Excited State Dynamics and Femtosecond Nonlinear Optical Studies of Novel Porphyrins, Phthalocyanines using Transient Absorption Spectroscopy and Z-scan Techniques

Abstract

Organic molecules such as porphyrins (P), phthalocyanines (Pc) and their derivatives are synthetic dyes with unique molecular structure possessing huge potential in the world of photonics and photomedicine [1, 2, 3, 4, 5]. Porphyrin molecules contain four pyrrole rings linked by the methane carbon bridges. On the other hand, Pc molecules are composed of four indole units which are pyrrole rings linked by nitrogen atoms conjugated with benzene rings [6]. These molecules have conjugated aromatic structure with 18 π delocalized electrons [7]. These electrons give rise to interesting optical and photophysical properties like very high absorption in visible to IR wavelength and long-lived excited state components which is ideal for photovoltaics [5] and biological applications [8]. For appropriate light intensity, the free electron cloud of a porphyrin or phthalocyanine can give rise to polarizability in electric field giving rise to nonlinear optical (NLO) effect [9]. These NLO properties find potential application in optical devices such as saturable absorbers, optical data storage, optical limiters, multiphoton filament formation, frequency mixing, and up-conversion, etc. [10, 11, 12, 13]. Porphyrins and phthalocyanines can exist as free bases or as metallo-complexes usually achieved by incorporating a metal at the center of their core structure in which case the central metal can accept or donate electron to the P or Pc moiety [14]. In photovoltaic cells, the dye molecules absorb solar light energy which in turn generates electronically excited states. The excited state can migrate to an interface where an electron transfer takes place, enabling the oxidized and reduced species (holes and electrons) to be able to migrate to the opposite sides of the cell, collected as electrical energy [5]. The first reported case of using organic dyes such as porphyrins and phthalocyanines for photovoltaics was in 1986 by C.W. Tang [15], who constructed an organic solar cell constituting a heterojunction bilayer. The bilayer was composed of Cu-phthalocyanine along with a perylene derivative. They later came to be termed as DSSC or dye sensitized solar cells. Compared to commercial silicon based solar cells, DSSCs have low cost, are low in toxicity and easy to produce [16]. However, DSSC comes with some major challenges which makes it difficult to be manufactured commercially like – low solar cell efficiency and no long-term stability. These challenges can be dealt with a detailed