Synthetic Metals 155 (2005) 291–294 Optical excitations of porphyrin J-aggregates A. Tonizzo a,∗ , M. Cerminara a , G. Macchi a , F. Meinardi a , N. Periasamy c , P. Sozzani b , R. Tubino a a INFM and Universit` a degli Studi di Milano-Bicocca, Dipartimento di Scienza dei Materiali, via Cozzi 53, I-20125 Milano, Italy b Universit` a degli Studi di Milano-Bicocca, Dipartimento di Scienza dei Materiali, via Cozzi 53, I-20125 Milano, Italy c Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, 400005 Mumbai, India Available online 4 November 2005 Abstract In this work, we will present a study of the optical properties of aggregated tetrakis(p-sulfonatophenyl)porphyrin (TSPP) in different environments which can have a dramatic influence on the spectroscopic features. We will show that when the porphyrins are free to self-organize (i.e. in saturated solutions) they show two distinct photoluminescence peaks, revealing the presence of two different species, with a relative population strongly dependent on the temperature. If aggregation occurs in an environment with restricted geometry (i.e. within the nanochannels of MCM-41) it is possible to modify the spontaneous self-assembly of the molecules and, as a consequence, the relative ratio of the two emissions intensity. We will report on the possibility to induce the preferential formation of the low-energy emitting species, aiming to tune in a controlled way the emission spectrum of the aggregate. © 2005 Elsevier B.V. All rights reserved. Keywords: Porphyrins; Photoluminescence; Collective phenomena; Inclusion compounds 1. Introduction Self-assembling processes of molecular components into large supramolecular structures are primarily investigated because of their involvement in many fundamental physico- chemical as well as biological processes. The possibility of changing the mesoscopic structure of the resulting species through a proper choice of the molecular components opens the way to the design and synthesis of materials capable to exhibit specific properties and functions [1]. From this point of view, porphyrins are well suited building blocks because, depending on their electronic and steric properties, they can spontaneously self-assemble into dimers or higher aggregates through non- covalent interactions [2]. In particular, water-soluble porphyrins are very interesting because aggregation can be conveniently controlled by screening the charge repulsion by changes in the ionic strength and pH. Many relevant physicochemical proper- ties of this class of compounds, including photophysical fea- tures, are strictly dependent on their aggregation state. On the basis of these properties, a challenging problem is the translation of the electron transfer high efficiency and speed, ∗ Corresponding author. often found for biological reactions, to the world of molecular photonic materials or photonic devices (i.e. biomimetics), aim- ing to optimize their performances. In particular, the tetrakis(p- sulfonatophenyl)porphyrin (TSPP) has recently received partic- ular attention because it is able to form, under proper experi- mental conditions, highly ordered J- and/or H-aggregates [3–5]. The H- and J-aggregates correspond to the limiting cases of parallel monomeric units stacked face-to-face or edge-to-edge, respectively. According to the excitonic splitting theory [6], H- aggregates exhibit a blue-shifted Soret absorption band, whereas J-aggregates are characterized by a red-shifted absorption band. In this paper, we present a detailed study of the spectroscopic properties of TSPP in monomeric and aggregated state, elucidat- ing the role of aggregation on the excitonic properties. Further- more, we examine the formation of porphyrin aggregates within a mesoporous cage compound (specifically an aluminosilicate mesostructure MCM-41), intended to create an encapsulated species where constraints in aggregate length and orientation, associated with the spatially constricted and directional charac- ter of the cavity, result in a new “ship-in-a-bottle-type” nanoma- terial, possessing novel spectroscopic properties. Additionally, in this kind of compound the aggregates assume a more robust and manipulable physical form, which would be more suitable for optical and optoelectronic applications. 0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.09.005