State-of-the-art ab initio Quantum Chemical Computational Methods for Describing the Challenging Photobehaviour of Cationic Dyes

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1 State-of-the-art ab initio Quantum Chemical Computational Methods for Describing the Challenging Photobehaviour of Cationic Dyes Enrico Benassi Scuola ormale Superiore, Pisa (Italy)

2 On the multipole moments r 1,... q i i i 1 Q 4 r Q l r l1 0 l0 l l qiri Pl cos i i1 system of point charges total electric (multipole) potential electric multipole moments of order l l = 0 l = 1 l = 2 etc. electric «monopole» (charge) electric dipole electric quadrupole

3 Quadrupolar Systems (QSs) Organic chromophores bearing electron donor (D) or acceptor (A) groups linked by π conjugated bridges Asymmetric, Dipolar Systems Symmetric, Quadrupolar Systems D π A D π A π D A π D π A extensively investigated still to be investigated in depth

4 Quadrupolar Systems (QSs) ew, highly performing, low cost materials in on-linear Optics (LO), particularly for their Two-Photon Absorption (TPA) and Emission (TPE) Sn hnuv hnir hnir S0 One-Photon Absorption Two-Photon Absorption Two-Photon Absorption spectra and cross-sections by using Two- Photon Excited Fluorescence Two-Photon excited fluorescence microscopy in vitro on cells loaded with the QSs

5 Applications and impact Imaging high fluorescence Compounds showing significant TPA coupled with... solvatochromic prop. Sensing One-Photon excitation Two-Photon excitation One-Photon fluorescence micros. Two-Photon fluorescence micros.

6 Applications and impact Photo-Dynamic Therapy high singlet oxygen generation Two-Photon excitation allows: higher penetration depth; decreased photodamage; enhanced spatial selectivity Imaging high fluorescence Compounds showing significant TPA coupled with... solvatochromic prop. Sensing

7 Applications and impact Drug Delivery, Ophtalmology, Photo-Dynamic Therapy high photoreactivity high singlet oxygen generation Imaging high fluorescence Compounds showing significant TPA coupled with... solvatochromic prop. Sensing

8 Applications and impact Photo-Dynamic Therapy Drug Delivery, Ophtalmology, Smart materials high singlet oxygen generation environment sensitivity Imaging high fluorescence Compounds showing significant TPA coupled with... solvatochromic prop. Sensing

9 Applications and impact Photo-Dynamic Therapy Drug Delivery, Ophtalmology, Smart materials high singlet oxygen generation environment sensitivity Imaging high fluorescence Compounds showing significant TPA coupled with... solvatochromic prop. Sensing high excited state absorption Optical Power Limiting

10 Applications and impact Photo-Dynamic Therapy Drug Delivery, Ophtalmology, Smart materials high singlet oxygen generation high fluorescence Imaging high triplet production Compounds showing significant TPA coupled with... environment sensitivity solvatochromic prop. Sensing high excited state absorption Micro- and ano- Fabrication Optical Power Limiting

11 rising TPA response Proposed Classification F. Terenziani, A. Painelli, C. Katan, M. Charlot, M. Blanchard-Desce, J. Am. Chem. Soc., 2006, 128, Modeling by essential-state calculations on neutral QSs:. class I: dipolar excited state (positive fluorosolvatochromism). class II: stable, non-dipolar states (no solvatochromism). class III: dipolar ground state (negative solvatochromism) The highly symmetric neutral QSs have surprisingly shown spectra significantly affected by the solvent coupled with enhanced TPA

12 Aim of the Work Spectral and photophysical behaviour of charged quadrupolar (and analogous dipolar) compounds in solvents of different polarity. Joint experimental and theoretical investigation about the role of ICT in the excited states of a charged QS Experimental investigation by using steady-state and transient absorption and emission spectroscopic techniques with ns- and fs- time resolution. Theoretical study of the structural and electronic properties by TD-DFT calculations.

13 Density Functional Theory Density functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure of many-body systems. With this theory, the properties of a manyelectron system can be determined by using functionals (functions of another function), which in this case is the spatially dependent electron density.

14 Hˆ E r, r, 1 2 r Many-Body Problem 2 ˆ H V U, i1 2mi i1 i j 2 i r i ri rj As usual in many-body electronic structure calculations, the nuclei of the treated molecules are seen as fixed (Born Oppenheimer approximation) DFT Many-body problem Single-body problem (without U ) ˆ

15 n r d r1 d r2 d r r1, r2, r r1, r2, r * n 0 0 A n n Aˆ n e.g.: E E n n Hˆ n d r r E n T n U n rv n min E n (Lagrange's multipliers)

16 We initially consider an energy functional that does not explicitly have an electron-electron interaction energy term E n n Tˆ Vˆ n 2 3 ns r s s s s Vˆ ˆ s r V r e d r Vxc n s r r r n r n r r s i1 2 2 Vˆ s i ii 2me i 2 r r r (Kohn-Sham equations). Iterative method.. The major problem with DFT is that the exact functionals for exchange and correlation are not known except for the free electron gas. However, approximations exist which permit the calculation of certain physical quantities quite accurately.

