Modeling of Nonlinear Absorption of 5,10-A₂B₂Porphyrins in the Nanosecond Regime

Abstract

The nonlinear absorption (NLA) properties of free base and metalated porphyrins with a unique 5,10-A(2)B(2) substitution pattern were studied with open Z-scan technique at 532 nm in the nanosecond regime over a broad range of input fluence. The NLA responses were found to be dependent on structural features and the solvent used. In most cases, the character of the response differed from that typically observed for other porphyrins under similar experimental conditions (i.e., reverse saturable absorption, RSA), suggesting distinct photophysical properties. A series of models was considered in order to understand the NLA responses. Although the character of the responses varied between compounds and solvents, it was found that one model fits best all of the recorded responses. Thus, a four-level model with simultaneous two-photon absorption arising from the higher excited state is proposed (1 + 1 + 2 photon absorption).

Solvent effect on the nonlinear absorption of 5,10-A2B2meso substituted porphyrins

“…As demonstrated previously, the population of the state cannot be neglected and the 35 population buil-up of the unabsorptive state leads to an RSA/SA switch. 11 increases with increasing for the four-level model (Fig. 8 H,K).…”

“…14,16,17 The remaining data for studies in DMF and toluene can be found in the previous publications. 11,12 The following trend in the solvent effect on could be 40 observed for most compounds: (DMF) < (toluene) ( respectively. The impact of these parameters on the NLA responses was studied.…”

“…High values of are desirable for OL. This is because the absorptive processes arising from the state are highly inefficient, 11 and high 20 will impede the contribution of these processes to NLA. Finally, high ratios of the excited to ground state absorption crosssections and enhance the nonlinear absorption coefficient, and therefore a transmission drop, which can be easily deduced from Eq.…”

“…12 The data obtained in the whole range of input energies applied were fitted with a four-level model which 30 we developed recently. 11 An illustration of the model and definition of the parameters used in the fitting are given in Fig. 3.…”

“…11 stands for ground state absorption coefficient at 532 nm, I -for input intensity and z' for the distance along the direction of the beam propagation. …”