Kasha, PNAS, 63 (1969) 253 UV light < 300 nm  DPT  1.1 ps Wavelength (nm) ~470 nm Tautomer emission Fluorescence spectra Wavelength (nm) ~320 nm >350 nm Normal emission Absorption spectra M M Double proton transfer phenomenon

UV light < 300 nm  DPT  1.1 ps Key Issues Double Proton Transfer sequence :Concerted vs Stepwise Role of exciton interactions Medium effect Double Proton Transfer Barrier (  1.5 kcal/mol) Double proton transfer phenomenon

UV light < 300 nm  DPT  1.1 ps Key Issues Double Proton Transfer sequence :Concerted vs Stepwise Role of exciton interactions Medium effect Double Proton Transfer Barrier (  1.5 kcal/mol) Double proton transfer phenomenon

Double proton transfer barrier (original expt.) Estimated barrier: 1.4 kcal/mol F2F2 F1F1

Tautomerization under jet-cooling condition (  10 K)

(7-azaindole) 2 is synthesized in Ar matrix

Inert gas (Ne, Ar, N 2, etc.) crystals pack in Face-Centered cubic lattice structures Pimentel ( ) (7-azaindole) 2 is synthesized in Ar matrix

Wavenumber (cm -1 ) a b c d Dimer in Ar Matrix Dimer in a supersonic jet expansion Dimer in CCl 4 solution (7-azaindole) 2 formation in Ar matrix is probed using infrared spectroscopy Mukherjee and Chakraborty, CPL 546 (2012) 74

Q e NORMAL S0S0 S1S1 Q e TAUTOMER Q V(Q) X DPT of (7-azaindole) 2 is inhibited in Ar matrix Mukherjee and Chakraborty, CPL 546 (2012) 74

Wavelength (nm) Intensity ex = nm ex = 310 nm X Annealed matrix Intensity Wavelength (nm) DPT of (7-azaindole) 2 is inhibited in Ar matrix Wavenumber (cm -1 ) d Pre-annealed matrix

7AICyclic amides O N N N H H C ThymineAdenine 7-azaindole∙∙∙amide complexes O N N N

7-azaindole∙∙∙δ-valerolactum complex (7AI  VL) Normal (7AI  VL) Tautomer S0S0 S1S1 6.0 kcal/mol 18.5 kcal/mol ~67.0 kcal/mol 91.7 kcal/mol (7AI 2 ) Normal (7AI 2 ) Tautomer S0S0 S1S kcal/mol 14.5 kcal/mol 67.5 kcal/mol 92.2 kcal/mol 7AI  -valerolactam (  VL) Normal formTautomeric form CIS/ G**

Wavelength (nm) ex = 312 nm M 7AI M 7AI M  VL AI+  VL Wavelength (nm) 1/[  VL] 1/A K a = 1.64  10 3 M -1 7AI+  VL  Efficient tautomeric conversion is observed in hydrocarbon solution at room temperature Tautomerization of 7-azaindole∙∙∙δ-valerolactum complex Mukherjee et al, JPCA, 2012

Wavelength (nm) 7AI+  VL Wavenumber (cm -1 )   prob = total fluorescence     Wavelength (nm) Energy Reaction Co-ordinate  v = 0 v =1 v = 2  v = n v  = 0 v  = 1 v  = 2 v  = n S0S0 S1S1 X X Tautomerization of the complex is inhibited under a jet-cooling condition

 Wavelength (nm) T(%)  prob = 300 – 400 nm (local emission)  Wavenumber (cm -1 )   prob = total fluorescence        prob = 400 – 750 nm (tautomer emission) Wavelength (nm) T(%)  No tautomeric conversion in jet cooled condition cm -1 S0S0 S1S1 Mukherjee et al, JPCA, 2012 Tautomerization of the complex is inhibited under a jet-cooling condition

Photophysics of 7-azaindole  Pyrazole complex S0 S0 S1 S1 S1S1 S0S0 8.2 kcal/mol 13.4 kcal/mol 99.6 kcal/mol 78.0 kcal.mol 16 kcal/mol Karmakar and Chakraborty, CPL, 2013

Tautomerization of 7-azaindole  Pyrazole complex is inhibited in a supersonic jet Karmakar and Chakraborty, CPL, 2013

Complex vs homodimer 7-azaindole  Pyrazole complex 7-azaindole  cyclic amide complex

Concluding remarks Excited state tautomerization of 7-azaindole dimer is easily affected by external perturbations Compared to homodimer, the tautomerization barriers of mixed dimeric complexes appear much larger, and further studies are essential to understand the origin.

Acknowledgement: Co-workers: Dr. Moitrayee Mukherjee Ms. Shreetama Karmakar Funding: Department of Science and Technology, Govt. of India