17 Investigated system: cationic QS (E) (4 dimethylamino styryl) 1 methyl pyridinium (2,6(E,E) 2 (4 dimethylamino styryl) 1 methyl pyridinium A π D D π A π D CATIOIC QSs: only a couple of very recent experimental studies on positively charged quadrupolar (A π D π A) + class III polymethine dyes were reported in the literature.

18 Investigated system: cationic QS 2 + CHCl 3 water

19 Intensity/ a.u. Investigated system: cationic QS normalised absorption and emission spectra 1 2 Absorption Emission DCE low polarity solvent 1.0 Absorption Emission W highly polar solvent / nm

20 Fluorescence Intensity/ a.u. Absorbance DCM DCE CHCl 3 A DCM DCE 2-PrOH/DCE 50/50 2-PrOH EtOH MeOH W/EtOH 50/50 W/EtOH 70/30 W ~ 640 nm B Solvent effects on the spectral properties of 2 Solvents employed in this study: (i) chlorinated solvents; (ii) protic solvents; (iii) non protic and non chlorinated solvents PrOH EtOH 1.0 W ~ 590 nm (nm) ~ 625 nm C D Within each solvent class, the solvatochromic shifts show a correlation with solvent polarity. Small effects due to H-bonding interactions in the highly protic water. o evidence of ionic pairs emerged in the considered media (the obtained electronic properties can be ascribed uniquely to the cation).

21 Fluorescence Intensity/ a.u. Absorbance PrOH EtOH 1.0 DCM DCE W ~ 640 nm ~ 625 nm CHCl 3 A DCM DCE 2-PrOH/DCE 50/50 2-PrOH EtOH MeOH W/EtOH 50/50 W/EtOH 70/30 W B C D Solvent effects on the spectral properties of 2 With increasing the solvent polarity: Blue shift of the absorption band (from 530 nm in DCM to 464 nm in W). Enlargement of the absorption band. Decrease of the molar extinction coefficient εmax (M 1 cm 1 ) FWHMabs (cm 1 ) DCM W ~ 590 nm (nm) charge transfer nature of the transition

22 chlorinated solvents protic solvents non protic and non chlorinated solvents W n abs /cm THF W/EtOH 70/30 MeC Ac W/EtOH 50/50 DMSO MeOH 30/70 EtOHW/EtOH 2-PrOH DCE DCM f(ε,n 2 ) = [2(ε-1)/(2ε+1)]-[2(n f (n 2 ) 2-1)/(2n 2 +1)]

23 Fluorescence Intensity/ a.u. Absorbance PrOH EtOH DCM DCE ~ 640 nm ~ 625 nm CHCl 3 A DCM DCE 2-PrOH/DCE 50/50 2-PrOH EtOH MeOH W/EtOH 50/50 W/EtOH 70/30 W B C Solvent effects on the spectral properties of 2 Rationalising the solvatochromism by QM calculations of the electronic properties of 2: complex behaviour!. Monopole (charge) contribution. Dipole contribution (negligible and not solvent sensitive). Quadrupole contribution 1.0 W D ~ 590 nm (nm)

24 Fluorescence Intensity/ a.u. solvent polarity Absorbance PrOH EtOH 1.0 DCM DCE W ~ 640 nm ~ 625 nm CHCl 3 A DCM DCE 2-PrOH/DCE 50/50 2-PrOH EtOH MeOH W/EtOH 50/50 W/EtOH 70/30 W 640 nm 620 nm B C D Solvent effects on the spectral properties of 2 The emission spectrum of 1 shows a negligible dependence on solvent polarity (λ max = 590 nm in DCM, 589 nm in W). The emission spectrum of 2 is affected by the solvent polarity (hypsochromic shift of 50 nm, responsible for a fluorescence profile similar to that observed for the dipolar compound 1 in W). ~ 590 nm 590 nm (nm)

25 Fluorescence Quantum Yields 1 2 Solvent f(ε,n 2 ) ϕf CHCl DCM DCE PrOH EtOH MeC MeOH EtOH/W W, ph=

26 Rotameric equilibria H 3 C H 3 C H 3 C A B C E U H E / kcal mol ZPE / kcal mol / kcal mol / kcal mol G / kcal mol S 1 1 / cal K mol A B C (TD-)DFT CAM-B3LYP / cc-pvtz (PCM)

27 Rotameric equilibria H 3 C H 3 C H 3 C A B C

28 Transition Energies and Oscillator Strengths 2 Experimental Theoretical λabs/nm λem/nm f01 λabs/nm λem/nm f01 f10 DCM MeC (W)

29 S 0 Solvent effects on the geometry change DCM δ MeC δ S 1 DCM MeC δ δ δ δ S S

30 Sketch of the ground (red) and excited (blue) electronic states in low polar (dashed) and polar (solid) solvents cationic QSs.

31 Frontier Molecular Orbitals DCM LUMO HOMO optimised S0 optimised S1 LUMO MeC HOMO optimised S0 optimised S1

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33 Acknowledgments Prof. Vincenzo Barone (SS, Pisa) Prof. Chiara Cappelli (University of Pisa) Dr. Franco Egidi (SS, Pisa) Prof. Fausto Elisei (University of Perugia) Prof. Anna Spalletti (University of Perugia) Dr. Benedetta Carlotti (University of Perugia)

34 THAKS! Quod si deficiant vires audacia certe laus erit, in magnis et voluisse sat est [Sextius Propertius]